Lifecycle Hooks

A component has a lifecycle managed by Angular.

Angular creates it, renders it, creates and renders its children, checks it when its data-bound properties change, and destroys it before removing it from the DOM.

Angular offers lifecycle hooks that provide visibility into these key life moments and the ability to act when they occur.

A directive has the same set of lifecycle hooks.

Component lifecycle hooks overview

Directive and component instances have a lifecycle as Angular creates, updates, and destroys them. Developers can tap into key moments in that lifecycle by implementing one or more of the lifecycle hook interfaces in the Angular corelibrary.

Each interface has a single hook method whose name is the interface name prefixed with ng. For example, the OnInitinterface has a hook method named ngOnInit() that Angular calls shortly after creating the component:

peek-a-boo.component.ts (excerpt)

content_copyexport class PeekABoo implements OnInit {
  constructor(private logger: LoggerService) { }

  // implement OnInit's `ngOnInit` method
  ngOnInit() { this.logIt(`OnInit`); }

  logIt(msg: string) {
    this.logger.log(`#${nextId++} ${msg}`);
  }
}

No directive or component will implement all of the lifecycle hooks. Angular only calls a directive/component hook method if it is defined.

Lifecycle sequence

After creating a component/directive by calling its constructor, Angular calls the lifecycle hook methods in the following sequence at specific moments:

Hook	Purpose and Timing
`ngOnChanges()`	Respond when Angular (re)sets data-bound input properties. The method receives a `SimpleChanges` object of current and previous property values. Called before `ngOnInit()` and whenever one or more data-bound input properties change.
`ngOnInit()`	Initialize the directive/component after Angular first displays the data-bound properties and sets the directive/component's input properties. Called once, after the first `ngOnChanges()`.
`ngDoCheck()`	Detect and act upon changes that Angular can't or won't detect on its own. Called during every change detection run, immediately after `ngOnChanges()` and `ngOnInit()`.
`ngAfterContentInit()`	Respond after Angular projects external content into the component's view / the view that a directive is in. Called once after the first `ngDoCheck()`.
`ngAfterContentChecked()`	Respond after Angular checks the content projected into the directive/component. Called after the `ngAfterContentInit()` and every subsequent `ngDoCheck()`.
`ngAfterViewInit()`	Respond after Angular initializes the component's views and child views / the view that a directive is in. Called once after the first `ngAfterContentChecked()`.
`ngAfterViewChecked()`	Respond after Angular checks the component's views and child views / the view that a directive is in. Called after the `ngAfterViewInit` and every subsequent `ngAfterContentChecked()`.
`ngOnDestroy()`	Cleanup just before Angular destroys the directive/component. Unsubscribe Observables and detach event handlers to avoid memory leaks. Called just before Angular destroys the directive/component.

Interfaces are optional (technically)

The interfaces are optional for JavaScript and Typescript developers from a purely technical perspective. The JavaScript language doesn't have interfaces. Angular can't see TypeScript interfaces at runtime because they disappear from the transpiled JavaScript.

Fortunately, they aren't necessary. You don't have to add the lifecycle hook interfaces to directives and components to benefit from the hooks themselves.

Angular instead inspects directive and component classes and calls the hook methods if they are defined. Angular finds and calls methods like ngOnInit(), with or without the interfaces.

Nonetheless, it's good practice to add interfaces to TypeScript directive classes in order to benefit from strong typing and editor tooling.

Other Angular lifecycle hooks

Other Angular sub-systems may have their own lifecycle hooks apart from these component hooks.

3rd party libraries might implement their hooks as well in order to give developers more control over how these libraries are used.

Lifecycle examples

The live example / download example demonstrates the lifecycle hooks in action through a series of exercises presented as components under the control of the root AppComponent.

They follow a common pattern: a parent component serves as a test rig for a child component that illustrates one or more of the lifecycle hook methods.

Here's a brief description of each exercise:

Component	Description
Peek-a-boo	Demonstrates every lifecycle hook. Each hook method writes to the on-screen log.
Spy	Directives have lifecycle hooks too. A `SpyDirective` can log when the element it spies upon is created or destroyed using the `ngOnInit` and `ngOnDestroy` hooks. This example applies the `SpyDirective` to a `<div>` in an `ngFor` hero repeater managed by the parent `SpyComponent`.
OnChanges	See how Angular calls the `ngOnChanges()` hook with a `changes` object every time one of the component input properties changes. Shows how to interpret the `changes` object.
DoCheck	Implements an `ngDoCheck()` method with custom change detection. See how often Angular calls this hook and watch it post changes to a log.
AfterView	Shows what Angular means by a view. Demonstrates the `ngAfterViewInit` and `ngAfterViewChecked` hooks.
AfterContent	Shows how to project external content into a component and how to distinguish projected content from a component's view children. Demonstrates the `ngAfterContentInit` and `ngAfterContentChecked` hooks.
Counter	Demonstrates a combination of a component and a directive each with its own hooks. In this example, a `CounterComponent` logs a change (via `ngOnChanges`) every time the parent component increments its input counter property. Meanwhile, the `SpyDirective` from the previous example is applied to the `CounterComponent` log where it watches log entries being created and destroyed.

