reactjs - createContext 不接受 defaultValue

Question

我正在尝试使用 createContext 构建上下文来自 react API:

import React, { useState, createContext } from 'react';

export const MovieContext =  createContext();

export const MovieProvider = (props) => {
  const [movies, setMovies] = useState(
    [
      {
        original_title: 'name of movie',
        poster_path: 'path_to_poster',
        id: 1
      }
    ]
  );
  return (
    <MovieContext.Provider value={[movies, setMovies]}>
      {props.children}
    </MovieContext.Provider>
  );
}

export default MovieProvider;

createContext()函数显示此错误:

Expected 1 arguments, but got 0.ts(2554) index.d.ts(385, 9): An argument for 'defaultValue' was not provided.

如果我将其设为字符串，并将字符串作为应用程序构建的值传递:

export const MovieContext =  createContext('');

<MovieContext.Provider value={''}>
  {props.children}
</MovieContext.Provider>

我添加什么样的值作为 createContext()参数使其与 useState Hook ？我关注 this guys tutorial他在 JS 中执行此操作，但我在 TS 中构建我的应用程序，所以它更严格一些。

Answer 1

这些是 Typescript 类型，它们被描述为 here .代码中有一条注释为什么需要这个参数，尽管 documentation没有说它是必需的。

function createContext<T>(
    // If you thought this should be optional, see
    // https://github.com/DefinitelyTyped/DefinitelyTyped/pull/24509#issuecomment-382213106
    defaultValue: T,
): Context<T>;

在您的情况下，将正确设置默认参数，例如

type IMovie = {
    original_title: string;
    poster_path: string;
    id: number;
};

type IMovieContext = [IMovie[], React.Dispatch<React.SetStateAction<IMovie[]>>];

export const MovieContext = createContext<IMovieContext>([[], () => null]);

export const MovieProvider = props => {
    const [movies, setMovies] = useState<IMovie[]>([
        {
            original_title: 'name of movie',
            poster_path: 'path_to_poster',
            id: 1,
        },
    ]);
    return <MovieContext.Provider value={[movies, setMovies]}>{props.children}</MovieContext.Provider>;
};