事件处理：基于功能的组件与基于类的组件

Question

这是我的第一个React App（[email protected]）。我试图找出为什么在使用基于功能的组件与基于类的组件时，onClick事件的处理方式不同。

据我所知，我应该使用基于类的c。只有当我需要改变状态时，我是对的吗？

基于功能的组件抛出_this is undefined error但基于类 - 不是。

我在两种情况下都使用箭头函数而不是绑定函数。

功能为主：

import React from 'react';

const AnswersDisplay = (props) => {
// even with bind I still get "_this is undefined"
//this.onAnswer = this.onAnswer.bind(this); 

  const answerList = props.answerList.map( (option) => {
    return (
      <button
        onClick={this.onAnswer}
        value={option}
        className="ui basic green button">{option}
      </button>
    )
  }); 

  const onAnswer = (e) =>{
    console.log(e.target.value);
  }

  return(
    <div className="ui two buttons hSpace">{this.answerList}</div>
  );
};

export default AnswersDisplay;

vs基于类的工作。

import React from 'react';

class AnswersDisplay extends React.Component {
  constructor(props) {
    super(props);
    //this.onAnswer = this.onAnswer.bind(this);
  }

  answerList = this.props.answerList.map( (option) => {
    return (
      <button
        onClick={this.onAnswer}
        value={option}
        className="ui basic green button">{option}
      </button>
    )
  });

  onAnswer = (e) =>{
    console.log(e.target.value);
  }

  render() {
    return(
        <div className="ui two buttons hSpace">{this.answerList}</div>
    );
  }
};


export default AnswersDisplay;

Answer 1

在功能组件的情况下，您声明一个常量，在其中包含一个函数。单击按钮时要调用的内容。但请确保在函数中使用this。在这种情况下，this将引用全局执行上下文，并且在该特定上下文中，JavaScript引擎将无法找到属性onAnswer，因此它将返回undefined。

只是为了完成这项工作，你已经没有this传递回调。

像那样：onClick={onAnswer}

整体而言，代码如下所示：

import React from 'react';

const AnswersDisplay = (props) => {
// even with bind I still get "_this is undefined"
//this.onAnswer = this.onAnswer.bind(this); 

  const answerList = props.answerList.map( (option) => {
    return (
      <button
        onClick={onAnswer}
        value={option}
        className="ui basic green button">{option}
      </button>
    )
  }); 

  const onAnswer = (e) =>{
    console.log(e.target.value);
  }

  return(
    <div className="ui two buttons hSpace">{this.answerList}</div>
  );
};

Answer 2

这个MDN web doc涵盖了你需要了解的关于this的一切。

为了简单起见，请考虑objects，其中this是“未定义的”，除非：

• 方法/函数已经与ES5的object（见下面的注释）绑定到bind(this)，或者通过用object本身重新绑定外部函数来回弹object：bind(obj)。

注意：如下所示（在方法6中），有一个例外，在arrow functions中使用ES5 object意味着它将保留封闭的词法范围this而不需要绑定。

例如：

this.prop = "global prop"
const outsideArrowFunction = () => (this.prop)

function outsideFunction() {
    return this.prop;
};

const obj = {
  prop: "obj's prop",
  method: function() {
    return this.prop; // returns "obj's prop"
  },
  method2: function() {
    return this; // returns the entire "obj" and its properties
  },
  method3: function() {
    return this.method(); // returns "obj's prop"
  },
  method4: function() {
    return outsideFunction(); // returns "global prop" because the outsideFunction's lexical scope doesn't recognize the "obj"'s nor its properties      
  },
  method5: function() {
    return outsideArrowFunction(); // same as method4, utilizes global this
  },
  method6: function() {
    const x = () => this.method();
    return x(); // returns "obj's prop" because arrow functions take on "this" from the "obj"
  },
  method7: function() {
    const x = function() { 
       return this.prop; 
    };
    return x(); // returns "global prop" because "this" loses lexical scope upon execution
  },
  method8: function() {
   const x = this.method.bind(this);
   return x(); // returns "obj's prop" because "this" refers to the "obj" upon execution
  },
  method9: function(callback) {
    return callback(this.method);
  },
  method10: function() {
    return this.method9(function(callback) {
      return callback(); // returns "global prop" because "this" loses lexical scope upon execution
    });
  }
}; 


const a = outsideArrowFunction.bind(obj); // returns "global prop" because arrow functions take on whatever "this" is upon its creation, so "this" refers to the global "this"
const b = outsideFunction.bind(obj); // returns "obj's prop" since a traditional function can rebind "this", which has been rebound to "obj"   
    
console.log(`Method: ${obj.method()}`);
console.log(`Method2: ${obj.method2()}`);
console.log(`Method3: ${obj.method3()}`);
console.log(`Method4: ${obj.method4()}`);
console.log(`Method5: ${obj.method5()}`);
console.log(`Method6: ${obj.method6()}`);
console.log(`Method7: ${obj.method7()}`);
console.log(`Method8: ${obj.method8()}`);
console.log(`Method10: ${obj.method10()}`);
console.log(`arrowFunction: ${a()}`);
console.log(`outsideFunction: ${b()}`);

当谈到classes时，他们是objects的模板。所以this将是undefined或是一个全球this除非班级method已被绑定在constructor或你使用arrow function。通过单击每个按钮尝试下面的示例，注意所有3种方法如何工作，但这取决于它们的调用方式：

class Example extends React.Component {
  constructor() {
    super();
    this.state = { method: "" };
    this.boundMethod = this.boundMethod.bind(this);
  }

  componentDidMount() { 
    this.unboundMethod(); 
  };

  boundMethod() {
    this.setState({ method: "Bound Method" }); // this works because the method is bound to the class
  }

  unboundMethod() {
    try {
      this.setState({ method: "Unbound Method" }); // this only works if it's called within a bound method class (like componentDidMount)
    } catch (err) {
      alert(err); // however, if it's called within a callback (like in an onClick event), it fails.
    }
  }

  arrowMethod = () => {
    this.setState({ method: "Arrow Method" }); // this works because arrow methods are automatically bound to the class
  };

  render() {
    return (
      <div>
        <button onClick={this.boundMethod}>Bound Method</button>
        <button onClick={this.unboundMethod}>Unbound Method</button>
        <button onClick={this.arrowMethod}>Arrow Method</button>
        <p>The {this.state.method} was called</p>
     </div>
    );
  };
}

ReactDOM.render(<Example />, document.body);

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