使用 React 将状态值递增 1

Question

在 React 中，我试图让按钮递增存储在状态中的值。但是，使用下面的代码函数时，在使用 handleClick 时，我的值设置为未定义或 NaN。

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

    // This binding is necessary to make `this` work in the callback
    this.handleClick = this.handleClick.bind(this);
  }

   handleClick = (prevState) => {
    this.setState({value: prevState.value + 1});
    console.log(this.state.value)
  }

你能告诉我为什么会这样吗？根据此处的文档，它应该是正确的： https://facebook.github.io/react/docs/state-and-lifecycle.html

Answer 1

设置状态是异步的，因此当 console.log 发生时您不会看到值更新。你应该在 UI 上打印出状态值，这样你就可以看到发生了什么。要修复控制台日志，试试这个。

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

   handleClick = (prevState) => {
    this.setState({value: prevState.value + 1}, () => {
        console.log(this.state.value)
    });
  }

注意：当您为反应定义内联 lambda（箭头函数）时 class this 已正确绑定，因此您无需在构造函数中绑定它。

如果它只是像这样的状态增量，您也可以更改传递前一个数字的方式

handleClick = () => {
    this.setState({value: this.state.value + 1}, () => {
        console.log(this.state.value)
    });
}

Answer 2

试试这个

class QuestionList extends React.component {

    constructor(props){
        super(props)
        this.state = {
            value : 0
        }
    }

    handleClick(){
        this.setState({
            value : this.state.value + 1
        })
    }

   render(){
        return( <button type="button" onClick={this.handleClick.bind(this)}> {this.state.value} </button> )
   }
}

请注意，当您设置状态时，它会触发渲染功能，这将反映当前状态。在浏览器中试试吧！

Answer 3

因为您错误地使用了 handleClick 函数。这里：

handleClick = (prevState) => { .... }

prevState 将是一个事件对象传递给 handleClick 函数，你需要使用 prevState 和 setState，像这样：

handleClick = () => {
    this.setState(prevState => {
       return {count: prevState.count + 1}
    })
}

另一个问题是，setState 是异步的，所以 console.log(this.state.value) 不会打印更新的状态值，你需要使用 setState 的回调函数。

查看有关以及如何检查更新值的更多详细信息。

检查工作解决方案：

class App extends React.Component {
 
   constructor(props){
       super(props);
       this.state={ count: 1}
   }
 
  onclick(type){
      this.setState(prevState => {
         return {count: type == 'add' ? prevState.count + 1: prevState.count - 1}
      });
  }

   render() {
    return (
      <div>
        Count: {this.state.count}
        <br/>
        <div style={{marginTop: '100px'}}/>
        <input type='button' onClick={this.onclick.bind(this, 'add')} value='Inc'/>
        <input type='button' onClick={this.onclick.bind(this, 'sub')} value='Dec'/>
       </div>
     )
   }
}

ReactDOM.render(
  <App />,
  document.getElementById('container')
);

<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='container'></div>

Answer 4

class SkuVariantList extends React.Component {
    constructor(props) {
      super(props)
      this.state = {
        clicks: 0
      };
      this.clickHandler = this.clickHandler.bind(this)
    }

    componentDidMount() {
      this.refs.myComponentDiv.addEventListener('click', this.clickHandler);
    }

    componentWillUnmount() {
      //this.refs.myComponentDiv.removeEventListener('click', this.clickHandler);
    }

    clickHandler() {
      var clk = this.state.clicks
      this.setState({
        clicks: clk + 1
      });
    }

    render() {
      let children = this.props.children;

      return (
        <div className="my-component" ref="myComponentDiv">
          <h2>My Component ({this.state.clicks} clicks})</h2>
          <h3>{this.props.headerText}</h3>
          {children}
        </div>
      );
    }
  }

Answer 5

你好，试试这些代码来增加你的价值

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

addOne() {                              // addOne as HandleClick
  this.setState((preState) => {
    return {
      count : preState.count + 1
      };
   });
 }

render() {
   return (
      <div>
        <h1>Count : {this.state.count}</h1>
        <button onClick={this.addOne}>+1</button>
      </div>
     );
   }
 }

ReactDOM.render(<Counter />, document.getElementById('YOUR-ID'));

Answer 6

import React from 'react'

class App extends React.Component{
  constructor(){
    super()
    this.state = {
      count: 0
    }
    this.handleClick = this.handleClick.bind(this)
  }
  handleClick(){
    this.setState(prevState => {
      return {
        count: prevState.count + 1
      }
    })
  }
  render(){
    return(
      <div style = {{display: 'flex', fontSize: 30, flexDirection: 'column', alignItems:'center'}}>
        <h1>{this.state.count}</h1>
        <button onClick = {this.handleClick}>Change</button>
      </div>
    )
  }
}
export default App

Answer 7

这是最短的代码。先初始化state，再执行一个方法自增。

state = {
    counter: 0
  }

increaseHandler = () => {
    let counter = this.state.counter
    counter += 1
    this.setState({counter: counter})
  }

Answer 8

你也可以这样做，我们用相同的函数进行递增和递减操作，使其更加模块化和可重复

class CounterApp extends React.Component{

    constructor(){
        super();
        //here count is initially assigned with 0
        this.state ={
        count:0
        }
    }
    
    //when we click Increment or Decrement +1 or -1 is passed to step and the value gets changed it gets updated to the view
    increment = (step) =>{
        this.setState({
            count:this.state.count + step
        })
    }
    render(){
        const { count } = this.state;//same as const count = this.state.count;

        return(
            <div>
                <div className="counter-app">
                    <h2 className="value">{count}</h2>
                    <button onClick={() => this.increment(+1)}>Increment</button>
                    <button onClick={() => this.increment(-1)}>Decrement</button>
                </div>
            </div>
        )
    }
}

使用 React 将状态值递增 1

Incrementing state value by one using React

state

reactjs