使用 Context 深层传递参数 | Passing Data Deeply with Context

传递 props 带来的问题 | The problem with passing props

Passing props is a great way to explicitly pipe data through your UI tree to the components that use it. 传递 props 是将数据通过 UI 树显式传递到使用它的组件的好方法。

But passing props can become verbose and inconvenient when you need to pass some prop deeply through the tree, or if many components need the same prop. The nearest common ancestor could be far removed from the components that need data, and lifting state up that high can lead to a situation called “prop drilling”. 但是当你需要在组件树中深层传递参数以及需要在组件间复用相同的参数时，传递 props 就会变得很麻烦。最近的根节点父组件可能离需要数据的组件很远，状态提升 到太高的层级会导致 “逐层传递 props” 的情况。

Wouldn’t it be great if there were a way to “teleport” data to the components in the tree that need it without passing props? With React’s context feature, there is! 要是有一种方法可以在组件树中不需要 props 将数据“直达”到所需的组件中，那可就太好了。React 的 context 功能可以满足我们的这个心愿。

Context：传递 props 的另一种方法 | Context: an alternative to passing props

Context lets a parent component provide data to the entire tree below it. There are many uses for context. Here is one example. Consider this Heading component that accepts a level for its size: Context 让父组件可以为它下面的整个组件树提供数据。Context 有很多种用途。这里就有一个示例。思考一下这个 Heading 组件接收一个 level 参数来决定它标题尺寸的场景：

Let’s say you want multiple headings within the same Section to always have the same size: 假设你想让相同 Section 中的多个 Heading 具有相同的尺寸：

Currently, you pass the level prop to each <Heading> separately: 目前，你将 level 参数分别传递给每个 <Heading>：

<Section>
  <Heading level={3}>关于</Heading>
  <Heading level={3}>照片</Heading>
  <Heading level={3}>视频</Heading>
</Section>

It would be nice if you could pass the level prop to the <Section> component instead and remove it from the <Heading>. This way you could enforce that all headings in the same section have the same size: 将 level 参数传递给 <Section> 组件而不是传给 <Heading> 组件看起来更好一些。这样的话你可以强制使同一个 section 中的所有标题都有相同的尺寸：

<Section level={3}>
  <Heading>关于</Heading>
  <Heading>照片</Heading>
  <Heading>视频</Heading>
</Section>

But how can the <Heading> component know the level of its closest <Section>? That would require some way for a child to “ask” for data from somewhere above in the tree. 但是 <Heading> 组件是如何知道离它最近的 <Section> 的 level 的呢？这需要子组件可以通过某种方式“访问”到组件树中某处在其上层的数据。

You can’t do it with props alone. This is where context comes into play. You will do it in three steps: 你不能只通过 props 来实现它。这就是 context 大显身手的地方。你可以通过以下三个步骤来实现它：

1. Create a context. (You can call it LevelContext, since it’s for the heading level.)

• 创建 一个 context。（你可以将其命名为 LevelContext, 因为它表示的是标题级别。）

2. Use that context from the component that needs the data. (Heading will use LevelContext.)

• 在需要数据的组件内 使用 刚刚创建的 context。（Heading 将会使用 LevelContext。）

3. Provide that context from the component that specifies the data. (Section will provide LevelContext.)

• 在指定数据的组件中 提供 这个 context。 （Section 将会提供 LevelContext。）

Context lets a parent—even a distant one!—provide some data to the entire tree inside of it. Context 可以让父节点，甚至是很远的父节点都可以为其内部的整个组件树提供数据。

Step 1：创建 context | Step 1: Create the context

First, you need to create the context. You’ll need to export it from a file so that your components can use it: 首先，你需要创建这个 context，并 将其从一个文件中导出，这样你的组件才可以使用它：

The only argument to createContext is the default value. Here, 1 refers to the biggest heading level, but you could pass any kind of value (even an object). You will see the significance of the default value in the next step. createContext 只需默认值这么一个参数。在这里, 1 表示最大的标题级别，但是你可以传递任何类型的值（甚至可以传入一个对象）。你将在下一个步骤中见识到默认值的意义。

Step 2：使用 Context | Step 2: Use the context

Import the useContext Hook from React and your context: 从 React 中引入 useContext Hook 以及你刚刚创建的 context:

import { useContext } from 'react';
import { LevelContext } from './LevelContext.js';

