Children

const mappedChildren = Children.map(children, child =>
  <div className="Row">
    {child}
  </div>
);

• 参考
  • Children.count(children)
  • Children.forEach(children, fn, thisArg?)
  • Children.map(children, fn, thisArg?)
  • Children.only(children)
  • Children.toArray(children)
• 用法
  • 子组件变形
  • 为每个子组件运行部分代码
  • 计数子元素
  • 将子元素转换为数组
• 备选方案
  • 公开多个组件
  • 接受对象数组作为 prop
  • 调用渲染属性自定义渲染
• 故障排除
  • 我传递了一个自定义组件，但 Children 方法没有显示其渲染结果

参考

¥Reference

Children.count(children)

调用 Children.count(children) 来计算 children 数据结构中的子组件数量。

¥Call Children.count(children) to count the number of children in the children data structure.

import { Children } from 'react';

function RowList({ children }) {
  return (
    <>
      <h1>Total rows: {Children.count(children)}</h1>
      ...
    </>
  );
}

请参阅下面的更多示例。

¥See more examples below.

参数

¥Parameters

• children：你的组件接收的 children prop 值。

  ¥children: The value of the children prop received by your component.

返回

¥Returns

这些 children 中的节点数。

¥The number of nodes inside these children.

注意事项

¥Caveats

• 空节点（null、undefined 和布尔值）、字符串、数字和 React 元素 均计为独立节点。数组不算作单独的节点，但它们的子节点算。遍历深度不超过 React 元素：它们不会被渲染，并且它们的子元素不会被遍历。片段 不会被遍历。

  ¥Empty nodes (null, undefined, and Booleans), strings, numbers, and React elements count as individual nodes. Arrays don’t count as individual nodes, but their children do. The traversal does not go deeper than React elements: they don’t get rendered, and their children aren’t traversed. Fragments don’t get traversed.

Children.forEach(children, fn, thisArg?)

调用 Children.forEach(children, fn, thisArg?) 为 children 数据结构中的每个子组件运行一些代码。

¥Call Children.forEach(children, fn, thisArg?) to run some code for each child in the children data structure.

import { Children } from 'react';

function SeparatorList({ children }) {
  const result = [];
  Children.forEach(children, (child, index) => {
    result.push(child);
    result.push(<hr key={index} />);
  });
  // ...

请参阅下面的更多示例。

¥See more examples below.

参数

¥Parameters

• children：你的组件接收的 children prop 值。

  ¥children: The value of the children prop received by your component.

• fn：你希望为每个子组件运行的函数，类似于 数组 forEach 方法 回调。此方法将以子组件作为第一个参数，其索引作为第二个参数进行调用。索引从 0 开始，每次调用时递增。

  ¥fn: The function you want to run for each child, similar to the array forEach method callback. It will be called with the child as the first argument and its index as the second argument. The index starts at 0 and increments on each call.

• 可选 thisArg：调用 fn 函数时应使用的 this 值。如果省略，则为 undefined。

  ¥optional thisArg: The this value with which the fn function should be called. If omitted, it’s undefined.

返回

¥Returns

Children.forEach 返回 undefined。

¥Children.forEach returns undefined.

注意事项

¥Caveats

• 空节点（null、undefined 和布尔值）、字符串、数字和 React 元素 均计为独立节点。数组不算作单独的节点，但它们的子节点算。遍历深度不超过 React 元素：它们不会被渲染，并且它们的子元素不会被遍历。片段 不会被遍历。

  ¥Empty nodes (null, undefined, and Booleans), strings, numbers, and React elements count as individual nodes. Arrays don’t count as individual nodes, but their children do. The traversal does not go deeper than React elements: they don’t get rendered, and their children aren’t traversed. Fragments don’t get traversed.

Children.map(children, fn, thisArg?)

调用 Children.map(children, fn, thisArg?) 映射或转换 children 数据结构中的每个子组件。

¥Call Children.map(children, fn, thisArg?) to map or transform each child in the children data structure.

import { Children } from 'react';

function RowList({ children }) {
  return (
    <div className="RowList">
      {Children.map(children, child =>
        <div className="Row">
          {child}
        </div>
      )}
    </div>
  );
}

请参阅下面的更多示例。

¥See more examples below.

参数

¥Parameters

• children：你的组件接收的 children prop 值。

  ¥children: The value of the children prop received by your component.

• fn：映射函数，类似于 数组 map 方法 回调。此方法将以子组件作为第一个参数，其索引作为第二个参数进行调用。索引从 0 开始，每次调用时递增。你需要从此函数返回一个 React 节点。这可能是一个空节点（null、undefined 或布尔值）、字符串、数字、React 元素或其他 React 节点的数组。

  ¥fn: The mapping function, similar to the array map method callback. It will be called with the child as the first argument and its index as the second argument. The index starts at 0 and increments on each call. You need to return a React node from this function. This may be an empty node (null, undefined, or a Boolean), a string, a number, a React element, or an array of other React nodes.

