Angular官方文档学习笔记第2篇_数据绑定_模板语法

Displaying Data

You can display data by

binding controls in an HTML template

to properties of an Angular component.

In this page, you'll create a component with a list of girls.

You'll display the list of girl names

and conditionally show a message below the list.

The final UI looks like this:

The live example / download example demonstrates all of the syntax and code snippets described in this page.

Showing component properties with interpolation

The easiest way to display a component property

is to bind the property name through interpolation.

With interpolation, you put the property name in the view template,

enclosed in double curly braces: {{myGirl}}.

Follow the quickstart instructions for creating a new project named displaying-data.

ng create displaying-data

如图所示:(可能要花几分钟)

然后cd displaying-data

执行ng serve启动项目

打开浏览器http://localhost:4200可以访问项目的首页

Delete the app.component.html file. It is not needed for this example.

Then modify the app.component.ts file

by changing the template and the body of the component.

意思就是: 去掉templateUrl: './app.component.html'

换成

template: `<h1>未闻花名</h1>`

When you're done, it should look like this:

src/app/app.component.ts

import { Component } from '@angular/core';
@Component({
selector: 'app-root',
template: `
<h1>{{title}}</h1>
<h2>My favorite girl is: {{myGirl}}</h2>
`
})
export class AppComponent {
title = '未闻花名';
myGirl = '面码';
}

You added two properties to the formerly ( 之前 ) empty component: title and myHero.

The template displays the two component properties using double curly brace interpolation:

src/app/app.component.ts (template)

template: `
  <h1>{{title}}</h1>
  <h2>My favorite girl is: {{myGirl}}</h2>
  `

The template is a multi-line string within ECMAScript 2015 backticks (`).

The backtick (`)—which is not the same character as a single quote (')—

allows you to compose a string over several lines, which makes the HTML more readable.

Angular automatically pulls the value of the title and myHero properties from the component

and inserts those values into the browser.

Angular 自动 updates the display when these properties change.

More precisely, the redisplay occurs after some kind of asynchronous event related to the view,

such as a keystroke,

a timer completion,

or a response to an HTTP request.

Notice that you don't call new to create an instance of the AppComponent class.

Angular is creating an instance for you. How?

The CSS selector in the @Component decorator specifies an element named <app-root>.

That element is a placeholder in the body of your index.html file:

src/index.html (body)

<body>
  <app-root></app-root>
</body>

When you bootstrap with the AppComponent class (in main.ts),

Angular looks for a <app-root> in the index.html,

finds it, instantiates an instance of AppComponent,

and renders it inside the <app-root> tag.

Now run the app. It should display the title and hero name:

The next few sections review some of the coding choices in the app.

这句话什么意思???

Template inline or template file?

You can store your component's template in one of two places.

You can define it inline using the template property,

or you can define the template in a separate HTML file

and link to it in the component metadata using the @Component decorator's templateUrl property.

The choice between inline and separate HTML is a matter of taste, circumstances, and organization policy.

Here the app uses inline HTML because the template is small and the demo is simpler without the additional HTML file.

In either style, the template data bindings have the same access to the component's properties.

By default, the Angular CLI generates components with a template file. You can override that with:

什么意思???

-it 表示的是使用内联模板

inline template的缩写

ng generate component hero -it

Constructor or variable initialization?

Although this example uses variable assignment to initialize the components,

you could instead declare and initialize the properties using a constructor:

export class AppCtorComponent {
  title: string;
  myGirl: string;

  constructor() {
    this.title = '未闻花名';
    this.myGirl = '面码';
  }
}

This app uses more terse ( 简短的 ) "variable assignment" style simply for brevity.

Showing an array property with *ngFor

To display a list of heroes,

begin by adding an array of hero names to the component

and redefine myHero to be the first name in the array.

src/app/app.component.ts (class)

export class AppComponent {
  title = '世界萌王';
  heroes = ['面码', '逢坂大河', '艾拉', '古手梨花'];
  myHero = this.heroes[0];
}

Now use the Angular ngFor directive in the template

to display each item in the heroes list.

src/app/app.component.ts (template)

template: `
  <h1>{{title}}</h1>
  <h2>My favorite hero is: {{myHero}}</h2>
  <p>Heroes:</p>
  <ul>
    <li *ngFor="let hero of heroes">
      {{ hero }}
    </li>
  </ul>
`

This UI uses the HTML unordered list with <ul> and <li> tags.

The *ngFor in the <li> element is the Angular "repeater" directive.

It marks that <li> element (and its children) as the "repeater template":

src/app/app.component.ts (li)

<li *ngFor="let hero of heroes">
  {{ hero }}
</li>

Don't forget the leading asterisk (*) in *ngFor.

It is an essential part of the syntax.

For more information, see the Template Syntax page.

Notice the hero in the ngFor double-quoted instruction;

it is an example of a template input variable.

Read more about template input variables in the microsyntax ( 微语法 ) section of the Template Syntax page.

Angular duplicates the <li> for each item in the list,

setting the hero variable to the item (the hero) in the current iteration.

Angular uses that variable as the context for the interpolation in the double curly braces.

In this case, ngFor is displaying an array,

but ngFor can repeat items for any iterable object.

Now the girls appear in an unordered list.

Creating a class for the data

The app's code defines the data directly inside the component,

which isn't best practice.

In a simple demo, however, it's fine.

At the moment, the binding is to an array of strings.

In real applications, most bindings are to more specialized objects.

To convert this binding to use specialized objects,

turn the array of hero names into an array of Hero objects.

For that you'll need a Girl class:

ng generate class girl

With the following code:

src/app/hero.ts

export class Hero {
  constructor(
    public id: number,
    public name: string) { }
}

You've defined a class with a constructor and two properties: id and name.

It might not look like the class has properties, but it does.

The declaration of the constructor parameters takes advantage of a TypeScript shortcut.

Consider the first parameter:

src/app/hero.ts (id)

public id: number,

That brief syntax does a lot:

Declares a constructor parameter and its type.
Declares a public property of the same name.
Initializes that property with the corresponding argument when creating an instance of the class.

Using the Hero class

After importing the Hero class, the AppComponent.heroes property can return a typed array of Hero objects:

src/app/app.component.ts (heroes)

heroes = [
  new Hero(1, 'Windstorm'),
  new Hero(13, 'Bombasto'),
  new Hero(15, 'Magneta'),
  new Hero(20, 'Tornado')
];
myHero = this.heroes[0];

Next, update the template.

At the moment it displays the hero's id and name.

Fix that to display only the hero's name property.

src/app/app.component.ts (template)

template: `
  <h1>{{title}}</h1>
  <h2>My favorite hero is: {{myHero.name}}</h2>
  <p>Heroes:</p>
  <ul>
    <li *ngFor="let hero of heroes">
      {{ hero.name }}
    </li>
  </ul>
`

The display looks the same, but the code is clearer.

Conditional display with NgIf

Sometimes an app needs to display a view or a portion of a view only under specific circumstances.

Let's change the example to display a message if there are more than three heroes.

The Angular ngIf directive inserts or removes an element based on a truthy/falsy condition.

To see it in action, add the following paragraph at the bottom of the template:

src/app/app.component.ts (message)

<p *ngIf="heroes.length > 3">There are many heroes!</p>

Don't forget the leading asterisk (*) in *ngIf.

It is an essential part of the syntax.

Read more about ngIf and *in the ngIf section of the Template Syntax page.

The template expression inside the double quotes,

*ngIf="heroes.length > 3", looks and behaves much like TypeScript.

When the component's list of heroes has more than three items,

Angular adds the paragraph to the DOM and the message appears.

If there are three or fewer items,

Angular omits the paragraph, so no message appears.

For more information, see the template expressions section of the Template Syntax page.

Angular isn't showing and hiding the message.

划重点! Vue中指令有v-show和v-if

It is adding and removing the paragraph element from the DOM.

That improves performance, especially in larger projects

when conditionally including or excluding big chunks of HTML with many data bindings.

Try it out.

Because the array has four items, the message should appear.

Go back into app.component.ts and delete or comment out one of the elements from the hero array.

The browser should refresh automatically and the message should disappear.

Summary

Now you know how to use:

Interpolation with double curly braces to display a component property.
ngFor to display an array of items.
A TypeScript class to shape the model data for your component and display properties of that model.
ngIf to conditionally display a chunk of HTML based on a boolean expression.

