javascript - React - 子级到父级的回调不起作用

Question

之前成功地将数据从子组件发送到父组件的回调方法在此新项目中不起作用。

我正在开发一个 React Web 应用程序。有一个名为 Flags.js 的组件。它包含美国州旗和 targetFlag 州。 Flags 组件可以正确加载，但不会将该 targetFlag 选择发送到 App.js。我之前询问过 child 与家长的沟通，并得到了另一个项目的有效答案:React communication problem from child to parent 。我尝试将这种逻辑应用到这个应用程序中，但没有成功。

Github:https://github.com/irene-rojas/us-flags

App.js

import React, { Component } from 'react';
import './App.css';
import SVGMap from "./components/Map/Map.js";
import Flags from "./components/Flags/Flags";


class App extends Component {

state = {
    correct: 0,
    wrong: 0,
    targetFlag: "",
    selectedState: ""
}

onClick = (event) => {
    event.preventDefault();
    let clickedState = event.target.id;
    this.setState({
        selectedState: clickedState
    });  
    if (clickedState === this.state.selectedState) {
        this.setState({correct: this.state.correct + 1}, () => {
            console.log("correct");
            this.getFlag();
        });
    };
    if (clickedState === !this.state.selectedState) {
        this.setState({wrong: this.state.wrong + 1}, () => {
            console.log("wrong");
            this.getFlag();
        });
    }
}

getFlag = (targetFlag) => {
    this.setState({
        targetFlag: targetFlag.id
    });
    console.log(`App.js: ${this.state.targetFlag}`)
}


  render() {
    return (
  <div className="App">

    <div className="header">
        <h1>Match the Flag</h1>
    </div>

    <div className="flagsDiv">
        <Flags 
            sendFlag={this.getFlag}
        />
    </div>

    <div className="mapDiv">
        <SVGMap 
            onClick={this.onClick}
        />
    </div>

    <div className="scoreDiv">
        Correct: {this.state.correct}
        <br></br>
        Wrong: {this.state.wrong}
        <br></br>
        Target State: {this.state.targetFlag}
        <br></br>
        Selected State: {this.state.selectedState}
    </div>

  </div>
    );
  }
}

export default App;

Flags.js

import React, { Component } from 'react';
import "./Flags.css";
import alabama from "./images/Alabama.png";
import alaska from "./images/Alaska.png";
--- 48 more states in this import format ---

class Flags extends Component {

state = {
    flags: [
        {name: "Alaska", src: alaska, id: "AK"},
        {name: "Hawaii", src: hawaii, id: "HI"},
--- 48 more states in this state format ---
    targetFlag: [
        {name: "", src: "", id: ""}
    ],
};

componentDidMount() {
    let targetFlag = this.state.flags[Math.floor(Math.random()*this.state.flags.length)];
        this.setState({
            name: targetFlag.name,
            src: targetFlag.src,
            id: targetFlag.id
        });
        // console.log(`Flags.js: ${targetFlag.name}`);
}

sendFlag = (targetFlag) => {
    this.props.sendFlag(targetFlag);
}  


render() {
    return (

    <div>
        <img  
            src={this.state.src} 
            alt={this.state.id}
            onLoad={this.sendFlag}
        />
        <br></br>
        {this.state.name}
    </div>

    )
    }
}

export default Flags;

我希望将 Flags.js 中选择的 targetFlag 值传输到 App.js。如果代码还有其他问题，请告诉我。

Answer 1

我提取了你的 github 存储库， 您应该注意到 setstate 是异步的，因此您应该使用回调函数

//App.js line 36
//notice the targetFlag.target.id and setstate callback function
getFlag = (targetFlag) => {
        this.setState({
            targetFlag: targetFlag.target.id
        }, ()=>{
            console.log(`App.js: ${this.state.targetFlag}`)
        });

    }

//Flags.js line 133
//notice the addition of id attribute
<img  
  src={this.state.src} 
  alt={this.state.id}
  onLoad={this.sendFlag}
  id={this.state.id}
 />