The remainder of this page discusses selected exercises in further detail.

Peek-a-boo: all hooks

The PeekABooComponent demonstrates all of the hooks in one component.

You would rarely, if ever, implement all of the interfaces like this. The peek-a-boo exists to show how Angular calls the hooks in the expected order.

This snapshot reflects the state of the log after the user clicked the Create... button and then the Destroy... button.

The sequence of log messages follows the prescribed hook calling order: OnChanges, OnInit, DoCheck (3x), AfterContentInit, AfterContentChecked (3x), AfterViewInit, AfterViewChecked (3x), and OnDestroy.

The constructor isn't an Angular hook per se. The log confirms that input properties (the name property in this case) have no assigned values at construction.

Had the user clicked the Update Hero button, the log would show another OnChanges and two more triplets of DoCheck, AfterContentChecked and AfterViewChecked. Clearly these three hooks fire often. Keep the logic in these hooks as lean as possible!

The next examples focus on hook details.

Spying OnInit and OnDestroy

Go undercover with these two spy hooks to discover when an element is initialized or destroyed.

This is the perfect infiltration job for a directive. The heroes will never know they're being watched.

Kidding aside, pay attention to two key points:

Angular calls hook methods for directives as well as components.
A spy directive can provide insight into a DOM object that you cannot change directly. Obviously you can't touch the implementation of a native <div>. You can't modify a third party component either. But you can watch both with a directive.

The sneaky spy directive is simple, consisting almost entirely of ngOnInit() and ngOnDestroy() hooks that log messages to the parent via an injected LoggerService.

src/app/spy.directive.ts

content_copy// Spy on any element to which it is applied.
// Usage: <div mySpy>...</div>
@Directive({selector: '[mySpy]'})
export class SpyDirective implements OnInit, OnDestroy {

  constructor(private logger: LoggerService) { }

  ngOnInit()    { this.logIt(`onInit`); }

  ngOnDestroy() { this.logIt(`onDestroy`); }

  private logIt(msg: string) {
    this.logger.log(`Spy #${nextId++} ${msg}`);
  }
}

You can apply the spy to any native or component element and it'll be initialized and destroyed at the same time as that element. Here it is attached to the repeated hero <div>:

src/app/spy.component.html

content_copy<div *ngFor="let hero of heroes" mySpy class="heroes">
  {{hero}}
</div>

Each spy's birth and death marks the birth and death of the attached hero <div> with an entry in the Hook Log as seen here:

Adding a hero results in a new hero <div>. The spy's ngOnInit() logs that event.

The Reset button clears the heroes list. Angular removes all hero <div> elements from the DOM and destroys their spy directives at the same time. The spy's ngOnDestroy() method reports its last moments.

The ngOnInit() and ngOnDestroy() methods have more vital roles to play in real applications.

OnInit()

Use ngOnInit() for two main reasons:

To perform complex initializations shortly after construction.
To set up the component after Angular sets the input properties.

Experienced developers agree that components should be cheap and safe to construct.

Misko Hevery, Angular team lead, explains why you should avoid complex constructor logic.

Don't fetch data in a component constructor. You shouldn't worry that a new component will try to contact a remote server when created under test or before you decide to display it. Constructors should do no more than set the initial local variables to simple values.

An ngOnInit() is a good place for a component to fetch its initial data. The Tour of Heroes Tutorial guide shows how.

Remember also that a directive's data-bound input properties are not set until after construction. That's a problem if you need to initialize the directive based on those properties. They'll have been set when ngOnInit() runs.

The ngOnChanges() method is your first opportunity to access those properties. Angular calls ngOnChanges() before ngOnInit() and many times after that. It only calls ngOnInit() once.

You can count on Angular to call the ngOnInit() method soon after creating the component. That's where the heavy initialization logic belongs.

OnDestroy()

Put cleanup logic in ngOnDestroy(), the logic that must run before Angular destroys the directive.

This is the time to notify another part of the application that the component is going away.

This is the place to free resources that won't be garbage collected automatically. Unsubscribe from Observables and DOM events. Stop interval timers. Unregister all callbacks that this directive registered with global or application services. You risk memory leaks if you neglect to do so.

OnChanges()

Angular calls its ngOnChanges() method whenever it detects changes to input properties of the component (or directive). This example monitors the OnChanges hook.

on-changes.component.ts (excerpt)

content_copyngOnChanges(changes: SimpleChanges) {
  for (let propName in changes) {
    let chng = changes[propName];
    let cur  = JSON.stringify(chng.currentValue);
    let prev = JSON.stringify(chng.previousValue);
    this.changeLog.push(`${propName}: currentValue = ${cur}, previousValue = ${prev}`);
  }
}

The ngOnChanges() method takes an object that maps each changed property name to a SimpleChange object holding the current and previous property values. This hook iterates over the changed properties and logs them.

The example component, OnChangesComponent, has two input properties: hero and power.

src/app/on-changes.component.ts

content_copy@Input() hero: Hero;
@Input() power: string;

The host OnChangesParentComponent binds to them like this:

src/app/on-changes-parent.component.html

content_copy<on-changes [hero]="hero" [power]="power"></on-changes>

Here's the sample in action as the user makes changes.

