获取API后的未定义值,React

Question

我正在使用themoviedb API构建一个简单的电影目录,但是我遇到了一个我无法解决的问题.问题是获取后的结果总是未定义的.

我尝试使用方法componentWillMount来获取数据并在此方法中设置状态,但它不起作用.我试图获取内部构造函数,没有结果.

到目前为止这是我的代码

import React, { Component } from 'react';
import Header from './components/Header';
import MovieList from './components/MovieList';
import Footer from './components/Footer';

const MOVIE_API = "http://api.themoviedb.org/3/discover/movie?api_key=72049b7019c79f226fad8eec6e1ee889&language=en-US&sort_by=release_date.desc&include_adult=true&include_video=false&page=2&primary_release_year=2018";

//class
class App extends Component {
  constructor(props){
    super(props);

    this.state = {
      movies: [],
      movieName: ''
    }
  }


  componentWillMount(){
    this.fetchMovie();
  }


//fetching movie
  fetchMovie = () =>{
    const req = new Request(MOVIE_API, {
      method: 'GET',
      cache: 'default'
    });

    fetch(req).then(response =>{
      return response.json();
    }).then(data =>{
      console.log(data); //REF 1;
      this.setState({
        movies: data
      });
    }).catch(err => {
      console.log("ERROR: " + err);
    })
  }

  render() {
    return (
      <div className="root">
        <Header />
        <MovieList moviesRes={this.state.movies}/>
        <Footer />

      </div>
    );
  }
}

export default App;

正如您所看到的,我调用方法componentWillMount来获取数据,但它不起作用.值得注意的是,如果我记录数据(REF 1),我可以看到结果(json).

===========================

编辑

这是MovieList的代码

/*import React, { Component } from 'react';

export default class MovieList extends Component{
    constructor(props){
        super(props);
        this.state = {
            movies: this.props.movieRes
        }
    }
    render(){

        //if result is undefined 
        if(this.state.movieRes === undefined){
            return(
                <h1>Loading...</h1>
            );
        }else{
            return(
                <ul>
                {this.state.movieRes.map((movie, index)=>{
                    return (
                        <li key={index}>{movie.title}</li>
                    );
                })}
                </ul>
            );
        }
    }
}*/

=================

更新子代码

import React, { Component } from 'react';

export default class MovieList extends Component{

    render(){
        const { movieRes = [] } = this.props; // we are assigning a default prop here of an empty array.
        return(
            <ul>
                {
                    //return movie from array
                    movieRes.map((movie, index)=>{
                        return (
                            <li key={index}>
                                {movie.id}
                            </li>
                        );
                    })
                }
            </ul>
        );
    }
}

在这种方式我压制错误,但它仍然无法正常工作.根据我的了解,React应该在检测到变化时立即渲染,但由于某种原因不是这样.

图片

从图像中可以看出,当我将数组从父组件传递给子组件时,数组长度为20,但在子组件中,数组长度似乎为0

===================

解

我将组件从类更改为const并将数组传递给它,一切顺利.这是最终的代码:

import React from 'react';

const MovieList = ({movies}) =>{
    if(!movies){
        return <h1>Loading...</h1>
    }
    return (
        <ul>
            {
                movies.map((movie, index) => {
                    return (
                        <li key={index}>
                            <p>{movie.title}</p>
                        </li>
                    )
                })
            }
        </ul>
    );

}

export default MovieList;

Answer 1

最初我误解了你的问题但是在重新阅读之后我注意到你在构造函数中将电影定义为一个数组.

如果没有实际的错误消息,我将假设MovieList期待一个数组为它的prop movieRes,然后你可能会尝试做类似的事情.map或循环来渲染电影.

但是,您使用的API不会返回数组.它返回一个带有数组键的对象results.因此,我data.results在执行setState时将其更改为访问权限.

//fetching movie
  fetchMovie = () =>{
    const req = new Request(MOVIE_API, {
      method: 'GET',
      cache: 'default'
    });

    fetch(req).then(response =>{
      return response.json();
    }).then(data =>{
      console.log(data);
      this.setState({
        movies: data.results // <-- change made here.
      });
    }).catch(err => {
      console.log("ERROR: " + err);
    })
  }

这是一个有效的JSFiddle:

https://jsfiddle.net/patrickgordon/69z2wepo/99513/

编辑:在子组件中,只需使用道具和默认道具,而不是将道具分配给状态.

import React, { Component } from 'react';

export default class MovieList extends Component{
    render(){
        const { movieRes = [] } = this.props; // we are assigning a default prop here of an empty array.
        return(
            <ul>
                {movieRes.map((movie, index)=>{
                    return (
                        <li key={index}>{movie.title}</li>
                    );
                })}
            </ul>
        );
    }
}