React官網零基礎學習筆記（3）-創建交互組件

原文如下：

---

Making an Interactive Component

Let’s fill the Square component with an “X” when we click it. First, change the button tag that is returned from the Square component’s render() function to this:

class Square extends React.Component {
  render() {
    return (
      <button className="square" onClick={function() { alert('click'); }}>
        {this.props.value}
      </button>
    );
  }
}

If we click on a Square now, we should get an alert in our browser.

Note

To save typing and avoid the confusing behavior of this, we will use the arrow function syntax for event handlers here and further below:

class Square extends React.Component {
 render() {
   return (
     <button className="square" onClick={() => alert('click')}>
       {this.props.value}
     </button>
   );
 }
}

Notice how with onClick={() => alert('click')}, we’re passing a function as the onClick prop. It only fires after a click. Forgetting () => and writing onClick={alert('click')} is a common mistake, and would fire the alert every time the component re-renders.

As a next step, we want the Square component to “remember” that it got clicked, and fill it with an “X” mark. To “remember” things, components use state.

React components can have state by setting this.state in their constructors. this.state should be considered as private to a React component that it’s defined in. Let’s store the current value of the Square in this.state, and change it when the Square is clicked.

First, we’ll add a constructor to the class to initialize the state:

class Square extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: null,
    };
  }

  render() {
    return (
      <button className="square" onClick={() => alert('click')}>
        {this.props.value}
      </button>
    );
  }
}

Note

In JavaScript classes, you need to always call super when defining the constructor of a subclass. All React component classes that have a constructor should start it with a super(props) call.

扫描二维码关注公众号，回复： 5089998 查看本文章

Now we’ll change the Square’s render method to display the current state’s value when clicked:

• Replace this.props.value with this.state.value inside the <button> tag.
• Replace the () => alert() event handler with () => this.setState({value: 'X'}).
• Put the className and onClick props on separate lines for better readability.

After these changes, the <button> tag that is returned by the Square’s render method looks like this:

class Square extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: null,
    };
  }

  render() {
    return (
      <button
        className="square"
        onClick={() => this.setState({value: 'X'})}
      >
        {this.state.value}
      </button>
    );
  }
}

By calling this.setState from an onClick handler in the Square’s render method, we tell React to re-render that Square whenever its <button> is clicked. After the update, the Square’s this.state.value will be 'X', so we’ll see the X on the game board. If you click on any Square, an X should show up.

When you call setState in a component, React automatically updates the child components inside of it too.

View the full code at this point

---