React组件间传值及跨组件通信详解

父组件中的方法
// 成员方法 箭头函数来定义，这样就没有this指向问题
setNum = (n = 1) => this.setState(state => ({ num: state.num + n }))
<Child num={num} setNum={this.setNum} />

子组件中
<button onClick={() => setNum(2)}>+++++</button>
子去修改父传过来的数据,props它是单向数据流，子不能直接去修改

childRef = createRef()
<Child ref={this.childRef} />
++++++++++++++++++++++++++++++++
this.setState({
	title: this.childRef.current.state.title
})
this.childRef.current.setState({ title: this.state.msg })
this.childRef.current.setTitle(this.state.msg)

import React, { Component } from 'react'
class Child1 extends Component {
  render() {
    let { setMsg } = this.props
    return (
      <div>
        <input type="text" onChange={e => setMsg(e.target.value)} />
      </div>
    )
  }
}
class Child2 extends Component {
  render() {
    let { msg } = this.props
    return (
      <div>
        <h3>{msg}</h3>
      </div>
    )
  }
}
class App extends Component {
  state = {
    msg: ''
  }
  setMsg = msg => this.setState({ msg })
  render() {
    return (
      <div>
        <h1>{this.state.msg}</h1>
        <Child1 setMsg={this.setMsg} />
        <Child2 msg={this.state.msg} />
      </div>
    )
  }
}
export default App

class App extends Component {
  state = {
    msg: '',
    setMsg: msg => this.setState({ msg })
  }
  render() {
    let { msg, setMsg } = this.state
    return (
      <div>
        <h1>{msg}</h1>
        {/* <Child1 setMsg={setMsg} /> */}
        {/* <Child2 msg={msg} /> */}
        <Child1 {...this.state} />
        <Child2 {...this.state} />
      </div>
    )
  }
}

import { createConext } from 'react'
const ctx = createContext()
const { Provider, Consumer } = ctx
export {
	ctx as default,
	Provider,
	Consumer
}

import Child from './pages/Child'
import { Provider } from './context/appContext'
class App extends Component {
    state = {
        name: "张三",
        setName: (name) => this.setState({ name }),
    };
=======================================================
render() {
       return (
           <div>
               {/* value属性就是生产的数据源 */}
               <Provider value={this.state}>
                   <Child />
               </Provider>
           </div>
       );
   }
}

// 假设现在当前组件是App组件下面的很深的子孙组件
import ctx, { Consumer } from '../../context/appContext'
.....
<Consumer>{state => <h3>{state.name}</h3>}</Consumer>

static contextType = ctx

const battery = this.context

// 假设现在当前组件是App组件下面的很深的子孙组件
import ctx, { Consumer } from '../../context/appContext'
static contextType = ctx
const battery = this.context
//就可以通过成员属性 this.context来获取数据
.....
<h3>{this.context.name}</h3>
<h3>{battery.name}</h3>