Currently, the Heading component reads level from props: 目前，Heading 组件从 props 中读取 level：

export default function Heading({ level, children }) {
  // ...
}

Instead, remove the level prop and read the value from the context you just imported, LevelContext: 删掉 level 参数并从你刚刚引入的 LevelContext 中读取值：

export default function Heading({ children }) {
  const level = useContext(LevelContext);
  // ...
}

useContext is a Hook. Just like useState and useReducer, you can only call a Hook immediately inside a React component (not inside loops or conditions). useContext tells React that the Heading component wants to read the LevelContext. useContext 是一个 Hook。和 useState 以及 useReducer一样，你只能在 React 组件中（不是循环或者条件里）立即调用 Hook。useContext 告诉 React Heading 组件想要读取 LevelContext。

Now that the Heading component doesn’t have a level prop, you don’t need to pass the level prop to Heading in your JSX like this anymore: 现在 Heading 组件没有 level 参数，你不需要再像这样在你的 JSX 中将 level 参数传递给 Heading：

<Section>
  <Heading level={4}>子子标题</Heading>
  <Heading level={4}>子子标题</Heading>
  <Heading level={4}>子子标题</Heading>
</Section>

修改一下 JSX，让 Section 组件代替 Heading 组件接收 level 参数：

<Section level={4}>
  <Heading>子子标题</Heading>
  <Heading>子子标题</Heading>
  <Heading>子子标题</Heading>
</Section>

你将修改下边的代码直到它正常运行：

Notice this example doesn’t quite work, yet! All the headings have the same size because even though you’re using the context, you have not provided it yet. React doesn’t know where to get it! 注意！这个示例还不能运行。所有 headings 的尺寸都一样，因为 即使你正在使用 context，但是你还没有提供它。 React 不知道从哪里获取这个 context！

If you don’t provide the context, React will use the default value you’ve specified in the previous step. In this example, you specified 1 as the argument to createContext, so useContext(LevelContext) returns 1, setting all those headings to <h1>. Let’s fix this problem by having each Section provide its own context. 如果你不提供 context，React 会使用你在上一步指定的默认值。在这个例子中，你为 createContext 传入了 1 这个参数，所以 useContext(LevelContext) 会返回 1，把所有的标题都设置为<h1>。我们通过让每个 Section 提供它自己的 context 来修复这个问题。

Step 3：提供 context | Step 3: Provide the context

The Section component currently renders its children: Section 组件目前渲染传入它的子组件：

export default function Section({ children }) {
  return (
    <section className="section">
      {children}
    </section>
  );
}

Wrap them with a context provider to provide the LevelContext to them: 把它们用 context provider 包裹起来 以提供 LevelContext 给它们：

import { LevelContext } from './LevelContext.js';

export default function Section({ level, children }) {
  return (
    <section className="section">
      <LevelContext.Provider value={level}>
        {children}
      </LevelContext.Provider>
    </section>
  );
}

This tells React: “if any component inside this <Section> asks for LevelContext, give them this level.” The component will use the value of the nearest <LevelContext.Provider> in the UI tree above it. 这告诉 React：“如果在 <Section> 组件中的任何子组件请求 LevelContext，给他们这个 level。”组件会使用 UI 树中在它上层最近的那个 <LevelContext.Provider> 传递过来的值。

It’s the same result as the original code, but you did not need to pass the level prop to each Heading component! Instead, it “figures out” its heading level by asking the closest Section above: 这与原始代码的运行结果相同，但是你不需要向每个 Heading 组件传递 level 参数了！取而代之的是，它通过访问上层最近的 Section 来“断定”它的标题级别：

1. You pass a level prop to the <Section>.

• 你将一个 level 参数传递给 <Section>。

2. Section wraps its children into <LevelContext.Provider value={level}>.

• Section 把它的子元素包在 <LevelContext.Provider value={level}> 里面。

3. Heading asks the closest value of LevelContext above with useContext(LevelContext).

• Heading 使用 useContext(LevelContext) 访问上层最近的 LevelContext 提供的值。

在相同的组件中使用并提供 context | Using and providing context from the same component

Currently, you still have to specify each section’s level manually: 目前，你仍需要手动指定每个 section 的 level：

export default function Page() {
  return (
    <Section level={1}>
      ...
      <Section level={2}>
        ...
        <Section level={3}>
          ...