The log entries appear as the string value of the power property changes. But the ngOnChanges does not catch changes to hero.name That's surprising at first.

Angular only calls the hook when the value of the input property changes. The value of the hero property is the reference to the hero object. Angular doesn't care that the hero's own name property changed. The hero object reference didn't change so, from Angular's perspective, there is no change to report!

DoCheck()

Use the DoCheck hook to detect and act upon changes that Angular doesn't catch on its own.

Use this method to detect a change that Angular overlooked.

The DoCheck sample extends the OnChanges sample with the following ngDoCheck() hook:

DoCheckComponent (ngDoCheck)

content_copyngDoCheck() {

  if (this.hero.name !== this.oldHeroName) {
    this.changeDetected = true;
    this.changeLog.push(`DoCheck: Hero name changed to "${this.hero.name}" from "${this.oldHeroName}"`);
    this.oldHeroName = this.hero.name;
  }

  if (this.power !== this.oldPower) {
    this.changeDetected = true;
    this.changeLog.push(`DoCheck: Power changed to "${this.power}" from "${this.oldPower}"`);
    this.oldPower = this.power;
  }

  if (this.changeDetected) {
      this.noChangeCount = 0;
  } else {
      // log that hook was called when there was no relevant change.
      let count = this.noChangeCount += 1;
      let noChangeMsg = `DoCheck called ${count}x when no change to hero or power`;
      if (count === 1) {
        // add new "no change" message
        this.changeLog.push(noChangeMsg);
      } else {
        // update last "no change" message
        this.changeLog[this.changeLog.length - 1] = noChangeMsg;
      }
  }

  this.changeDetected = false;
}

This code inspects certain values of interest, capturing and comparing their current state against previous values. It writes a special message to the log when there are no substantive changes to the hero or the power so you can see how often DoCheck is called. The results are illuminating:

While the ngDoCheck() hook can detect when the hero's name has changed, it has a frightful cost. This hook is called with enormous frequency—after every change detection cycle no matter where the change occurred. It's called over twenty times in this example before the user can do anything.

Most of these initial checks are triggered by Angular's first rendering of unrelated data elsewhere on the page. Mere mousing into another <input> triggers a call. Relatively few calls reveal actual changes to pertinent data. Clearly our implementation must be very lightweight or the user experience suffers.

AfterView

The AfterView sample explores the AfterViewInit() and AfterViewChecked() hooks that Angular calls after it creates a component's child views.

Here's a child view that displays a hero's name in an <input>:

ChildComponent

content_copy@Component({
  selector: 'app-child-view',
  template: '<input [(ngModel)]="hero">'
})
export class ChildViewComponent {
  hero = 'Magneta';
}

The AfterViewComponent displays this child view within its template:

AfterViewComponent (template)

content_copytemplate: `
  <div>-- child view begins --</div>
    <app-child-view></app-child-view>
  <div>-- child view ends --</div>`

The following hooks take action based on changing values within the child view, which can only be reached by querying for the child view via the property decorated with @ViewChild.

AfterViewComponent (class excerpts)

content_copyexport class AfterViewComponent implements  AfterViewChecked, AfterViewInit {
  private prevHero = '';

  // Query for a VIEW child of type `ChildViewComponent`
  @ViewChild(ChildViewComponent) viewChild: ChildViewComponent;

  ngAfterViewInit() {
    // viewChild is set after the view has been initialized
    this.logIt('AfterViewInit');
    this.doSomething();
  }

  ngAfterViewChecked() {
    // viewChild is updated after the view has been checked
    if (this.prevHero === this.viewChild.hero) {
      this.logIt('AfterViewChecked (no change)');
    } else {
      this.prevHero = this.viewChild.hero;
      this.logIt('AfterViewChecked');
      this.doSomething();
    }
  }
  // ...
}

Abide by the unidirectional data flow rule

The doSomething() method updates the screen when the hero name exceeds 10 characters.

AfterViewComponent (doSomething)

content_copy// This surrogate for real business logic sets the `comment`
private doSomething() {
  let c = this.viewChild.hero.length > 10 ? `That's a long name` : '';
  if (c !== this.comment) {
    // Wait a tick because the component's view has already been checked
    this.logger.tick_then(() => this.comment = c);
  }
}

Why does the doSomething() method wait a tick before updating comment?

Angular's unidirectional data flow rule forbids updates to the view after it has been composed. Both of these hooks fire after the component's view has been composed.

Angular throws an error if the hook updates the component's data-bound comment property immediately (try it!). The LoggerService.tick_then() postpones the log update for one turn of the browser's JavaScript cycle and that's just long enough.

Here's AfterView in action:

Notice that Angular frequently calls AfterViewChecked(), often when there are no changes of interest. Write lean hook methods to avoid performance problems.

AfterContent

The AfterContent sample explores the AfterContentInit() and AfterContentChecked() hooks that Angular calls afterAngular projects external content into the component.

Content projection

Content projection is a way to import HTML content from outside the component and insert that content into the component's template in a designated spot.