Here's the final code:

Model 类 girl.ts

export class Girl {
    constructor(public id: number, public name: string){}
}

App Module ts文件

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';


@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

App Component ts文件

import { Component } from '@angular/core';
import { Girl } from './girl'
@Component({
  selector: 'app-root',
  template: `
  	<h1>{{title}}</h1>
  	<h2>My favorite girl is: <span class="class_span_girlName">{{firstGirl.name}}</span></h2>
      <ul>
          <li *ngFor="let girl of girls">
              {{ girl.name }}
          </li>
      </ul>
      
      <p *ngIf="girls.length > 3">
          There are many girls!
      </p>
  `,
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  // title: string
  // girlName: string

    title = '世界萌王'
    girls = [
        new Girl(1,'面码'),
        new Girl(2,'逢坂大河'),
        new Girl(3,'艾拉'),
        new Girl(4,'古手梨花')
    ]
    // 必须使用this
    firstGirl = this.girls[0]

  constructor(){
      // this.title = '未闻花名';
      // this.girlName = '面码';
  }
}

效果如下:

Template Syntax

The Angular application manages what the user sees and can do,

achieving this through the interaction of a component class instance (the component) and its user-facing template.

You may be familiar with the component/template duality

from your experience with model-view-controller (MVC)

or model-view-viewmodel (MVVM).

In Angular, the component plays the part of the controller/viewmodel ( ViewModel 驱动视图的数据 ),

and the template represents the view.

This page is a comprehensive technical reference to the Angular template language.

It explains basic principles of the template language

and describes most of the syntax that you'll encounter elsewhere in the documentation.

Many code snippets illustrate the points and concepts,

all of them available in the Template Syntax Live Code / download example.

HTML in templates

HTML is the language of the Angular template.

Almost all HTML syntax is valid template syntax.

The <script> element is a notable exception;

it is forbidden, eliminating the risk of script injection attacks.

In practice, <script> is ignored and a warning appears in the browser console.

See the Security page for details.

Some legal HTML doesn't make much sense in a template.

The <html>, <body>, and <base> elements have no useful role.

Pretty much everything else is fair game. 什么意思???

You can extend the HTML vocabulary of your templates

with components and directives that appear as new elements and attributes.

In the following sections, you'll learn

how to get and set DOM (Document Object Model) values dynamically through data binding.

Begin with the first form of data binding

—interpolation—to see how much richer template HTML can be.

Interpolation ( {{...}} )

You met the double-curly braces of interpolation, {{ and }}, early in your Angular education.

src/app/app.component.html

<p>My current girl is {{currentGirl.name}}</p>

You use interpolation to weave calculated strings into the text

between HTML element tags and within attribute assignments.

src/app/app.component.html

<h3>
  {{title}}
  <img src="{{heroImageUrl}}" style="height:30px">
</h3>

The text between the braces is often the name of a component property.

Angular replaces that name with the string value of the corresponding component property.

In the example above, Angular evaluates the title and heroImageUrl properties and "fills in the blanks",

first displaying a bold application title and then a heroic image.

More generally, the text between the braces is a template expression

that Angular first evaluates and then converts to a string.

The following interpolation illustrates the point by adding the two numbers:

src/app/app.component.html

<!-- "The sum of 1 + 1 is 2" -->
<p>The sum of 1 + 1 is {{1 + 1}}</p>

The expression can invoke methods of the host component such as getVal(), seen here:

src/app/app.component.html

<!-- "The sum of 1 + 1 is not 4" -->
<p>The sum of 1 + 1 is not {{1 + 1 + getVal()}}</p>

Angular evaluates all expressions in double curly braces,

converts the expression results to strings,

and links them with neighboring literal strings.

Finally, it assigns this composite interpolated result to an element or directive property.

You appear to be inserting the result between element tags and assigning it to attributes.

It's convenient to think so, and you rarely suffer for this mistake.

Though this is not exactly true.

Interpolation is a special syntax that Angular converts into a property binding, as is explained below.

But first, let's take a closer look at template expressions and statements.

Template expressions

A template expression produces a value.

Angular executes the expression and assigns it to a property of a binding target;

the target might be an HTML element, a component, or a directive.

The interpolation braces in {{1 + 1}} surround the template expression 1 + 1.

In the property binding section below,

a template expression appears in quotes to the right of the = symbol as in [property]="expression".

You write these template expressions in a language that looks like JavaScript.

Many JavaScript expressions are legal template expressions, but not all.

JavaScript expressions that have or promote side effects are prohibited, including:

assignments (=, +=, -=, ...)
new
chaining expressions with ; or ,
increment and decrement operators (++ and --)

Other notable differences from JavaScript syntax include:

no support for the bitwise operators | and &
new template expression operators, such as |, ?. and !.

Expression context

The expression context is typically the component instance.

In the following snippets, the title within double-curly braces

and the isUnchanged in quotes refer to properties of the AppComponent.

src/app/app.component.html

{{title}}
<span [hidden]="isUnchanged">changed</span>

An expression may also refer to properties of the template's context

such as a template input variable (let hero) or a template reference variable (#heroInput).

src/app/app.component.html

<div *ngFor="let hero of heroes">{{hero.name}}</div>
<input #heroInput> {{heroInput.value}}

The context for terms in an expression is a blend of

1. the template variables,

2. the directive's context object (if it has one),

3. and the component's members.

下面说说命名冲突的优先级:

If you reference a name that belongs to more than one of these namespaces,

1. the template variable name takes precedence 优先,

2. followed by a name in the directive's context, 其次

3. and, lastly, the component's member names. 最后

The previous example presents such a name collision.

The component has a hero property and the *ngFor defines a hero template variable.

The hero in {{hero.name}} refers to the template input variable, not the component's property.

Template expressions cannot refer to anything in the global namespace (except undefined).

They can't refer to window or document.

They can't call console.log or Math.max.

They are restricted to referencing members of the expression context.

Expression guidelines

Template expressions can make or break an application.

Please follow these guidelines:

No visible side effects
Quick execution
Simplicity
Idempotence

The only exceptions to these guidelines should be in specific circumstances that you thoroughly understand.

No visible side effects

A template expression should not change any application state other than the value of the target property.

This rule is essential to Angular's "unidirectional data flow ( 单向数据流 )" policy.

You should never worry that reading a component value might change some other displayed value.

The view should be stable throughout a single rendering pass.

Quick execution

Angular executes template expressions after every change detection cycle.

Change detection cycles are triggered by many asynchronous activities

such as promise resolutions, http results, timer events, keypresses and mouse moves.

Expressions should finish quickly

or ( 否则 ) the user experience may drag, especially on slower devices.

Consider caching values when their computation is expensive. 同Vue的计算属性

Simplicity

Although it's possible to write quite complex template expressions, you should avoid them.

A property name or method call should be the norm ( 常见 ).

An occasional Boolean negation (!) is OK.

Otherwise, confine application and business logic to the component itself,

where it will be easier to develop and test.

Idempotence

An idempotent expression is ideal ( 最佳 )

because it is free of side effects

and improves Angular's change detection performance.

In Angular terms,

an idempotent expression always returns exactly the same thing

until one of its dependent values changes.

Dependent values should not change during a single turn of the event loop.

If an idempotent expression returns a string or a number,

it returns the same string or number when called twice in a row.

If the expression returns an object (including an array),

it returns the same object reference when called twice in a row.

Template statements

A template statement responds to an event raised by a binding target

such as an element, component, or directive.

You'll see template statements in the event binding section,

appearing in quotes to the right of the = symbol as in (event)="statement".

src/app/app.component.html

<button (click)="deleteHero()"> Delete hero</button>

A template statement has a side effect.

That's the whole point of an event.

It's how you update application state from user action.

Responding to events is the other side of Angular's "unidirectional data flow".

You're free to change anything, anywhere, during this turn of the event loop.

Like template expressions,

template statements use a language that looks like JavaScript.

The template statement parser differs from the template expression parser

and specifically supports both basic assignment (=) and chaining expressions (with ; or ,).

However, certain JavaScript syntax is not allowed:

new
increment and decrement operators, ++ and --
operator assignment, such as += and -=
the bitwise operators | and &
the template expression operators

Statement context

As with expressions, statements can refer only to what's in the statement context

such as an event handling method of the component instance.

The statement context is typically the component instance.

The deleteHero in (click)="deleteHero()" is a method of the data-bound component.

src/app/app.component.html

<button (click)="deleteHero()"> Delete hero</button>

The statement context may also refer to properties of the template's own context.

In the following examples,

1. the template $event object,

2. a template input variable (let hero),

3. and a template reference variable (#heroForm)

are passed to an event handling method of the component.

src/app/app.component.html

<button (click)="onSave($event)">Save</button>
<button *ngFor="let hero of heroes" (click)="deleteHero(hero)">{{hero.name}}</button>
<form #heroForm (ngSubmit)="onSubmit(heroForm)"> ... </form>

Template context names take precedence over component context names.

In deleteHero(hero) above,

the hero is the template input variable,

not the component's hero property.

Template statements cannot refer to anything in the global namespace.

They can't refer to window or document.

They can't call console.log or Math.max.

