React-为什么在此示例中不需要绑定? [英] React - Why is binding this not required in this example?

问题描述

试图弄清React的基础.

Trying to figure out the basics of React.

请看此页面上的第二个示例: https://facebook.github.io/react/ 我看到tick()函数设置了Timer类的状态，将前一个值增加了一个.

Looking at the second example on this page: https://facebook.github.io/react/ I see that the tick() function sets the state of the Timer class, incrementing the previous value by one.

class Timer extends React.Component {
  constructor(props) {
    super(props);
    this.state = {secondsElapsed: 0};
  }

  tick() {
    this.setState((prevState) => ({
      secondsElapsed: prevState.secondsElapsed + 1
    }));
  }

  componentDidMount() {
    this.interval = setInterval(() => this.tick(), 1000);
  }

  componentWillUnmount() {
    clearInterval(this.interval);
  }

  render() {
    return (
      <div>Seconds Elapsed: {this.state.secondsElapsed}</div>
    );
  }
}

ReactDOM.render(<Timer />, mountNode);

但是，当我尝试实现自己的简单Counter类时，它失败了，并且出现了控制台错误，提示无法读取未定义的setState属性.

However, when I tried to implement my own simple Counter class, it failed and I got a console error saying Cannot read property setState of undefined.

class Counter extends React.Component {
    constructor(props) {
        super(props);
        this.state = {count: 0};
    }

    increment(prevState) {
        this.setState((prevState) => ({
            count: prevState.count + 1
        }));
    }

  render() {
    return (
            <div className="main">
                <button onClick={this.increment}>{this.state.count}</button>
            </div>
    )
  }
}

一些谷歌搜索显示我必须将其绑定到增量函数.但是，为什么在我看到的第一个示例中不需要这样做?我将代码复制到CodePen，并在React 15.3.1上运行良好，在该示例中找不到任何类似于绑定的内容.只有在构造函数中添加绑定代码后，示例中的内容才开始起作用.

Some Googling reveals that I have to bind this to the increment function. But why was that not required in the first example that I saw? I copied the code to CodePen and it ran fine with React 15.3.1 I cannot find anything resembling binding in that example. Only after I added binding code in the constructor did things start working in my example.

class Counter extends React.Component {
    constructor(props) {
        super(props);
        this.state = {count: 0};

        // THIS ONE LINE FIXED THE ISSUE
        this.increment = this.increment.bind(this);
    }

    increment(prevState) {
        this.setState((prevState) => ({
      count: prevState.count + 1
    }));
    }

  render() {
    return (
            <div className="main">
                <button onClick={this.increment}>{this.state.count}</button>
            </div>
    )
  }
}

推荐答案

回答您的问题:第一个示例使用arrow functions，它会自动执行

Answering your question: the first example uses arrow functions, that automatically performs context binding. From the docs:

箭头函数不会自行创建此上下文，因此具有 它的原始含义来自封闭的上下文.

An arrow function does not create its own this context, so this has its original meaning from the enclosing context.

实际上，React中有一些绑定方法:

Indeed there are some ways of binding in React:

1)您可以绑定构造函数中的所有函数，如您所说:

1) you can bind all functions in your constructor, like you said:

constructor(props) {
    /* ... */
    this.increment = this.increment.bind(this);
}

2)使用箭头函数调用回调:

2) invoke your callbacks with arrow functions:

<button onClick={(e) => this.increment(e)}>

3)附加 .bind ，如下所示:

3) append .bind at the end of your method reference each time you set it as a callback, like this:

<button onClick={this.increment.bind(this)}>

4)在您的类中，使用箭头函数定义方法:

4) In your class, define the method with arrow functions:

increment = (e) => {
  /* your class function defined as ES6 arrow function */
}

/* ... */

<button onClick={this.increment}>

为了与babel一起使用此语法，您必须启用此插件或使用 stage-2 预设.

In order to use this syntax with babel, you have to enable this plugin or use stage-2 preset.

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