AngularJS developers know this technique as transclusion.

Consider this variation on the previous AfterView example. This time, instead of including the child view within the template, it imports the content from the AfterContentComponent's parent. Here's the parent's template:

AfterContentParentComponent (template excerpt)

content_copy`<after-content>
   <app-child></app-child>
 </after-content>`

Notice that the <app-child> tag is tucked between the <after-content> tags. Never put content between a component's element tags unless you intend to project that content into the component.

Now look at the component's template:

AfterContentComponent (template)

content_copytemplate: `
  <div>-- projected content begins --</div>
    <ng-content></ng-content>
  <div>-- projected content ends --</div>`

The <ng-content> tag is a placeholder for the external content. It tells Angular where to insert that content. In this case, the projected content is the <app-child> from the parent.

The telltale signs of content projection are twofold:

HTML between component element tags.
The presence of <ng-content> tags in the component's template.

AfterContent hooks

AfterContent hooks are similar to the AfterView hooks. The key difference is in the child component.

The AfterView hooks concern ViewChildren, the child components whose element tags appear within the component's template.
The AfterContent hooks concern ContentChildren, the child components that Angular projected into the component.

The following AfterContent hooks take action based on changing values in a content child, which can only be reached by querying for them via the property decorated with @ContentChild.

AfterContentComponent (class excerpts)

content_copyexport class AfterContentComponent implements AfterContentChecked, AfterContentInit {
  private prevHero = '';
  comment = '';

  // Query for a CONTENT child of type `ChildComponent`
  @ContentChild(ChildComponent) contentChild: ChildComponent;

  ngAfterContentInit() {
    // contentChild is set after the content has been initialized
    this.logIt('AfterContentInit');
    this.doSomething();
  }

  ngAfterContentChecked() {
    // contentChild is updated after the content has been checked
    if (this.prevHero === this.contentChild.hero) {
      this.logIt('AfterContentChecked (no change)');
    } else {
      this.prevHero = this.contentChild.hero;
      this.logIt('AfterContentChecked');
      this.doSomething();
    }
  }
  // ...
}

No unidirectional flow worries with AfterContent

This component's doSomething() method update's the component's data-bound comment property immediately. There's no need to wait.

Recall that Angular calls both AfterContent hooks before calling either of the AfterView hooks. Angular completes composition of the projected content before finishing the composition of this component's view. There is a small window between the AfterContent... and AfterView... hooks to modify the host view.

Component Interaction

This cookbook contains recipes for common component communication scenarios in which two or more components share information.

See the live example / download example.

Pass data from parent to child with input binding

HeroChildComponent has two input properties, typically adorned with @Input decorations.

component-interaction/src/app/hero-child.component.ts

content_copy

import { Component, Input } from '@angular/core';
import { Hero } from './hero';
@Component({
selector: 'app-hero-child',
template: `
<h3>{{hero.name}} says:</h3>
<p>I, {{hero.name}}, am at your service, {{masterName}}.</p>
`
})
export class HeroChildComponent {
@Input() hero: Hero;
@Input('master') masterName: string;
}

The second @Input aliases the child component property name masterName as 'master'.

The HeroParentComponent nests the child HeroChildComponent inside an *ngFor repeater, binding its master string property to the child's master alias, and each iteration's hero instance to the child's hero property.

component-interaction/src/app/hero-parent.component.ts

content_copy

import { Component } from '@angular/core';
import { HEROES } from './hero';
@Component({
selector: 'app-hero-parent',
template: `
<h2>{{master}} controls {{heroes.length}} heroes</h2>
<app-hero-child *ngFor="let hero of heroes"
[hero]="hero"
[master]="master">
</app-hero-child>
`
})
export class HeroParentComponent {
heroes = HEROES;
master = 'Master';
}

The running application displays three heroes:

Test it

E2E test that all children were instantiated and displayed as expected:

component-interaction/e2e/src/app.e2e-spec.ts

content_copy

// ...
let _heroNames = ['Mr. IQ', 'Magneta', 'Bombasto'];
let _masterName = 'Master';
it('should pass properties to children properly', function () {
let parent = element.all(by.tagName('app-hero-parent')).get(0);
let heroes = parent.all(by.tagName('app-hero-child'));
for (let i = 0; i < _heroNames.length; i++) {
let childTitle = heroes.get(i).element(by.tagName('h3')).getText();
let childDetail = heroes.get(i).element(by.tagName('p')).getText();
expect(childTitle).toEqual(_heroNames[i] + ' says:');
expect(childDetail).toContain(_masterName);
}
});
// ...

Intercept input property changes with a setter

Use an input property setter to intercept and act upon a value from the parent.