Statement guidelines

As with expressions, avoid writing complex template statements.

A method call or simple property assignment should be the norm ( 常规,常态 ).

Now that you have a feel for template expressions and statements,

you're ready to learn about the varieties of data binding syntax beyond interpolation.

Binding syntax: An overview

Data binding is a mechanism for coordinating what users see, with application data values.

While you could push values to and pull values from HTML,

the application is easier to write, read, and maintain

if you turn these chores over to a binding framework.

You simply declare bindings between binding sources

and target HTML elements and let the framework do the work.

Angular provides many kinds of data binding.

This guide covers most of them, after a high-level view of Angular data binding and its syntax.

Binding types can be grouped into three categories

distinguished by the direction of data flow:

1. from the source-to-view,

2. from view-to-source,

3. and in the two-way sequence: view-to-source-to-view:

Data direction	Syntax	Type
One-way from data source to view target	`{{expression}}` `[target]="expression"` `bind-target="expression"`	Interpolation Property Attribute Class Style
One-way from view target to data source	`(target)="statement"` `on-target="statement"`	Event
Two-way	`[(target)]="expression"` `bindon-target="expression"`	Two-way

Binding types other than interpolation have a target name to the left of the equal sign,

either surrounded by punctuation ([], ())

or preceded by a prefix (bind-, on-, bindon-).

The target name is the name of a property.

It may look like the name of an attribute but it never is.

To appreciate the difference,

you must develop a new way to think about template HTML.

A new mental model

With all the power of data binding and the ability to extend the HTML vocabulary with custom markup,

it is tempting to think of template HTML as HTML Plus.

It really is HTML Plus.

But it's also significantly different than the HTML you're used to.

It requires a new mental model.

In the normal course of HTML development, you create a visual structure with HTML elements,

and you modify those elements by setting element attributes with string constants.

src/app/app.component.html

<div class="special">Mental Model</div>
<img src="assets/images/hero.png">
<button disabled>Save</button>

You still create a structure and initialize attribute values this way in Angular templates.

Then you learn to create new elements with components

that encapsulate HTML

and drop them into templates as if they were native HTML elements.

src/app/app.component.html

<!-- Normal HTML -->
<div class="special">Mental Model</div>
<!-- Wow! A new element! -->
<app-hero-detail></app-hero-detail>

That's HTML Plus.

Then you learn about data binding.

The first binding you meet might look like this:

src/app/app.component.html

<!-- Bind button disabled state to `isUnchanged` property -->
<button [disabled]="isUnchanged">Save</button>

You'll get to that peculiar bracket notation in a moment ( 等过一会儿 ).

Looking beyond it, your intuition ( 直觉 ) suggests that you're binding to the button's disabled attribute

and setting it to the current value of the component's isUnchanged property.

Your intuition is incorrect! 大错特错

Your everyday HTML mental model is misleading. 误导

In fact, once you start data binding,

you are no longer working with HTML attributes.

You aren't setting attributes.

You are setting the properties of DOM elements, components, and directives.

HTML attribute vs. DOM property

The distinction between an HTML attribute and a DOM property

is crucial to understanding how Angular binding works.

Attributes are defined by HTML.

Properties are defined by the DOM (Document Object Model).

A few HTML attributes have 1:1 mapping to properties. id is one example.
Some HTML attributes don't have corresponding properties. colspan is one example.
Some DOM properties don't have corresponding attributes. textContent is one example.
Many HTML attributes appear to map to properties ... but not in the way you might think!

That last category is confusing until you grasp this general rule:

Attributes initialize DOM properties

and then they are done.

Property values can change; attribute values can't.

For example, when the browser renders <input type="text" value="Bob">,

it creates a corresponding DOM node with a value property initialized to "Bob".

When the user enters "Sally" into the input box,

the DOM element value property becomes "Sally".

But the HTML value attribute remains unchanged

as you discover if you ask the input element about that attribute: input.getAttribute('value') returns "Bob".

The HTML attribute value specifies the initial value;

the DOM value property is the current value.

The disabled attribute is another peculiar example.

A button's disabled property is false by default so the button is enabled.

When you add the disabled attribute,

its presence alone initializes the button's disabled property to true so the button is disabled.

Adding and removing the disabled attribute disables and enables the button.

The value of the attribute is irrelevant,

which is why you cannot enable a button by writing <button disabled="false">Still Disabled</button>.

Setting the button's disabled property (say, with an Angular binding)

disables or enables the button.

The value of the property matters.

The HTML attribute and the DOM property are not the same thing,

even when they have the same name.

This fact bears ( 值得 ) repeating( 重申 ):

Template binding works with properties and events, not attributes.

A WORLD WITHOUT ATTRIBUTES

In the world of Angular,

the only role of attributes is to initialize element and directive state.

When you write a data binding, you're dealing exclusively with properties and events of the target object.

HTML attributes effectively disappear.

With this model firmly in mind, read on to learn about binding targets.

Binding targets

The target of a data binding is something in the DOM.

Depending on the binding type,

the target can be

an (element | component | directive) property,

an (element | component | directive) event,

or (rarely) an attribute name.

The following table summarizes:

Type	Target	Examples
Property	Element property Component property Directive property	src/app/app.component.html `<img [src]="heroImageUrl">` `<app-hero-detail [hero]="currentHero"></app-hero-detail>` `<div [ngClass]="{'special': isSpecial}"></div>`
Event	Element event Component event Directive event	src/app/app.component.html `<button (click)="onSave()">Save</button>` `<app-hero-detail (deleteRequest)="deleteHero()"></app-hero-detail>` `<div (myClick)="clicked=$event" clickable>click me</div> ???`
Two-way	Event and property	src/app/app.component.html `<input [(ngModel)]="name">`
Attribute	Attribute (the exception)	src/app/app.component.html `<button [attr.aria-label]="help">help</button>`
Class	`class` property	src/app/app.component.html `<div [class.special]="isSpecial">Special</div>`
Style	`style` property	src/app/app.component.html `<button [style.color]="isSpecial ? 'red' : 'green'">`

With this broad view in mind,

you're ready to look at binding types in detail.

Property binding ( [property] )

Write a template property binding to set a property of a view element.

The binding sets the property to the value of a template expression.

The most common property binding sets an element property to a component property value.

An example is binding the src property of an image element to a component's heroImageUrl property:

src/app/app.component.html

<img [src]="heroImageUrl">

Another example is

disabling a button when the component says that it isUnchanged:

src/app/app.component.html

<button [disabled]="isUnchanged">Cancel is disabled</button>

Another is

setting a property of a directive:

src/app/app.component.html

<div [ngClass]="classes"> [ngClass] binding to the classes property</div>

Yet another is

setting the model property of a custom component (a great way for parent and child components to communicate):

src/app/app.component.html

<app-hero-detail [hero]="currentHero"></app-hero-detail>

One-way in

People often describe property binding as one-way data binding

because it flows a value in one direction,

from a component's data property into a target element property.

You cannot use property binding to pull values out of the target element.

You can't bind to a property of the target element to read it.

You can only set it.

Similarly, you cannot use property binding to call a method on the target element.

If the element raises events,

you can listen to them with an event binding.

If you must read a target element property

or call one of its methods,

you'll need a different technique.

See the API reference for ViewChild and ContentChild.

Binding target

An element property

between enclosing square brackets

identifies the target property.

The target property in the following code is the image element's src property.

src/app/app.component.html

<img [src]="heroImageUrl">

Some people prefer the bind- prefix alternative,

known as the canonical form:

src/app/app.component.html

<img bind-src="heroImageUrl">

The target name is always the name of a property,

even when it appears to be the name of something else.

You see src and may think it's the name of an attribute.

No. It's the name of an image element property.

Element properties may be the more common targets,

but Angular looks first to see if the name is a property of a known directive,

as it is in the following example:

src/app/app.component.html

<div [ngClass]="classes">[ngClass] binding to the classes property</div>

Technically, Angular is matching the name to a directive input,

one of the property names listed in the directive's inputs array

or a property decorated with @Input().

Such inputs map to the directive's own properties.

If the name fails to match a property of a known directive or element,

Angular reports an “unknown directive” error.

Avoid side effects

As mentioned previously,

evaluation of a template expression should have no visible side effects.

The expression language itself does its part to keep you safe.

You can't assign a value to anything in a property binding expression

nor use the increment and decrement operators.

Of course, the expression might invoke a property or method that has side effects.

Angular has no way of knowing that or stopping you.

The expression could call something like getFoo().

Only you know what getFoo() does.

If getFoo() changes something and you happen to be binding to that something,

you risk an unpleasant experience.

Angular may or may not display the changed value.

Angular may detect the change and throw a warning error.

In general, stick to data properties and to methods that return values and do no more.