Since context lets you read information from a component above, each Section could read the level from the Section above, and pass level + 1 down automatically. Here is how you could do it: 由于 context 让你可以从上层的组件读取信息，每个 Section 都会从上层的 Section 读取 level，并自动向下层传递 level + 1。 你可以像下面这样做：

import { useContext } from 'react';
import { LevelContext } from './LevelContext.js';

export default function Section({ children }) {
  const level = useContext(LevelContext);
  return (
    <section className="section">
      <LevelContext.Provider value={level + 1}>
        {children}
      </LevelContext.Provider>
    </section>
  );
}

With this change, you don’t need to pass the level prop either to the <Section> or to the <Heading>: 这样修改之后，你不用将 level 参数传给 <Section> 或者是 <Heading> 了：

Now both Heading and Section read the LevelContext to figure out how “deep” they are. And the Section wraps its children into the LevelContext to specify that anything inside of it is at a “deeper” level. 现在，Heading 和 Section 都通过读取 LevelContext 来判断它们的深度。而且 Section 把它的子组件都包在 LevelContext 中来指定其中的任何内容都处于一个“更深”的级别。

Context 会穿过中间层级的组件 | Context passes through intermediate components

You can insert as many components as you like between the component that provides context and the one that uses it. This includes both built-in components like <div> and components you might build yourself. 你可以在提供 context 的组件和使用它的组件之间的层级插入任意数量的组件。这包括像 <div> 这样的内置组件和你自己创建的组件。

In this example, the same Post component (with a dashed border) is rendered at two different nesting levels. Notice that the <Heading> inside of it gets its level automatically from the closest <Section>: 在这个示例中，相同的 Post 组件（带有虚线边框）在两个不同的嵌套层级上渲染。注意，它内部的 <Heading> 会自动从最近的 <Section> 获取它的级别：

You didn’t do anything special for this to work. A Section specifies the context for the tree inside it, so you can insert a <Heading> anywhere, and it will have the correct size. Try it in the sandbox above! 你不需要做任何特殊的操作。Section 为它内部的树指定一个 context，所以你可以在任何地方插入一个 <Heading>，而且它会有正确的尺寸。在上边的沙箱中尝试一下！

Context lets you write components that “adapt to their surroundings” and display themselves differently depending on where (or, in other words, in which context) they are being rendered. Context 让你可以编写“适应周围环境”的组件，并且根据 在哪 （或者说 在哪个 context 中）来渲染它们不同的样子。

How context works might remind you of CSS property inheritance. In CSS, you can specify color: blue for a <div>, and any DOM node inside of it, no matter how deep, will inherit that color unless some other DOM node in the middle overrides it with color: green. Similarly, in React, the only way to override some context coming from above is to wrap children into a context provider with a different value. Context 的工作方式可能会让你想起 CSS 属性继承。在 CSS 中，你可以为一个 <div> 手动指定 color: blue，并且其中的任何 DOM 节点，无论多深，都会继承那个颜色，除非中间的其他 DOM 节点用 color: green 来覆盖它。类似地，在 React 中，覆盖来自上层的某些 context 的唯一方法是将子组件包裹到一个提供不同值的 context provider 中。

In CSS, different properties like color and background-color don’t override each other. You can set all <div>’s color to red without impacting background-color. Similarly, different React contexts don’t override each other. Each context that you make with createContext() is completely separate from other ones, and ties together components using and providing that particular context. One component may use or provide many different contexts without a problem. 在 CSS 中，诸如 color 和 background-color 之类的不同属性不会覆盖彼此。你可以设置所有 <div> 的 color 为红色，而不会影响 background-color。类似地，不同的 React context 不会覆盖彼此。你通过 createContext() 创建的每个 context 都和其他 context 完全分离，只有使用和提供 那个特定的 context 的组件才会联系在一起。一个组件可以轻松地使用或者提供许多不同的 context。

写在你使用 context 之前 | Before you use context

Context is very tempting to use! However, this also means it’s too easy to overuse it. Just because you need to pass some props several levels deep doesn’t mean you should put that information into context. 使用 Context 看起来非常诱人！然而，这也意味着它也太容易被过度使用了。如果你只想把一些 props 传递到多个层级中，这并不意味着你需要把这些信息放到 context 里。

Here’s a few alternatives you should consider before using context: 在使用 context 之前，你可以考虑以下几种替代方案：

1. Start by passing props. If your components are not trivial, it’s not unusual to pass a dozen props down through a dozen components. It may feel like a slog, but it makes it very clear which components use which data! The person maintaining your code will be glad you’ve made the data flow explicit with props.

