如何在reactjs中从一个组件（子组件）到另一个组件（父组件）获取值？

Question

我正在从事Reactjs Weather项目。提交后，我正在子组件（Form.js）中的一种方法中获取值，我需要在父组件（App.js）中获取该值。如何获得该价值？

1. App.js文件（父组件）

getWeather = async (e) => {
    e.preventDefault();

}

2. Form.js文件（子组件）

import React, { Component } from 'react'
import Background from '../video/rain_drop2.jpg';
import { ToastContainer, toast } from 'react-toastify';
import 'react-toastify/dist/ReactToastify.css';

import { MDBContainer,
    MDBRow,
    MDBCol,
    MDBCard,
    MDBCardBody,
    MDBCardHeader,
    MDBBtn,
    MDBInput } from 'mdbreact'


const sectionStyle = {
    backgroundImage: `url(${Background})`,
    opacity: '0.92',
    width: '90%',
    backgroundSize: 'cover',
    backgroundPosition: 'center',
    height:'99%',
    position:'absolute',
    filter:'blur(1.8px)'
    };

class Form extends React.Component {
  constructor(props){
    super(props);

    this.state = {
      fields: {},
      errors: {}
    }
  }

  handleValidation(){
    let fields = this.state.fields;
    let errors = {};
    let formIsValid = true;

    //Name
    if(!fields["city"]){
      formIsValid = false;
      errors["city"] = "Cannot be empty";
    }

    this.setState({errors: errors});
    return formIsValid;
  }

  submitDetails = (e) => {
    e.preventDefault();

    // I want this value to be accessed in App.js file
    const city = e.target.city.value
    console.log('VALUE', city)

    browserHistory.push()
    if(this.handleValidation()){
      toast("Form Submitted!.")
    }else{
      toast("Form has errors.")
    }

  }

  handleChange(field, e){           
    let fields = this.state.fields;
    fields[field] = e.target.value;        
    this.setState({fields});
  }

  render(){
    return (

      <div>  
<MDBContainer>
<MDBRow>
  <MDBCol>
  <MDBCard  style={sectionStyle}></MDBCard>
  <MDBCard className="card" style={{zIndex:'1', background: 'none'}}>
  <MDBCardBody >
    <MDBCardHeader style={{background: '#ecf0f1', opacity:'0.7', borderRadius: '10px',
    fontFamily: 'Josefin Sans'}}>
      <h3 className="my-3 text-center" style={{color: '#535c68'}}>
         Weather Finder
      </h3>
    </MDBCardHeader>
    <br/>
    <form onSubmit= {this.submitDetails.bind(this)}>
      <div className="blue-grey-text">
      <span className="error">{this.state.errors["name"]}</span>
        <MDBInput
          ref="city"
          label="City"
          icon="city"
          type="text"
          name="city"
          id="text-input"
          onChange={this.handleChange.bind(this, "city")}
          value={this.state.fields["city"]}
        />
        <span style={{color: "red"}}>{this.state.errors["city"]}</span>

        </div>
        <br/>

      <div className="text-center mt-4">
        <MDBBtn
          color="blue-grey"
          className="mb-3"
          type="submit"
          value="Submit"
        >
          Search Weather
        </MDBBtn>
        <ToastContainer />
      </div>
      </form>
    </MDBCardBody>
    </MDBCard>
    </MDBCol>
  </MDBRow>
  </MDBContainer>
  </div>
    )
  }
}

export default Form;

任何建议，高度赞赏...在此先感谢

Answer 1

当在父组件中渲染子组件时，将在父组件中定义的函数作为道具传递给子组件。

当您在子组件中获得所需的值时，调用从父组件传递的函数作为prop，并将该值作为参数传递给该函数。

父组件

// function to be called from child component
foo = (value) => {
   // code
};

render() {
   return (
      <ChildComponent foo={this.foo}/>
   );
}

子组件

bar = () => {
   const value = // get the value
   // call the foo function of parent component and pass the value
   this.props.foo(value);
};

Answer 2

最常用的方法是使用回调：

• 在子表单组件中，您定义了新的道具'onSubmit'

• 在父级App组件中，您使用'data'参数定义了'submitHandler'函数：

  handleSubmit = data => {/ *您的逻辑在这里* /}

  渲染=>（）

• 以及子组件的方法'submitDetails'的最后一步，您调用this.props.onSubmit(data);

Answer 3

您可以为此提供来自父组件的回调：

const Parent = () => {
  const [ weather, setWeather ] = useState(null);

  return (
    <Child setWeather={ setWeather } />
  );
};

const Child = ({ setWeather }) => {
  const getWeather = () => { ... };
  const onClick = () => {
    const weather = getWeather();
    setWeather(weather);
  };

  return (
    <button onClick={ onClick }>Get Weather</button>
  );
}