Return the proper type

The template expression should evaluate to the type of value expected by the target property.

Return a string if the target property expects a string.

Return a number if the target property expects a number.

Return an object if the target property expects an object.

The hero property of the HeroDetail component expects a Hero object,

which is exactly what you're sending in the property binding:

src/app/app.component.html

<app-hero-detail [hero]="currentHero"></app-hero-detail>

Remember the brackets [ ]

The brackets [ ] tell Angular to evaluate the template expression.

If you omit the brackets,

Angular treats the string as a constant and initializes the target property with that string.

It does not evaluate the string!

Don't make the following mistake:

src/app/app.component.html

<!-- ERROR: HeroDetailComponent.hero expects a
     Hero object, not the string "currentHero" -->
  <app-hero-detail hero="currentHero"></app-hero-detail>

One-time string initialization

意思是: 字符串就别用 [ ] 了,浪费性能!

You should omit the brackets [ ] when all of the following are true:

The target property accepts a string value.
The string is a fixed value that you can bake into the template.
This initial value never changes.

You routinely initialize attributes this way in standard HTML,

and it works just as well for directive and component property initialization.

The following example initializes the prefix property of the HeroDetailComponent to a fixed string, not a template expression.

Angular sets it and forgets about it.

src/app/app.component.html

<app-hero-detail prefix="You are my" [hero]="currentHero"></app-hero-detail>

The [hero] binding,

on the other hand,

remains a live binding to the component's currentHero property.

Property binding or interpolation?

You often have a choice between interpolation and property binding.

The following binding pairs do the same thing:

src/app/app.component.html

<p><img src="{{heroImageUrl}}"> is the <i>interpolated</i> image.</p>
<p><img [src]="heroImageUrl"> is the <i>property bound</i> image.</p>


<p><span>"{{title}}" is the <i>interpolated</i> title.</span></p>
<p>"<span [innerHTML]="title"></span>" is the <i>property bound</i> title.</p>

Interpolation is a convenient alternative to property binding in many cases.

When rendering data values as strings,

there is no technical reason to prefer one form to the other.

You lean toward ( 倾向 ) readability,

which tends to favor interpolation.

You suggest establishing coding style rules

and choosing the form that both conforms to the rules and feels most natural for the task at hand.

When setting an element property to a non-string data value, you must use property binding.

Content security

Imagine the following malicious content.

src/app/app.component.ts

evilTitle = 'Template <script>alert("evil never sleeps")</script>Syntax';

Fortunately, Angular data binding is on alert for dangerous HTML.

It sanitizes the values before displaying them.

It will not allow HTML with script tags to leak into the browser,

neither with interpolation nor property binding.

src/app/app.component.html

<!--
  Angular generates warnings for these two lines as it sanitizes them
  WARNING: sanitizing HTML stripped some content (see http://g.co/ng/security#xss).
 -->
<p><span>"{{evilTitle}}" is the <i>interpolated</i> evil title.</span></p>
<p>"<span [innerHTML]="evilTitle"></span>" is the <i>property bound</i> evil title.</p>

Interpolation handles the script tags differently than property binding but

both approaches render the content harmlessly.

效果如下:

Attribute, class, and style bindings

The template syntax provides specialized one-way bindings for

scenarios ( 场景 ) less well suited to property binding.

Attribute binding

You can set the value of an attribute directly with an attribute binding.

This is the only exception to the rule that a binding sets a target property.

This is the only binding that creates and sets an attribute.

This guide stresses repeatedly that setting an element property with a property binding

is always preferred to ( 优于 ) setting the attribute with a string.

Why does Angular offer attribute binding?

You must use attribute binding when there is no element property to bind.

Consider the ARIA, SVG, and table span attributes.

ARIA: Accessible Rich Internet Applications

They are pure attributes.

They do not correspond to element properties,

and they do not set element properties.

There are no property targets to bind to.

This fact becomes painfully obvious when you write something like this.

<tr><td colspan="{{1 + 1}}">Three-Four</td></tr>

And you get this error:

Template parse errors:
Can't bind to 'colspan' since it isn't a known native property

As the message says, the <td> element does not have a colspan property.

It has the "colspan" attribute,

but interpolation and property binding can set only properties, not attributes.

You need attribute bindings to create and bind to such attributes.

注意: property binding 与 attribute binding的区别

Attribute binding syntax resembles property binding.

Instead of an element property between brackets,

1. start with the prefix attr,

2. followed by a dot (.)

3. and the name of the attribute.

You then set the attribute value, using an expression that resolves to a string.

Bind [attr.colspan] to a calculated value:

src/app/app.component.html

content_copy<table border=1>
  <!--  expression calculates colspan=2 -->
  <tr><td [attr.colspan]="1 + 1">One-Two</td></tr>

  <!-- ERROR: There is no `colspan` property to set!
    <tr><td colspan="{{1 + 1}}">Three-Four</td></tr>
  -->

  <tr><td>Five</td><td>Six</td></tr>
</table>

Here's how the table renders:

One of the primary use cases for attribute binding

is to set ARIA attributes, as in this example:

Accessible Rich Internet Applications (ARIA)

规定了能够让 Web 内容和 Web 应用

（特别是那些由 Ajax 和 JavaScript 开发的）

对于残障人士更易使用的各种机制。

src/app/app.component.html

<!-- create and set an aria attribute for assistive technology -->
<button [attr.aria-label]="actionName">{{actionName}} with Aria</button>

Class binding

You can add and remove CSS class names

from an element's class attribute with a class binding.

Class binding syntax resembles property binding.

Instead of an element property between brackets,

1. start with the prefix class,

2. optionally followed by a dot (.)

3. and the name of a CSS class: [class.class-name].

2和3为可选的

The following examples show

how to add and remove the application's "special" class with class bindings.

Here's how to set the attribute without binding:

src/app/app.component.html

<!-- standard class attribute setting  -->
<div class="bad curly special">Bad curly special</div>

You can replace that with a binding to a string of the desired class names;

this is an all-or-nothing, replacement binding.

src/app/app.component.html

<!-- reset/override all class names with a binding  -->
<div class="bad curly special"
     [class]="badCurly">Bad curly</div>

Finally, you can bind to a specific class name.

Angular adds the class when the template expression evaluates to truthy.

It removes the class when the expression is falsy.

src/app/app.component.html

<!-- toggle the "special" class on/off with a property -->
<div [class.special]="isSpecial">The class binding is special</div>

<!-- binding to `class.special` trumps the class attribute -->
<div class="special"
     [class.special]="!isSpecial">This one is not so special</div>

While 尽管 this is a fine way to toggle a single class name,

但是, the NgClass directive is usually preferred

when managing multiple class names at the same time.

Style binding asdf

You can set inline styles with a style binding.

Style binding syntax resembles property binding. Instead of an element property between brackets, start with the prefix style, followed by a dot (.) and the name of a CSS style property: [style.style-property].

src/app/app.component.html

content_copy<button [style.color]="isSpecial ? 'red': 'green'">Red</button>
<button [style.background-color]="canSave ? 'cyan': 'grey'" >Save</button>

Some style binding styles have a unit extension. The following example conditionally sets the font size in “em” and “%” units .

src/app/app.component.html

content_copy<button [style.font-size.em]="isSpecial ? 3 : 1" >Big</button>
<button [style.font-size.%]="!isSpecial ? 150 : 50" >Small</button>

While this is a fine way to set a single style, the NgStyle directive is generally preferred when setting several inline styles at the same time.

Note that a style property name can be written in either dash-case, as shown above, or camelCase, such as fontSize.

Event binding ( (event) )

The bindings directives you've met so far flow data in one direction: from a component to an element.

Users don't just stare at the screen. They enter text into input boxes. They pick items from lists. They click buttons. Such user actions may result in a flow of data in the opposite direction: from an element to a component.

The only way to know about a user action is to listen for certain events such as keystrokes, mouse movements, clicks, and touches. You declare your interest in user actions through Angular event binding.

Event binding syntax consists of a target event name within parentheses on the left of an equal sign, and a quotedtemplate statement on the right. The following event binding listens for the button's click events, calling the component's onSave() method whenever a click occurs:

src/app/app.component.html

content_copy<button (click)="onSave()">Save</button>

Target event

A name between parentheses — for example, (click) — identifies the target event. In the following example, the target is the button's click event.

src/app/app.component.html

content_copy<button (click)="onSave()">Save</button>

Some people prefer the on- prefix alternative, known as the canonical form:

src/app/app.component.html

content_copy<button on-click="onSave()">On Save</button>

Element events may be the more common targets, but Angular looks first to see if the name matches an event property of a known directive, as it does in the following example:

src/app/app.component.html

content_copy<!-- `myClick` is an event on the custom `ClickDirective` -->
<div (myClick)="clickMessage=$event" clickable>click with myClick</div>

The myClick directive is further described in the section on aliasing input/output properties.