• 从 传递 props 开始。 如果你的组件看起来不起眼，那么通过十几个组件向下传递一堆 props 并不罕见。这有点像是在埋头苦干，但是这样做可以让哪些组件用了哪些数据变得十分清晰！维护你代码的人会很高兴你用 props 让数据流变得更加清晰。

2. Extract components and pass JSX as children to them. If you pass some data through many layers of intermediate components that don’t use that data (and only pass it further down), this often means that you forgot to extract some components along the way. For example, maybe you pass data props like posts to visual components that don’t use them directly, like <Layout posts={posts} />. Instead, make Layout take children as a prop, and render <Layout><Posts posts={posts} /></Layout>. This reduces the number of layers between the component specifying the data and the one that needs it.

• 抽象组件并 将 JSX 作为 children 传递 给它们。 如果你通过很多层不使用该数据的中间组件（并且只会向下传递）来传递数据，这通常意味着你在此过程中忘记了抽象组件。举个例子，你可能想传递一些像 posts 的数据 props 到不会直接使用这个参数的组件，类似 <Layout posts={posts} />。取而代之的是，让 Layout 把 children 当做一个参数，然后渲染 <Layout><Posts posts={posts} /></Layout>。这样就减少了定义数据的组件和使用数据的组件之间的层级。

If neither of these approaches works well for you, consider context. 如果这两种方法都不适合你，再考虑使用 context。

Context 的使用场景 | Use cases for context

• Theming: If your app lets the user change its appearance (e.g. dark mode), you can put a context provider at the top of your app, and use that context in components that need to adjust their visual look.
• 主题： 如果你的应用允许用户更改其外观（例如暗夜模式），你可以在应用顶层放一个 context provider，并在需要调整其外观的组件中使用该 context。
• Current account: Many components might need to know the currently logged in user. Putting it in context makes it convenient to read it anywhere in the tree. Some apps also let you operate multiple accounts at the same time (e.g. to leave a comment as a different user). In those cases, it can be convenient to wrap a part of the UI into a nested provider with a different current account value.
• 当前账户： 许多组件可能需要知道当前登录的用户信息。将它放到 context 中可以方便地在树中的任何位置读取它。某些应用还允许你同时操作多个账户（例如，以不同用户的身份发表评论）。在这些情况下，将 UI 的一部分包裹到具有不同账户数据的 provider 中会很方便。
• Routing: Most routing solutions use context internally to hold the current route. This is how every link “knows” whether it’s active or not. If you build your own router, you might want to do it too.
• 路由： 大多数路由解决方案在其内部使用 context 来保存当前路由。这就是每个链接“知道”它是否处于活动状态的方式。如果你创建自己的路由库，你可能也会这么做。
• Managing state: As your app grows, you might end up with a lot of state closer to the top of your app. Many distant components below may want to change it. It is common to use a reducer together with context to manage complex state and pass it down to distant components without too much hassle.
• 状态管理： 随着你的应用的增长，最终在靠近应用顶部的位置可能会有很多 state。许多遥远的下层组件可能想要修改它们。通常 将 reducer 与 context 搭配使用来管理复杂的状态并将其传递给深层的组件来避免过多的麻烦。

Context is not limited to static values. If you pass a different value on the next render, React will update all the components reading it below! This is why context is often used in combination with state. Context 不局限于静态值。如果你在下一次渲染时传递不同的值，React 将会更新读取它的所有下层组件！这就是 context 经常和 state 结合使用的原因。

In general, if some information is needed by distant components in different parts of the tree, it’s a good indication that context will help you. 一般而言，如果树中不同部分的远距离组件需要某些信息，context 将会对你大有帮助。