如何在React上正确绑定onhtml处理程序到this

Question

解释为什么这不是一个重复：我的代码已经在工作了，我已经作为一个注释包括在内。问题是，当我将它包含到click handler函数中时，这个上下文为什么会发生变化。
我正在尝试在React中创建一个计算器项目。目标是将onclick处理程序附加到数字按钮，以便在计算器显示区域显示数字。但是，如果直接将处理程序写入render方法，则该处理程序正在工作，如果尝试使用ComponentDidMount，则会出现错误this.inputDigit不是函数。如何正确绑定此.inputDigit（数字）？

import React from 'react';
import './App.css';

export default class Calculator extends React.Component {

// display of calculator initially zero
state = {
  displayValue: '0'
}

//click handler function
inputDigit(digit){
  const { displayValue } = this.state;
  this.setState({
    displayValue: displayValue+String(digit)
  })
}

componentDidMount(){

  //Get all number keys and attach click handler function
  var numberKeys = document.getElementsByClassName("number-keys");
  var myFunction = function() {
    var targetNumber = Number(this.innerHTML);
    return this.inputDigit(targetNumber); // This is not working
  };
  for (var i = 0; i < numberKeys.length; i++) {
    numberKeys[i].onclick = myFunction;
  }

}

render() {
  const { displayValue } = this.state;
  return (
    <div className="calculator">
      <div className="calculator-display">{displayValue}</div>
      <div className="calculator-keypad">
        <div className="input-keys">

          <div className="digit-keys">
            {/*<button className="number-keys" onClick={()=> this.inputDigit(0)}>0</button> This will Work*/}}
            <button className="number-keys">0</button>                
            <button className="number-keys1">1</button>
            <button className="number-keys">2</button>
            <button className="number-keys">3</button>
            <button className="number-keys">4</button>
            <button className="number-keys">5</button>
            <button className="number-keys">6</button>
            <button className="number-keys">7</button>
            <button className="number-keys">8</button>
            <button className="number-keys">9</button>
          </div>
        </div>          
      </div>
    </div>
  )
 }
}

Answer 1

在构造函数中绑定它

constructor(props) {
    super(props);
    this.inputDigit = this.inputDigit.bind(this);
}

Answer 2

这是因为你把它写在一个没有绑定的函数里，

使用

var myFunction = function() {
    var targetNumber = Number(this.innerHTML);
    return this.inputDigit(targetNumber); 
  }.bind(this);

或者

const myFunction = () => {
    var targetNumber = Number(this.innerHTML);
    return this.inputDigit(targetNumber); 
}

在此之后，您需要绑定inputDigit函数，因为它也使用setState

//click handler function
inputDigit = (digit) => {
  const { displayValue } = this.state;
  this.setState({
    displayValue: displayValue+String(digit)
  })
}

由于您也希望使用按钮文本，因此在这种情况下，您应该使用一个单独的变量代替this来调用inputDigit函数，如

class Calculator extends React.Component {

// display of calculator initially zero
state = {
  displayValue: '0'
}

//click handler function
inputDigit(digit){
  const { displayValue } = this.state;
  this.setState({
    displayValue: displayValue+String(digit)
  })
}

componentDidMount(){

  //Get all number keys and attach click handler function
  var numberKeys = document.getElementsByClassName("number-keys");
  var that = this;
  var myFunction = function() {
    var targetNumber = Number(this.innerHTML);
    console.log(targetNumber);
    return that.inputDigit(targetNumber); // This is not working
  };
  for (var i = 0; i < numberKeys.length; i++) {
    numberKeys[i].onclick = myFunction;
  }

}

render() {
  const { displayValue } = this.state;
  return (
    <div className="calculator">
      <div className="calculator-display">{displayValue}</div>
      <div className="calculator-keypad">
        <div className="input-keys">

          <div className="digit-keys">
            {/*<button className="number-keys" onClick={()=> this.inputDigit(0)}>0</button> This will Work*/}
            <button className="number-keys">0</button>                
            <button className="number-keys">1</button>
            <button className="number-keys">2</button>
            <button className="number-keys">3</button>
            <button className="number-keys">4</button>
            <button className="number-keys">5</button>
            <button className="number-keys">6</button>
            <button className="number-keys">7</button>
            <button className="number-keys">8</button>
            <button className="number-keys">9</button>
          </div>
        </div>          
      </div>
    </div>
  )
 }
} 

ReactDOM.render(<Calculator/>, document.getElementById('app'))

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>