If the name fails to match an element event or an output property of a known directive, Angular reports an “unknown directive” error.

$event and event handling statements

In an event binding, Angular sets up an event handler for the target event.

When the event is raised, the handler executes the template statement. The template statement typically involves a receiver, which performs an action in response to the event, such as storing a value from the HTML control into a model.

The binding conveys information about the event, including data values, through an event object named $event.

The shape of the event object is determined by the target event. If the target event is a native DOM element event, then $event is a DOM event object, with properties such as target and target.value.

Consider this example:

src/app/app.component.html

content_copy<input [value]="currentHero.name"
       (input)="currentHero.name=$event.target.value" >

This code sets the input box value property by binding to the name property. To listen for changes to the value, the code binds to the input box's input event. When the user makes changes, the input event is raised, and the binding executes the statement within a context that includes the DOM event object, $event.

To update the name property, the changed text is retrieved by following the path $event.target.value.

If the event belongs to a directive (recall that components are directives), $event has whatever shape the directive decides to produce.

Custom events with EventEmitter

Directives typically raise custom events with an Angular EventEmitter. The directive creates an EventEmitter and exposes it as a property. The directive calls EventEmitter.emit(payload) to fire an event, passing in a message payload, which can be anything. Parent directives listen for the event by binding to this property and accessing the payload through the $event object.

Consider a HeroDetailComponent that presents hero information and responds to user actions. Although the HeroDetailComponent has a delete button it doesn't know how to delete the hero itself. The best it can do is raise an event reporting the user's delete request.

Here are the pertinent excerpts from that HeroDetailComponent:

src/app/hero-detail.component.ts (template)

content_copytemplate: `
<div>
  <img src="{{heroImageUrl}}">
  <span [style.text-decoration]="lineThrough">
    {{prefix}} {{hero?.name}}
  </span>
  <button (click)="delete()">Delete</button>
</div>`

src/app/hero-detail.component.ts (deleteRequest)

content_copy// This component makes a request but it can't actually delete a hero.
deleteRequest = new EventEmitter<Hero>();

delete() {
  this.deleteRequest.emit(this.hero);
}

The component defines a deleteRequest property that returns an EventEmitter. When the user clicks delete, the component invokes the delete() method, telling the EventEmitter to emit a Hero object.

Now imagine a hosting parent component that binds to the HeroDetailComponent's deleteRequest event.

src/app/app.component.html (event-binding-to-component)

content_copy<app-hero-detail (deleteRequest)="deleteHero($event)" [hero]="currentHero"></app-hero-detail>

When the deleteRequest event fires, Angular calls the parent component's deleteHero method, passing the hero-to-delete (emitted by HeroDetail) in the $event variable.

Template statements have side effects

The deleteHero method has a side effect: it deletes a hero. Template statement side effects are not just OK, but expected.

Deleting the hero updates the model, perhaps triggering other changes including queries and saves to a remote server. These changes percolate through the system and are ultimately displayed in this and other views.

Two-way binding ( [(...)] )

You often want to both display a data property and update that property when the user makes changes.

On the element side that takes a combination of setting a specific element property and listening for an element change event.

Angular offers a special two-way data binding syntax for this purpose, [(x)]. The [(x)] syntax combines the brackets of property binding, [x], with the parentheses of event binding, (x).

[( )] = BANANA IN A BOX

Visualize a banana in a box to remember that the parentheses go inside the brackets.

The [(x)] syntax is easy to demonstrate when the element has a settable property called x and a corresponding event named xChange. Here's a SizerComponent that fits the pattern. It has a size value property and a companion sizeChangeevent:

src/app/sizer.component.ts

content_copy

import { Component, EventEmitter, Input, Output } from '@angular/core';
@Component({
selector: 'app-sizer',
template: `
<div>
<button (click)="dec()" title="smaller">-</button>
<button (click)="inc()" title="bigger">+</button>
<label [style.font-size.px]="size">FontSize: {{size}}px</label>
</div>`
})
export class SizerComponent {
@Input() size: number | string;
@Output() sizeChange = new EventEmitter<number>();
dec() { this.resize(-1); }
inc() { this.resize(+1); }
resize(delta: number) {
this.size = Math.min(40, Math.max(8, +this.size + delta));
this.sizeChange.emit(this.size);
}
}

The initial size is an input value from a property binding. Clicking the buttons increases or decreases the size, within min/max values constraints, and then raises (emits) the sizeChange event with the adjusted size.

Here's an example in which the AppComponent.fontSizePx is two-way bound to the SizerComponent:

src/app/app.component.html (two-way-1)

content_copy<app-sizer [(size)]="fontSizePx"></app-sizer>
<div [style.font-size.px]="fontSizePx">Resizable Text</div>

The AppComponent.fontSizePx establishes the initial SizerComponent.size value. Clicking the buttons updates the AppComponent.fontSizePx via the two-way binding. The revised AppComponent.fontSizePx value flows through to the stylebinding, making the displayed text bigger or smaller.

The two-way binding syntax is really just syntactic sugar for a property binding and an event binding. Angular desugarsthe SizerComponent binding into this:

src/app/app.component.html (two-way-2)

content_copy<app-sizer [size]="fontSizePx" (sizeChange)="fontSizePx=$event"></app-sizer>

The $event variable contains the payload of the SizerComponent.sizeChange event. Angular assigns the $event value to the AppComponent.fontSizePx when the user clicks the buttons.

Clearly the two-way binding syntax is a great convenience compared to separate property and event bindings.

It would be convenient to use two-way binding with HTML form elements like <input> and <select>. However, no native HTML element follows the x value and xChange event pattern.

Fortunately, the Angular NgModel directive is a bridge that enables two-way binding to form elements.

Built-in directives

Earlier versions of Angular included over seventy built-in directives. The community contributed many more, and countless private directives have been created for internal applications.

You don't need many of those directives in Angular. You can often achieve the same results with the more capable and expressive Angular binding system. Why create a directive to handle a click when you can write a simple binding such as this?

src/app/app.component.html

content_copy<button (click)="onSave()">Save</button>

You still benefit from directives that simplify complex tasks. Angular still ships with built-in directives; just not as many. You'll write your own directives, just not as many.

This segment reviews some of the most frequently used built-in directives, classified as either attribute directives or structural directives.

Built-in attribute directives

Attribute directives listen to and modify the behavior of other HTML elements, attributes, properties, and components. They are usually applied to elements as if they were HTML attributes, hence the name.

Many details are covered in the Attribute Directives guide. Many NgModules such as the RouterModule and the FormsModule define their own attribute directives. This section is an introduction to the most commonly used attribute directives:

NgClass - add and remove a set of CSS classes
NgStyle - add and remove a set of HTML styles
NgModel - two-way data binding to an HTML form element

NgClass

You typically control how elements appear by adding and removing CSS classes dynamically. You can bind to the ngClass to add or remove several classes simultaneously.

A class binding is a good way to add or remove a single class.

src/app/app.component.html

content_copy<!-- toggle the "special" class on/off with a property -->
<div [class.special]="isSpecial">The class binding is special</div>

To add or remove many CSS classes at the same time, the NgClass directive may be the better choice.

Try binding ngClass to a key:value control object. Each key of the object is a CSS class name; its value is true if the class should be added, false if it should be removed.

Consider a setCurrentClasses component method that sets a component property, currentClasses with an object that adds or removes three classes based on the true/false state of three other component properties:

src/app/app.component.ts

content_copycurrentClasses: {};
setCurrentClasses() {
  // CSS classes: added/removed per current state of component properties
  this.currentClasses =  {
    'saveable': this.canSave,
    'modified': !this.isUnchanged,
    'special':  this.isSpecial
  };
}

Adding an ngClass property binding to currentClasses sets the element's classes accordingly:

src/app/app.component.html

content_copy<div [ngClass]="currentClasses">This div is initially saveable, unchanged, and special</div>

It's up to you to call setCurrentClasses(), both initially and when the dependent properties change.

NgStyle

You can set inline styles dynamically, based on the state of the component. With NgStyle you can set many inline styles simultaneously.

A style binding is an easy way to set a single style value.

src/app/app.component.html

content_copy<div [style.font-size]="isSpecial ? 'x-large' : 'smaller'" >
  This div is x-large or smaller.
</div>

To set many inline styles at the same time, the NgStyle directive may be the better choice.

Try binding ngStyle to a key:value control object. Each key of the object is a style name; its value is whatever is appropriate for that style.