The setter of the name input property in the child NameChildComponent trims the whitespace from a name and replaces an empty value with default text.

component-interaction/src/app/name-child.component.ts

content_copy

import { Component, Input } from '@angular/core';
@Component({
selector: 'app-name-child',
template: '<h3>"{{name}}"</h3>'
})
export class NameChildComponent {
private _name = '';
@Input()
set name(name: string) {
this._name = (name && name.trim()) || '<no name set>';
}
get name(): string { return this._name; }
}

Here's the NameParentComponent demonstrating name variations including a name with all spaces:

component-interaction/src/app/name-parent.component.ts

content_copy

import { Component } from '@angular/core';
@Component({
selector: 'app-name-parent',
template: `
<h2>Master controls {{names.length}} names</h2>
<app-name-child *ngFor="let name of names" [name]="name"></app-name-child>
`
})
export class NameParentComponent {
// Displays 'Mr. IQ', '<no name set>', 'Bombasto'
names = ['Mr. IQ', ' ', ' Bombasto '];
}

Test it

E2E tests of input property setter with empty and non-empty names:

component-interaction/e2e/src/app.e2e-spec.ts

content_copy

// ...
it('should display trimmed, non-empty names', function () {
let _nonEmptyNameIndex = 0;
let _nonEmptyName = '"Mr. IQ"';
let parent = element.all(by.tagName('app-name-parent')).get(0);
let hero = parent.all(by.tagName('app-name-child')).get(_nonEmptyNameIndex);
let displayName = hero.element(by.tagName('h3')).getText();
expect(displayName).toEqual(_nonEmptyName);
});
it('should replace empty name with default name', function () {
let _emptyNameIndex = 1;
let _defaultName = '"<no name set>"';
let parent = element.all(by.tagName('app-name-parent')).get(0);
let hero = parent.all(by.tagName('app-name-child')).get(_emptyNameIndex);
let displayName = hero.element(by.tagName('h3')).getText();
expect(displayName).toEqual(_defaultName);
});
// ...

Intercept input property changes with ngOnChanges()

Detect and act upon changes to input property values with the ngOnChanges() method of the OnChanges lifecycle hook interface.

You may prefer this approach to the property setter when watching multiple, interacting input properties.

Learn about ngOnChanges() in the LifeCycle Hooks chapter.

This VersionChildComponent detects changes to the major and minor input properties and composes a log message reporting these changes:

component-interaction/src/app/version-child.component.ts

content_copy

import { Component, Input, OnChanges, SimpleChange } from '@angular/core';
@Component({
selector: 'app-version-child',
template: `
<h3>Version {{major}}.{{minor}}</h3>
<h4>Change log:</h4>
<ul>
<li *ngFor="let change of changeLog">{{change}}</li>
</ul>
`
})
export class VersionChildComponent implements OnChanges {
@Input() major: number;
@Input() minor: number;
changeLog: string[] = [];
ngOnChanges(changes: {[propKey: string]: SimpleChange}) {
let log: string[] = [];
for (let propName in changes) {
let changedProp = changes[propName];
let to = JSON.stringify(changedProp.currentValue);
if (changedProp.isFirstChange()) {
log.push(`Initial value of ${propName} set to ${to}`);
} else {
let from = JSON.stringify(changedProp.previousValue);
log.push(`${propName} changed from ${from} to ${to}`);
}
}
this.changeLog.push(log.join(', '));
}
}

The VersionParentComponent supplies the minor and major values and binds buttons to methods that change them.

component-interaction/src/app/version-parent.component.ts

content_copy

import { Component } from '@angular/core';
@Component({
selector: 'app-version-parent',
template: `
<h2>Source code version</h2>
<button (click)="newMinor()">New minor version</button>
<button (click)="newMajor()">New major version</button>
<app-version-child [major]="major" [minor]="minor"></app-version-child>
`
})
export class VersionParentComponent {
major = 1;
minor = 23;
newMinor() {
this.minor++;
}
newMajor() {
this.major++;
this.minor = 0;
}
}

Here's the output of a button-pushing sequence:

Test it

Test that both input properties are set initially and that button clicks trigger the expected ngOnChanges calls and values:

component-interaction/e2e/src/app.e2e-spec.ts

content_copy

// ...
// Test must all execute in this exact order
it('should set expected initial values', function () {
let actual = getActual();
let initialLabel = 'Version 1.23';
let initialLog = 'Initial value of major set to 1, Initial value of minor set to 23';
expect(actual.label).toBe(initialLabel);
expect(actual.count).toBe(1);
expect(actual.logs.get(0).getText()).toBe(initialLog);
});
it('should set expected values after clicking \'Minor\' twice', function () {
let repoTag = element(by.tagName('app-version-parent'));
let newMinorButton = repoTag.all(by.tagName('button')).get(0);
newMinorButton.click().then(function() {
newMinorButton.click().then(function() {
let actual = getActual();
let labelAfter2Minor = 'Version 1.25';
let logAfter2Minor = 'minor changed from 24 to 25';
expect(actual.label).toBe(labelAfter2Minor);
expect(actual.count).toBe(3);
expect(actual.logs.get(2).getText()).toBe(logAfter2Minor);
});
});
});
it('should set expected values after clicking \'Major\' once', function () {
let repoTag = element(by.tagName('app-version-parent'));
let newMajorButton = repoTag.all(by.tagName('button')).get(1);
newMajorButton.click().then(function() {
let actual = getActual();
let labelAfterMajor = 'Version 2.0';
let logAfterMajor = 'major changed from 1 to 2, minor changed from 25 to 0';
expect(actual.label).toBe(labelAfterMajor);
expect(actual.count).toBe(4);
expect(actual.logs.get(3).getText()).toBe(logAfterMajor);
});
});
function getActual() {
let versionTag = element(by.tagName('app-version-child'));
let label = versionTag.element(by.tagName('h3')).getText();
let ul = versionTag.element((by.tagName('ul')));
let logs = ul.all(by.tagName('li'));
return {
label: label,
logs: logs,
count: logs.count()
};
}
// ...

