javascript - 我无法从 this.state 获取数据

Question

我从后端收到 JSON，我将其保存在我的状态中，并且我想在另一个 React 组件的 props 中使用它，但它不起作用。

我尝试在组件的 Prop 中显示需要日期 - {this.state.movi​​es["0"]["title"]}，但它不起作用。

  constructor() {
    super();
    this.state = {
      movies: []
    }
  }

  componentDidMount() {
    this.getAllMoviesForMainPage();
  }

  getAllMoviesForMainPage() {
    axios.get("http://localhost:8080/showAll")
      .then(response => {
        this.setState({ movies: response.data })
      })
  }

  render() {
    return (
      <div className="qwerty">
        <MainPageComponent />
        <div className='moviePreviewGrid'>
          <Router>
            <div className="moviePreviewGrid-row">
              <div className="moviePreviewGrid-col">
                <MoviePreview
                  Title={this.state.movies["2"]["title"]}
                  MoviePreviewAvatar={DrivePoster}
                  SeeMore="unrelated long string here"
                />
                <NavLink to="/showByTitle/Драйв">
                  <button type="button" className="myBtn">See more</button>
                </NavLink>
              </div>

我的 JSON 的结构

[
    {
        "id": 1,
        "title": "Джокер",
        "releaseDate": "2019",
        "genre": "Триллер, драма, криминал",
        "duration": "122 минуты",
        "rating": 8.7,
        "criticReviews": [
            {
                "criticName": "Anton",
                "review": "anton review"
            },
            {
                "criticName": "OldCritic",
                "review": "old review"
            }
        ],
        "userReviews": [
            {
                "nickName": "Igor",
                "review": "igor review"
            },
            {
                "nickName": "Nik",
                "review": "nik review"
            }
        ]
    },
    {
        "id": 2,
        "title": "Ирландец",
        "releaseDate": "2019",
        "genre": "Драма, триллер, криминал, биографический",
        "duration": "209 минут",
        "rating": 8.4,
        "criticReviews": [
            {
                "criticName": "YoungCritic",
                "review": "young review"
            }
        ],
        "userReviews": [
            {
                "nickName": "Gambit",
                "review": "gambit review"
            },
            {
                "nickName": "Andrew",
                "review": "andrew review"
            }
        ]
    },
    {
        "id": 3,
        "title": "Драйв",
        "releaseDate": "2011",
        "genre": "Боевик, драма",
        "duration": "100 минут",
        "rating": 7.8,
        "criticReviews": [
            {
                "criticName": "Critic",
                "review": "review"
            }
        ],
        "userReviews": [
            {
                "nickName": "Anton",
                "review": "anton review"
            }
        ]
    },
    {
        "id": 4,
        "title": "Последний человек на Земле",
        "releaseDate": "2015",
        "genre": "Комедия",
        "duration": "22 минуты",
        "rating": 7.3,
        "criticReviews": [
            {
                "criticName": "NewCritic",
                "review": "new review"
            }
        ],
        "userReviews": [
            {
                "nickName": "Atomf7",
                "review": "atomf7 review"
            }
        ]
    },
    {
        "id": 5,
        "title": "Интерстеллар",
        "releaseDate": "2014",
        "genre": "Фантастика, драма, приключения",
        "duration": "169 минут",
        "rating": 8.6,
        "criticReviews": [
            {
                "criticName": "Nik",
                "review": "nik review"
            }
        ],
        "userReviews": [
            {
                "nickName": "Alice",
                "review": "alice review"
            }
        ]
    }
]

我不会有第一部电影的标题

Answer 1

在第一个 init 中，this.state.movi​​es 的长度为 0，因此 this.state.movi​​es["2"]["title"] 为类(class)根本没有任何值(value)因为getAllMoviesForMainPage是异步(axios调用)并且需要更长的时间才能完成，所以首先你必须给它一个初始值，只有当请求完成时你才能给它真正的值(value)。

示例:

<MoviePreview
  Title={this.state.movies.length > 0 ? this.state.movies[2].title : ""}
  MoviePreviewAvatar={DrivePoster}
/>

通常是一个状态，isLoading 用于这种情况。这样您就可以知道何时收到数据。