Consider a setCurrentStyles component method that sets a component property, currentStyles with an object that defines three styles, based on the state of three other component properties:

src/app/app.component.ts

content_copycurrentStyles: {};
setCurrentStyles() {
  // CSS styles: set per current state of component properties
  this.currentStyles = {
    'font-style':  this.canSave      ? 'italic' : 'normal',
    'font-weight': !this.isUnchanged ? 'bold'   : 'normal',
    'font-size':   this.isSpecial    ? '24px'   : '12px'
  };
}

Adding an ngStyle property binding to currentStyles sets the element's styles accordingly:

src/app/app.component.html

content_copy<div [ngStyle]="currentStyles">
  This div is initially italic, normal weight, and extra large (24px).
</div>

It's up to you to call setCurrentStyles(), both initially and when the dependent properties change.

NgModel - Two-way binding to form elements with [(ngModel)]

When developing data entry forms, you often both display a data property and update that property when the user makes changes.

Two-way data binding with the NgModel directive makes that easy. Here's an example:

src/app/app.component.html (NgModel-1)

content_copy<input [(ngModel)]="currentHero.name">

FormsModule is required to use ngModel

Before using the ngModel directive in a two-way data binding, you must import the FormsModule and add it to the NgModule's imports list. Learn more about the FormsModule and ngModel in the Forms guide.

Here's how to import the FormsModule to make [(ngModel)] available.

src/app/app.module.ts (FormsModule import)

content_copyimport { NgModule } from '@angular/core';
import { BrowserModule }  from '@angular/platform-browser';
import { FormsModule } from '@angular/forms'; // <--- JavaScript import from Angular

/* Other imports */

@NgModule({
  imports: [
    BrowserModule,
    FormsModule  // <--- import into the NgModule
  ],
  /* Other module metadata */
})
export class AppModule { }

Inside [(ngModel)]

Looking back at the name binding, note that you could have achieved the same result with separate bindings to the <input> element's value property and input event.

src/app/app.component.html

content_copy<input [value]="currentHero.name"
       (input)="currentHero.name=$event.target.value" >

That's cumbersome. Who can remember which element property to set and which element event emits user changes? How do you extract the currently displayed text from the input box so you can update the data property? Who wants to look that up each time?

That ngModel directive hides these onerous details behind its own ngModel input and ngModelChange output properties.

src/app/app.component.html

content_copy<input
  [ngModel]="currentHero.name"
  (ngModelChange)="currentHero.name=$event">

The ngModel data property sets the element's value property and the ngModelChange event property listens for changes to the element's value.

The details are specific to each kind of element and therefore the NgModel directive only works for an element supported by a ControlValueAccessor that adapts an element to this protocol. The <input> box is one of those elements. Angular provides value accessors for all of the basic HTML form elements and the Forms guide shows how to bind to them.

You can't apply [(ngModel)] to a non-form native element or a third-party custom component until you write a suitable value accessor, a technique that is beyond the scope of this guide.

You don't need a value accessor for an Angular component that you write because you can name the value and event properties to suit Angular's basic two-way binding syntax and skip NgModel altogether. The sizer shown above is an example of this technique.

Separate ngModel bindings is an improvement over binding to the element's native properties. You can do better.

You shouldn't have to mention the data property twice. Angular should be able to capture the component's data property and set it with a single declaration, which it can with the [(ngModel)] syntax:

src/app/app.component.html

content_copy<input [(ngModel)]="currentHero.name">

Is [(ngModel)] all you need? Is there ever a reason to fall back to its expanded form?

The [(ngModel)] syntax can only set a data-bound property. If you need to do something more or something different, you can write the expanded form.

The following contrived example forces the input value to uppercase:

src/app/app.component.html

content_copy<input
  [ngModel]="currentHero.name"
  (ngModelChange)="setUppercaseName($event)">

Here are all variations in action, including the uppercase version:

Built-in structural directives

Structural directives are responsible for HTML layout. They shape or reshape the DOM's structure, typically by adding, removing, and manipulating the host elements to which they are attached.

The deep details of structural directives are covered in the Structural Directives guide where you'll learn:

why you prefix the directive name with an asterisk (*).
to use <ng-container> to group elements when there is no suitable host element for the directive.
how to write your own structural directive.
that you can only apply one structural directive to an element.

This section is an introduction to the common structural directives:

NgIf - conditionally add or remove an element from the DOM
NgSwitch - a set of directives that switch among alternative views
NgForOf - repeat a template for each item in a list

NgIf

You can add or remove an element from the DOM by applying an NgIf directive to that element (called the host element). Bind the directive to a condition expression like isActive in this example.

src/app/app.component.html

content_copy<app-hero-detail *ngIf="isActive"></app-hero-detail>

Don't forget the asterisk (*) in front of ngIf.

When the isActive expression returns a truthy value, NgIf adds the HeroDetailComponent to the DOM. When the expression is falsy, NgIf removes the HeroDetailComponent from the DOM, destroying that component and all of its sub-components.

Show/hide is not the same thing

You can control the visibility of an element with a class or style binding:

src/app/app.component.html

content_copy<!-- isSpecial is true -->
<div [class.hidden]="!isSpecial">Show with class</div>
<div [class.hidden]="isSpecial">Hide with class</div>

<!-- HeroDetail is in the DOM but hidden -->
<app-hero-detail [class.hidden]="isSpecial"></app-hero-detail>

<div [style.display]="isSpecial ? 'block' : 'none'">Show with style</div>
<div [style.display]="isSpecial ? 'none'  : 'block'">Hide with style</div>

Hiding an element is quite different from removing an element with NgIf.

When you hide an element, that element and all of its descendents remain in the DOM. All components for those elements stay in memory and Angular may continue to check for changes. You could be holding onto considerable computing resources and degrading performance, for something the user can't see.

When NgIf is false, Angular removes the element and its descendents from the DOM. It destroys their components, potentially freeing up substantial resources, resulting in a more responsive user experience.

The show/hide technique is fine for a few elements with few children. You should be wary when hiding large component trees; NgIf may be the safer choice.

Guard against null

The ngIf directive is often used to guard against null. Show/hide is useless as a guard. Angular will throw an error if a nested expression tries to access a property of null.

Here we see NgIf guarding two <div>s. The currentHero name will appear only when there is a currentHero. The nullHero will never be displayed.

src/app/app.component.html

content_copy<div *ngIf="currentHero">Hello, {{currentHero.name}}</div>
<div *ngIf="nullHero">Hello, {{nullHero.name}}</div>

See also the safe navigation operator described below.

NgForOf

NgForOf is a repeater directive — a way to present a list of items. You define a block of HTML that defines how a single item should be displayed. You tell Angular to use that block as a template for rendering each item in the list.

Here is an example of NgForOf applied to a simple <div>:

src/app/app.component.html

content_copy<div *ngFor="let hero of heroes">{{hero.name}}</div>

You can also apply an NgForOf to a component element, as in this example:

src/app/app.component.html

content_copy<app-hero-detail *ngFor="let hero of heroes" [hero]="hero"></app-hero-detail>

Don't forget the asterisk (*) in front of ngFor.

The text assigned to *ngFor is the instruction that guides the repeater process.

*ngFor microsyntax

The string assigned to *ngFor is not a template expression. It's a microsyntax — a little language of its own that Angular interprets. The string "let hero of heroes" means:

Take each hero in the heroes array, store it in the local hero looping variable, and make it available to the templated HTML for each iteration.

Angular translates this instruction into a <ng-template> around the host element, then uses this template repeatedly to create a new set of elements and bindings for each hero in the list.

Learn about the microsyntax in the Structural Directives guide.

Template input variables

The let keyword before hero creates a template input variable called hero. The NgForOf directive iterates over the heroes array returned by the parent component's heroes property and sets hero to the current item from the array during each iteration.

You reference the hero input variable within the NgForOf host element (and within its descendants) to access the hero's properties. Here it is referenced first in an interpolation and then passed in a binding to the hero property of the <hero-detail> component.

src/app/app.component.html

content_copy<div *ngFor="let hero of heroes">{{hero.name}}</div>
<app-hero-detail *ngFor="let hero of heroes" [hero]="hero"></app-hero-detail>

Learn more about template input variables in the Structural Directives guide.

ngFor with index*

The index property of the NgForOf directive context returns the zero-based index of the item in each iteration. You can capture the index in a template input variable and use it in the template.

The next example captures the index in a variable named i and displays it with the hero name like this.

src/app/app.component.html

content_copy<div *ngFor="let hero of heroes; let i=index">{{i + 1}} - {{hero.name}}</div>

NgFor is implemented by the NgForOf directive. Read more about the other NgForOf context values such as last, even, and odd in the NgForOf API reference.

ngFor with trackBy*

The NgForOf directive may perform poorly, especially with large lists. A small change to one item, an item removed, or an item added can trigger a cascade of DOM manipulations.

For example, re-querying the server could reset the list with all new hero objects.