Parent listens for child event

The child component exposes an EventEmitter property with which it emits events when something happens. The parent binds to that event property and reacts to those events.

The child's EventEmitter property is an output property, typically adorned with an @Output decoration as seen in this VoterComponent:

component-interaction/src/app/voter.component.ts

content_copy

import { Component, EventEmitter, Input, Output } from '@angular/core';
@Component({
selector: 'app-voter',
template: `
<h4>{{name}}</h4>
<button (click)="vote(true)" [disabled]="didVote">Agree</button>
<button (click)="vote(false)" [disabled]="didVote">Disagree</button>
`
})
export class VoterComponent {
@Input() name: string;
@Output() voted = new EventEmitter<boolean>();
didVote = false;
vote(agreed: boolean) {
this.voted.emit(agreed);
this.didVote = true;
}
}

Clicking a button triggers emission of a true or false, the boolean payload.

The parent VoteTakerComponent binds an event handler called onVoted() that responds to the child event payload $eventand updates a counter.

component-interaction/src/app/votetaker.component.ts

content_copy

import { Component } from '@angular/core';
@Component({
selector: 'app-vote-taker',
template: `
<h2>Should mankind colonize the Universe?</h2>
<h3>Agree: {{agreed}}, Disagree: {{disagreed}}</h3>
<app-voter *ngFor="let voter of voters"
[name]="voter"
(voted)="onVoted($event)">
</app-voter>
`
})
export class VoteTakerComponent {
agreed = 0;
disagreed = 0;
voters = ['Mr. IQ', 'Ms. Universe', 'Bombasto'];
onVoted(agreed: boolean) {
agreed ? this.agreed++ : this.disagreed++;
}
}

The framework passes the event argument—represented by $event—to the handler method, and the method processes it:

Test it

Test that clicking the Agree and Disagree buttons update the appropriate counters:

component-interaction/e2e/src/app.e2e-spec.ts

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// ...
it('should not emit the event initially', function () {
let voteLabel = element(by.tagName('app-vote-taker'))
.element(by.tagName('h3')).getText();
expect(voteLabel).toBe('Agree: 0, Disagree: 0');
});
it('should process Agree vote', function () {
let agreeButton1 = element.all(by.tagName('app-voter')).get(0)
.all(by.tagName('button')).get(0);
agreeButton1.click().then(function() {
let voteLabel = element(by.tagName('app-vote-taker'))
.element(by.tagName('h3')).getText();
expect(voteLabel).toBe('Agree: 1, Disagree: 0');
});
});
it('should process Disagree vote', function () {
let agreeButton1 = element.all(by.tagName('app-voter')).get(1)
.all(by.tagName('button')).get(1);
agreeButton1.click().then(function() {
let voteLabel = element(by.tagName('app-vote-taker'))
.element(by.tagName('h3')).getText();
expect(voteLabel).toBe('Agree: 1, Disagree: 1');
});
});
// ...

Parent interacts with child via local variable

A parent component cannot use data binding to read child properties or invoke child methods. You can do both by creating a template reference variable for the child element and then reference that variable within the parent templateas seen in the following example.

The following is a child CountdownTimerComponent that repeatedly counts down to zero and launches a rocket. It has start and stop methods that control the clock and it displays a countdown status message in its own template.

component-interaction/src/app/countdown-timer.component.ts

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import { Component, OnDestroy, OnInit } from '@angular/core';
@Component({
selector: 'app-countdown-timer',
template: '<p>{{message}}</p>'
})
export class CountdownTimerComponent implements OnInit, OnDestroy {
intervalId = 0;
message = '';
seconds = 11;
clearTimer() { clearInterval(this.intervalId); }
ngOnInit() { this.start(); }
ngOnDestroy() { this.clearTimer(); }
start() { this.countDown(); }
stop() {
this.clearTimer();
this.message = `Holding at T-${this.seconds} seconds`;
}
private countDown() {
this.clearTimer();
this.intervalId = window.setInterval(() => {
this.seconds -= 1;
if (this.seconds === 0) {
this.message = 'Blast off!';
} else {
if (this.seconds < 0) { this.seconds = 10; } // reset
this.message = `T-${this.seconds} seconds and counting`;
}
}, 1000);
}
}

The CountdownLocalVarParentComponent that hosts the timer component is as follows:

component-interaction/src/app/countdown-parent.component.ts

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import { Component } from '@angular/core';
import { CountdownTimerComponent } from './countdown-timer.component';
@Component({
selector: 'app-countdown-parent-lv',
template: `
<h3>Countdown to Liftoff (via local variable)</h3>
<button (click)="timer.start()">Start</button>
<button (click)="timer.stop()">Stop</button>
<div class="seconds">{{timer.seconds}}</div>
<app-countdown-timer #timer></app-countdown-timer>
`,
styleUrls: ['../assets/demo.css']
})
export class CountdownLocalVarParentComponent { }

The parent component cannot data bind to the child's start and stop methods nor to its seconds property.