Most, if not all, are previously displayed heroes. You know this because the id of each hero hasn't changed. But Angular sees only a fresh list of new object references. It has no choice but to tear down the old DOM elements and insert all new DOM elements.

Angular can avoid this churn with trackBy. Add a method to the component that returns the value NgForOf should track. In this case, that value is the hero's id.

src/app/app.component.ts

content_copytrackByHeroes(index: number, hero: Hero): number { return hero.id; }

In the microsyntax expression, set trackBy to this method.

src/app/app.component.html

content_copy<div *ngFor="let hero of heroes; trackBy: trackByHeroes">
  ({{hero.id}}) {{hero.name}}
</div>

Here is an illustration of the trackBy effect. "Reset heroes" creates new heroes with the same hero.ids. "Change ids" creates new heroes with new hero.ids.

With no trackBy, both buttons trigger complete DOM element replacement.
With trackBy, only changing the id triggers element replacement.

The NgSwitch directives

NgSwitch is like the JavaScript switch statement. It can display one element from among several possible elements, based on a switch condition. Angular puts only the selected element into the DOM.

NgSwitch is actually a set of three, cooperating directives: NgSwitch, NgSwitchCase, and NgSwitchDefault as seen in this example.

src/app/app.component.html

content_copy<div [ngSwitch]="currentHero.emotion">
  <app-happy-hero    *ngSwitchCase="'happy'"    [hero]="currentHero"></app-happy-hero>
  <app-sad-hero      *ngSwitchCase="'sad'"      [hero]="currentHero"></app-sad-hero>
  <app-confused-hero *ngSwitchCase="'confused'" [hero]="currentHero"></app-confused-hero>
  <app-unknown-hero  *ngSwitchDefault           [hero]="currentHero"></app-unknown-hero>
</div>

NgSwitch is the controller directive. Bind it to an expression that returns the switch value. The emotion value in this example is a string, but the switch value can be of any type.

Bind to [ngSwitch]. You'll get an error if you try to set *ngSwitch because NgSwitch is an attribute directive, not a structural directive. It changes the behavior of its companion directives. It doesn't touch the DOM directly.

Bind to *ngSwitchCase and *ngSwitchDefault. The NgSwitchCase and NgSwitchDefault directives are structuraldirectives because they add or remove elements from the DOM.

NgSwitchCase adds its element to the DOM when its bound value equals the switch value.
NgSwitchDefault adds its element to the DOM when there is no selected NgSwitchCase.

The switch directives are particularly useful for adding and removing component elements. This example switches among four "emotional hero" components defined in the hero-switch.components.ts file. Each component has a heroinput property which is bound to the currentHero of the parent component.

Switch directives work as well with native elements and web components too. For example, you could replace the <confused-hero> switch case with the following.

src/app/app.component.html

content_copy<div *ngSwitchCase="'confused'">Are you as confused as {{currentHero.name}}?</div>

Template reference variables ( #var )

A template reference variable is often a reference to a DOM element within a template. It can also be a reference to an Angular component or directive or a web component.

Use the hash symbol (#) to declare a reference variable. The #phone declares a phone variable on an <input> element.

src/app/app.component.html

content_copy<input #phone placeholder="phone number">

You can refer to a template reference variable anywhere in the template. The phone variable declared on this <input> is consumed in a <button> on the other side of the template

src/app/app.component.html

content_copy<input #phone placeholder="phone number">

<!-- lots of other elements -->

<!-- phone refers to the input element; pass its `value` to an event handler -->
<button (click)="callPhone(phone.value)">Call</button>

How a reference variable gets its value

In most cases, Angular sets the reference variable's value to the element on which it was declared. In the previous example, phone refers to the phone number <input> box. The phone button click handler passes the input value to the component's callPhone method. But a directive can change that behavior and set the value to something else, such as itself. The NgForm directive does that.

The following is a simplified version of the form example in the Forms guide.

src/app/hero-form.component.html

content_copy<form (ngSubmit)="onSubmit(heroForm)" #heroForm="ngForm">
  <div class="form-group">
    <label for="name">Name
      <input class="form-control" name="name" required [(ngModel)]="hero.name">
    </label>
  </div>
  <button type="submit" [disabled]="!heroForm.form.valid">Submit</button>
</form>
<div [hidden]="!heroForm.form.valid">
  {{submitMessage}}
</div>

A template reference variable, heroForm, appears three times in this example, separated by a large amount of HTML. What is the value of heroForm?

If Angular hadn't taken it over when you imported the FormsModule, it would be the HTMLFormElement. The heroForm is actually a reference to an Angular NgForm directive with the ability to track the value and validity of every control in the form.

The native <form> element doesn't have a form property. But the NgForm directive does, which explains how you can disable the submit button if the heroForm.form.valid is invalid and pass the entire form control tree to the parent component's onSubmit method.

Template reference variable warning notes

A template reference variable (#phone) is not the same as a template input variable (let phone) such as you might see in an *ngFor. Learn the difference in the Structural Directives guide.

The scope of a reference variable is the entire template. Do not define the same variable name more than once in the same template. The runtime value will be unpredictable.

You can use the ref- prefix alternative to #. This example declares the fax variable as ref-fax instead of #fax.

src/app/app.component.html

content_copy<input ref-fax placeholder="fax number">
<button (click)="callFax(fax.value)">Fax</button>

Input and Output properties

An Input property is a settable property annotated with an @Input decorator. Values flow into the property when it is data bound with a property binding

An Output property is an observable property annotated with an @Output decorator. The property almost always returns an Angular EventEmitter. Values flow out of the component as events bound with an event binding.

You can only bind to another component or directive through its Input and Output properties.

Remember that all components are directives.

The following discussion refers to components for brevity and because this topic is mostly a concern for component authors.

Discussion

You are usually binding a template to its own component class. In such binding expressions, the component's property or method is to the right of the (=).

src/app/app.component.html

content_copy<img [src]="iconUrl"/>
<button (click)="onSave()">Save</button>

The iconUrl and onSave are members of the AppComponent class. They are not decorated with @Input() or @Output. Angular does not object.

You can always bind to a public property of a component in its own template. It doesn't have to be an Input or Outputproperty

A component's class and template are closely coupled. They are both parts of the same thing. Together they are the component. Exchanges between a component class and its template are internal implementation details.

Binding to a different component

You can also bind to a property of a different component. In such bindings, the other component's property is to the leftof the (=).

In the following example, the AppComponent template binds AppComponent class members to properties of the HeroDetailComponent whose selector is 'app-hero-detail'.

src/app/app.component.html

content_copy<app-hero-detail [hero]="currentHero" (deleteRequest)="deleteHero($event)">
</app-hero-detail>

The Angular compiler may reject these bindings with errors like this one:

content_copyUncaught Error: Template parse errors:
Can't bind to 'hero' since it isn't a known property of 'app-hero-detail'

You know that HeroDetailComponent has hero and deleteRequest properties. But the Angular compiler refuses to recognize them.

The Angular compiler won't bind to properties of a different component unless they are Input or Output properties.

There's a good reason for this rule.

It's OK for a component to bind to its own properties. The component author is in complete control of those bindings.

But other components shouldn't have that kind of unrestricted access. You'd have a hard time supporting your component if anyone could bind to any of its properties. Outside components should only be able to bind to the component's public binding API.

Angular asks you to be explicit about that API. It's up to you to decide which properties are available for binding by external components.

TypeScript public doesn't matter

You can't use the TypeScript public and private access modifiers to shape the component's public binding API.

All data bound properties must be TypeScript public properties. Angular never binds to a TypeScript privateproperty.

Angular requires some other way to identify properties that outside components are allowed to bind to. That other wayis the @Input() and @Output() decorators.

Declaring Input and Output properties

In the sample for this guide, the bindings to HeroDetailComponent do not fail because the data bound properties are annotated with @Input() and @Output() decorators.

src/app/hero-detail.component.ts

content_copy@Input()  hero: Hero;
@Output() deleteRequest = new EventEmitter<Hero>();

Alternatively, you can identify members in the inputs and outputs arrays of the directive metadata, as in this example:

src/app/hero-detail.component.ts

content_copy@Component({
  inputs: ['hero'],
  outputs: ['deleteRequest'],
})

Input or output?

Input properties usually receive data values. Output properties expose event producers, such as EventEmitter objects.

The terms input and output reflect the perspective of the target directive.

HeroDetailComponent.hero is an input property from the perspective of HeroDetailComponent because data flows intothat property from a template binding expression.

HeroDetailComponent.deleteRequest is an output property from the perspective of HeroDetailComponent because events stream out of that property and toward the handler in a template binding statement.

Aliasing input/output properties

Sometimes the public name of an input/output property should be different from the internal name.