You can place a local variable, #timer, on the tag <countdown-timer> representing the child component. That gives you a reference to the child component and the ability to access any of its properties or methods from within the parent template.

This example wires parent buttons to the child's start and stop and uses interpolation to display the child's secondsproperty.

Here we see the parent and child working together.

Test it

Test that the seconds displayed in the parent template match the seconds displayed in the child's status message. Test also that clicking the Stop button pauses the countdown timer:

component-interaction/e2e/src/app.e2e-spec.ts

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// ...
it('timer and parent seconds should match', function () {
let parent = element(by.tagName(parentTag));
let message = parent.element(by.tagName('app-countdown-timer')).getText();
browser.sleep(10); // give `seconds` a chance to catchup with `message`
let seconds = parent.element(by.className('seconds')).getText();
expect(message).toContain(seconds);
});
it('should stop the countdown', function () {
let parent = element(by.tagName(parentTag));
let stopButton = parent.all(by.tagName('button')).get(1);
stopButton.click().then(function() {
let message = parent.element(by.tagName('app-countdown-timer')).getText();
expect(message).toContain('Holding');
});
});
// ...

Parent calls an @ViewChild()

The local variable approach is simple and easy. But it is limited because the parent-child wiring must be done entirely within the parent template. The parent component itself has no access to the child.

You can't use the local variable technique if an instance of the parent component class must read or write child component values or must call child component methods.

When the parent component class requires that kind of access, inject the child component into the parent as a ViewChild.

The following example illustrates this technique with the same Countdown Timer example. Neither its appearance nor its behavior will change. The child CountdownTimerComponent is the same as well.

The switch from the local variable to the ViewChild technique is solely for the purpose of demonstration.

Here is the parent, CountdownViewChildParentComponent:

component-interaction/src/app/countdown-parent.component.ts

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import { AfterViewInit, ViewChild } from '@angular/core';
import { Component } from '@angular/core';
import { CountdownTimerComponent } from './countdown-timer.component';
@Component({
selector: 'app-countdown-parent-vc',
template: `
<h3>Countdown to Liftoff (via ViewChild)</h3>
<button (click)="start()">Start</button>
<button (click)="stop()">Stop</button>
<div class="seconds">{{ seconds() }}</div>
<app-countdown-timer></app-countdown-timer>
`,
styleUrls: ['../assets/demo.css']
})
export class CountdownViewChildParentComponent implements AfterViewInit {
@ViewChild(CountdownTimerComponent)
private timerComponent: CountdownTimerComponent;
seconds() { return 0; }
ngAfterViewInit() {
// Redefine `seconds()` to get from the `CountdownTimerComponent.seconds` ...
// but wait a tick first to avoid one-time devMode
// unidirectional-data-flow-violation error
setTimeout(() => this.seconds = () => this.timerComponent.seconds, 0);
}
start() { this.timerComponent.start(); }
stop() { this.timerComponent.stop(); }
}

It takes a bit more work to get the child view into the parent component class.

First, you have to import references to the ViewChild decorator and the AfterViewInit lifecycle hook.

Next, inject the child CountdownTimerComponent into the private timerComponent property via the @ViewChild property decoration.

The #timer local variable is gone from the component metadata. Instead, bind the buttons to the parent component's own start and stop methods and present the ticking seconds in an interpolation around the parent component's seconds method.

These methods access the injected timer component directly.

The ngAfterViewInit() lifecycle hook is an important wrinkle. The timer component isn't available until after Angular displays the parent view. So it displays 0 seconds initially.

Then Angular calls the ngAfterViewInit lifecycle hook at which time it is too late to update the parent view's display of the countdown seconds. Angular's unidirectional data flow rule prevents updating the parent view's in the same cycle. The app has to wait one turn before it can display the seconds.

Use setTimeout() to wait one tick and then revise the seconds() method so that it takes future values from the timer component.

Test it

Use the same countdown timer tests as before.

Parent and children communicate via a service

A parent component and its children share a service whose interface enables bi-directional communication within the family.

The scope of the service instance is the parent component and its children. Components outside this component subtree have no access to the service or their communications.

This MissionService connects the MissionControlComponent to multiple AstronautComponent children.

component-interaction/src/app/mission.service.ts

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import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';
@Injectable()
export class MissionService {
// Observable string sources
private missionAnnouncedSource = new Subject<string>();
private missionConfirmedSource = new Subject<string>();
// Observable string streams
missionAnnounced$ = this.missionAnnouncedSource.asObservable();
missionConfirmed$ = this.missionConfirmedSource.asObservable();
// Service message commands
announceMission(mission: string) {
this.missionAnnouncedSource.next(mission);
}
confirmMission(astronaut: string) {
this.missionConfirmedSource.next(astronaut);
}
}

The MissionControlComponent both provides the instance of the service that it shares with its children (through the providers metadata array) and injects that instance into itself through its constructor:

component-interaction/src/app/missioncontrol.component.ts

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import { Component } from '@angular/core';
import { MissionService } from './mission.service';
@Component({
selector: 'app-mission-control',
template: `
<h2>Mission Control</h2>
<button (click)="announce()">Announce mission</button>
<app-astronaut *ngFor="let astronaut of astronauts"
[astronaut]="astronaut">
</app-astronaut>
<h3>History</h3>
<ul>
<li *ngFor="let event of history">{{event}}</li>
</ul>
`,
providers: [MissionService]
})
export class MissionControlComponent {
astronauts = ['Lovell', 'Swigert', 'Haise'];
history: string[] = [];
missions = ['Fly to the moon!',
'Fly to mars!',
'Fly to Vegas!'];
nextMission = 0;
constructor(private missionService: MissionService) {
missionService.missionConfirmed$.subscribe(
astronaut => {
this.history.push(`${astronaut} confirmed the mission`);
});
}
announce() {
let mission = this.missions[this.nextMission++];
this.missionService.announceMission(mission);
this.history.push(`Mission "${mission}" announced`);
if (this.nextMission >= this.missions.length) { this.nextMission = 0; }
}
}

The AstronautComponent also injects the service in its constructor. Each AstronautComponent is a child of the MissionControlComponent and therefore receives its parent's service instance:

component-interaction/src/app/astronaut.component.ts

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import { Component, Input, OnDestroy } from '@angular/core';
import { MissionService } from './mission.service';
import { Subscription } from 'rxjs';
@Component({
selector: 'app-astronaut',
template: `
<p>
{{astronaut}}: <strong>{{mission}}</strong>
<button
(click)="confirm()"
[disabled]="!announced || confirmed">
Confirm
</button>
</p>
`
})
export class AstronautComponent implements OnDestroy {
@Input() astronaut: string;
mission = '<no mission announced>';
confirmed = false;
announced = false;
subscription: Subscription;
constructor(private missionService: MissionService) {
this.subscription = missionService.missionAnnounced$.subscribe(
mission => {
this.mission = mission;
this.announced = true;
this.confirmed = false;
});
}
confirm() {
this.confirmed = true;
this.missionService.confirmMission(this.astronaut);
}
ngOnDestroy() {
// prevent memory leak when component destroyed
this.subscription.unsubscribe();
}
}

Notice that this example captures the subscription and unsubscribe() when the AstronautComponent is destroyed. This is a memory-leak guard step. There is no actual risk in this app because the lifetime of a AstronautComponent is the same as the lifetime of the app itself. That would not always be true in a more complex application.

You don't add this guard to the MissionControlComponent because, as the parent, it controls the lifetime of the MissionService.

The History log demonstrates that messages travel in both directions between the parent MissionControlComponent and the AstronautComponent children, facilitated by the service:

Test it

Tests click buttons of both the parent MissionControlComponent and the AstronautComponent children and verify that the history meets expectations:

component-interaction/e2e/src/app.e2e-spec.ts

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// ...
it('should announce a mission', function () {
let missionControl = element(by.tagName('app-mission-control'));
let announceButton = missionControl.all(by.tagName('button')).get(0);
announceButton.click().then(function () {
let history = missionControl.all(by.tagName('li'));
expect(history.count()).toBe(1);
expect(history.get(0).getText()).toMatch(/Mission.* announced/);
});
});
it('should confirm the mission by Lovell', function () {
testConfirmMission(1, 2, 'Lovell');
});
it('should confirm the mission by Haise', function () {
testConfirmMission(3, 3, 'Haise');
});
it('should confirm the mission by Swigert', function () {
testConfirmMission(2, 4, 'Swigert');
});
function testConfirmMission(buttonIndex: number, expectedLogCount: number, astronaut: string) {
let _confirmedLog = ' confirmed the mission';
let missionControl = element(by.tagName('app-mission-control'));
let confirmButton = missionControl.all(by.tagName('button')).get(buttonIndex);
confirmButton.click().then(function () {
let history = missionControl.all(by.tagName('li'));
expect(history.count()).toBe(expectedLogCount);
expect(history.get(expectedLogCount - 1).getText()).toBe(astronaut + _confirmedLog);
});
}
// ...

未完待续,下一章节,つづく

Angular_文档_03_生命周期_组件交互

Lifecycle Hooks

Component lifecycle hooks overview

Lifecycle sequence

Interfaces are optional (technically)

Other Angular lifecycle hooks

Lifecycle examples

Peek-a-boo: all hooks

Spying OnInit and OnDestroy

OnInit()

OnDestroy()

OnChanges()

DoCheck()

AfterView

Abide by the unidirectional data flow rule

AfterContent

Content projection

AfterContent hooks

No unidirectional flow worries with AfterContent

Component Interaction

Pass data from parent to child with input binding

Test it

Intercept input property changes with a setter

Test it

Intercept input property changes with ngOnChanges()

Test it

Parent listens for child event

Test it

Parent interacts with child via local variable

Test it

Parent calls an @ViewChild()

Test it

Parent and children communicate via a service

Test it

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