This is frequently the case with attribute directives. Directive consumers expect to bind to the name of the directive. For example, when you apply a directive with a myClick selector to a <div> tag, you expect to bind to an event property that is also called myClick.

src/app/app.component.html

content_copy<div (myClick)="clickMessage=$event" clickable>click with myClick</div>

However, the directive name is often a poor choice for the name of a property within the directive class. The directive name rarely describes what the property does. The myClick directive name is not a good name for a property that emits click messages.

Fortunately, you can have a public name for the property that meets conventional expectations, while using a different name internally. In the example immediately above, you are actually binding through the myClick alias to the directive's own clicks property.

You can specify the alias for the property name by passing it into the input/output decorator like this:

src/app/click.directive.ts

content_copy@Output('myClick') clicks = new EventEmitter<string>(); //  @Output(alias) propertyName = ...

You can also alias property names in the inputs and outputs arrays. You write a colon-delimited (:) string with the directive property name on the left and the public alias on the right:

src/app/click.directive.ts

content_copy@Directive({
  outputs: ['clicks:myClick']  // propertyName:alias
})

Template expression operators

The template expression language employs a subset of JavaScript syntax supplemented with a few special operators for specific scenarios. The next sections cover two of these operators: pipe and safe navigation operator.

The pipe operator ( | )

The result of an expression might require some transformation before you're ready to use it in a binding. For example, you might display a number as a currency, force text to uppercase, or filter a list and sort it.

Angular pipes are a good choice for small transformations such as these. Pipes are simple functions that accept an input value and return a transformed value. They're easy to apply within template expressions, using the pipe operator (|):

src/app/app.component.html

content_copy<div>Title through uppercase pipe: {{title | uppercase}}</div>

The pipe operator passes the result of an expression on the left to a pipe function on the right.

You can chain expressions through multiple pipes:

src/app/app.component.html

content_copy<!-- Pipe chaining: convert title to uppercase, then to lowercase -->
<div>
  Title through a pipe chain:
  {{title | uppercase | lowercase}}
</div>

And you can also apply parameters to a pipe:

src/app/app.component.html

content_copy<!-- pipe with configuration argument => "February 25, 1970" -->
<div>Birthdate: {{currentHero?.birthdate | date:'longDate'}}</div>

The json pipe is particularly helpful for debugging bindings:

src/app/app.component.html (pipes-json)

content_copy<div>{{currentHero | json}}</div>

The generated output would look something like this

content_copy{ "id": 0, "name": "Hercules", "emotion": "happy",
  "birthdate": "1970-02-25T08:00:00.000Z",
  "url": "http://www.imdb.com/title/tt0065832/",
  "rate": 325 }

The safe navigation operator ( ?. ) and null property paths

The Angular safe navigation operator (?.) is a fluent and convenient way to guard against null and undefined values in property paths. Here it is, protecting against a view render failure if the currentHero is null.

src/app/app.component.html

content_copyThe current hero's name is {{currentHero?.name}}

What happens when the following data bound title property is null?

src/app/app.component.html

content_copyThe title is {{title}}

The view still renders but the displayed value is blank; you see only "The title is" with nothing after it. That is reasonable behavior. At least the app doesn't crash.

Suppose the template expression involves a property path, as in this next example that displays the name of a null hero.

content_copyThe null hero's name is {{nullHero.name}}

JavaScript throws a null reference error, and so does Angular:

content_copyTypeError: Cannot read property 'name' of null in [null].

Worse, the entire view disappears.

This would be reasonable behavior if the hero property could never be null. If it must never be null and yet it is null, that's a programming error that should be caught and fixed. Throwing an exception is the right thing to do.

On the other hand, null values in the property path may be OK from time to time, especially when the data are null now and will arrive eventually.

While waiting for data, the view should render without complaint, and the null property path should display as blank just as the title property does.

Unfortunately, the app crashes when the currentHero is null.

You could code around that problem with *ngIf.

src/app/app.component.html

content_copy<!--No hero, div not displayed, no error -->
<div *ngIf="nullHero">The null hero's name is {{nullHero.name}}</div>

You could try to chain parts of the property path with &&, knowing that the expression bails out when it encounters the first null.

src/app/app.component.html

content_copyThe null hero's name is {{nullHero && nullHero.name}}

These approaches have merit but can be cumbersome, especially if the property path is long. Imagine guarding against a null somewhere in a long property path such as a.b.c.d.

The Angular safe navigation operator (?.) is a more fluent and convenient way to guard against nulls in property paths. The expression bails out when it hits the first null value. The display is blank, but the app keeps rolling without errors.

src/app/app.component.html

content_copy<!-- No hero, no problem! -->
The null hero's name is {{nullHero?.name}}

It works perfectly with long property paths such as a?.b?.c?.d.

The non-null assertion operator ( ! )

As of Typescript 2.0, you can enforce strict null checking with the --strictNullChecks flag. TypeScript then ensures that no variable is unintentionally null or undefined.

In this mode, typed variables disallow null and undefined by default. The type checker throws an error if you leave a variable unassigned or try to assign null or undefined to a variable whose type disallows null and undefined.

The type checker also throws an error if it can't determine whether a variable will be null or undefined at runtime. You may know that can't happen but the type checker doesn't know. You tell the type checker that it can't happen by applying the post-fix non-null assertion operator (!).

The Angular non-null assertion operator (!) serves the same purpose in an Angular template.

For example, after you use *ngIf to check that hero is defined, you can assert that hero properties are also defined.

src/app/app.component.html

content_copy<!--No hero, no text -->
<div *ngIf="hero">
  The hero's name is {{hero!.name}}
</div>

When the Angular compiler turns your template into TypeScript code, it prevents TypeScript from reporting that hero.name might be null or undefined.

Unlike the safe navigation operator, the non-null assertion operator does not guard against null or undefined. Rather it tells the TypeScript type checker to suspend strict null checks for a specific property expression.

You'll need this template operator when you turn on strict null checks. It's optional otherwise.

The `$any` type cast function (`$any( <expression> )`)

Sometimes a binding expression will be reported as a type error and it is not possible or difficult to fully specify the type. To silence the error, you can use the $any cast function to cast the expression to the any type.

src/app/app.component.html

content_copy<!-- Accessing an undeclared member -->
<div>
  The hero's marker is {{$any(hero).marker}}
</div>

In this example, when the Angular compiler turns your template into TypeScript code, it prevents TypeScript from reporting that marker is not a member of the Hero interface.

The $any cast function can be used in conjunction with this to allow access to undeclared members of the component.

src/app/app.component.html

content_copy<!-- Accessing an undeclared member -->
<div>
  Undeclared members is {{$any(this).member}}
</div>

The $any cast function can be used anywhere in a binding expression where a method call is valid.

Summary

You've completed this survey of template syntax. Now it's time to put that knowledge to work on your own components and directives.

未完待续,下一章节,つづく

Angular_文档_02_数据绑定_模板语法

Displaying Data

Showing component properties with interpolation

Template inline or template file?

Constructor or variable initialization?

Showing an array property with *ngFor

Creating a class for the data

Using the Hero class

Conditional display with NgIf

Summary

Template Syntax

HTML in templates

Interpolation ( {﻿{...}} )

Template expressions

Expression context

Expression guidelines

No visible side effects

Quick execution

Simplicity

Idempotence

Template statements

Statement context

Statement guidelines

Binding syntax: An overview

A new mental model

HTML attribute vs. DOM property

Binding targets

Property binding ( [property] )

One-way in

Binding target

Avoid side effects

Return the proper type

Remember the brackets [ ]

One-time string initialization

Property binding or interpolation?

Content security

Attribute, class, and style bindings

Attribute binding

Class binding

Style binding asdf

Event binding ( (event) )

Target event

$event and event handling statements

Custom events with EventEmitter

Template statements have side effects

Two-way binding ( [(...)] )

Built-in directives

Built-in attribute directives

NgClass

NgStyle

NgModel - Two-way binding to form elements with [(ngModel)]

FormsModule is required to use ngModel

Inside [(ngModel)]

Built-in structural directives

NgIf

Show/hide is not the same thing

Guard against null

NgForOf

*ngFor microsyntax

Template input variables

*ngFor with index

*ngFor with trackBy

The NgSwitch directives

Template reference variables ( #var )

How a reference variable gets its value

Template reference variable warning notes

Input and Output properties

Discussion

Binding to a different component

TypeScript public doesn't matter

Declaring Input and Output properties

Input or output?

Aliasing input/output properties

Template expression operators

The pipe operator ( | )

The safe navigation operator ( ?. ) and null property paths

The non-null assertion operator ( ! )

The $any type cast function ($any( <expression> ))

Summary

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Interpolation ( {{...}} )

ngFor with index*

ngFor with trackBy*

The `$any` type cast function (`$any( <expression> )`)