• 可选 thisArg：调用 fn 函数时应使用的 this 值。如果省略，则为 undefined。

  ¥optional thisArg: The this value with which the fn function should be called. If omitted, it’s undefined.

返回

¥Returns

如果 children 是 null 或 undefined，则返回相同的值。

¥If children is null or undefined, returns the same value.

否则，返回一个由 fn 函数返回的节点组成的扁平数组。返回的数组将包含除 null 和 undefined 之外的所有返回节点。

¥Otherwise, returns a flat array consisting of the nodes you’ve returned from the fn function. The returned array will contain all nodes you returned except for null and undefined.

注意事项

¥Caveats

• 空节点（null、undefined 和布尔值）、字符串、数字和 React 元素 均计为独立节点。数组不算作单独的节点，但它们的子节点算。遍历深度不超过 React 元素：它们不会被渲染，并且它们的子元素不会被遍历。片段 不会被遍历。

  ¥Empty nodes (null, undefined, and Booleans), strings, numbers, and React elements count as individual nodes. Arrays don’t count as individual nodes, but their children do. The traversal does not go deeper than React elements: they don’t get rendered, and their children aren’t traversed. Fragments don’t get traversed.

• 如果你从 fn 返回一个元素或一个包含键的元素数组，则返回元素的键将自动与 children 中相应原始项的键合并。当你从数组中的 fn 返回多个元素时，它们的键只需在本地彼此唯一即可。

  ¥If you return an element or an array of elements with keys from fn, the returned elements’ keys will be automatically combined with the key of the corresponding original item from children. When you return multiple elements from fn in an array, their keys only need to be unique locally amongst each other.

Children.only(children)

调用 Children.only(children) 来断言 children 代表单个 React 元素。

¥Call Children.only(children) to assert that children represent a single React element.

function Box({ children }) {
  const element = Children.only(children);
  // ...

参数

¥Parameters

• children：你的组件接收的 children prop 值。

  ¥children: The value of the children prop received by your component.

返回

¥Returns

如果 children 是有效元素 返回该元素。

¥If children is a valid element, returns that element.

否则，抛出错误。

¥Otherwise, throws an error.

注意事项

¥Caveats

• 如果你将数组（例如 Children.map 的返回值）作为 children 传递，则此方法始终会抛出异常。换句话说，它强制 children 是一个单独的 React 元素，而不是一个包含单个元素的数组。

  ¥This method always throws if you pass an array (such as the return value of Children.map) as children. In other words, it enforces that children is a single React element, not that it’s an array with a single element.

Children.toArray(children)

调用 Children.toArray(children) 创建一个基于 children 数据结构的数组。

¥Call Children.toArray(children) to create an array out of the children data structure.

import { Children } from 'react';

export default function ReversedList({ children }) {
  const result = Children.toArray(children);
  result.reverse();
  // ...

参数

¥Parameters

• children：你的组件接收的 children prop 值。

  ¥children: The value of the children prop received by your component.

返回

¥Returns

返回 children 中的元素的扁平数组。

¥Returns a flat array of elements in children.

注意事项

¥Caveats

• 返回的数组中将省略空节点（null、undefined 和布尔值）。返回元素的 ‘键将根据原始元素计算得出’ 键及其嵌套级别和位置。这确保展平数组不会引起行为变化。

  ¥Empty nodes (null, undefined, and Booleans) will be omitted in the returned array. The returned elements’ keys will be calculated from the original elements’ keys and their level of nesting and position. This ensures that flattening the array does not introduce changes in behavior.

用法

¥Usage

子组件变形

¥Transforming children

要转换组件 作为 children 属性接收， 调用 Children.map 的子 JSX：

¥To transform the children JSX that your component receives as the children prop, call Children.map:

import { Children } from 'react';

function RowList({ children }) {
  return (
    <div className="RowList">
      {Children.map(children, child =>
        <div className="Row">
          {child}
        </div>
      )}
    </div>
  );
}

在上面的例子中，RowList 将其接收的每个子组件封装到 <div className="Row"> 容器中。例如，假设父组件将三个 <p> 标签作为 children 属性传递给 RowList：

¥In the example above, the RowList wraps every child it receives into a <div className="Row"> container. For example, let’s say the parent component passes three <p> tags as the children prop to RowList:

<RowList>
  <p>This is the first item.</p>
  <p>This is the second item.</p>
  <p>This is the third item.</p>
</RowList>

然后，使用上面的 RowList 实现，最终渲染结果将如下所示：

¥Then, with the RowList implementation above, the final rendered result will look like this:

<div className="RowList">
  <div className="Row">
    <p>This is the first item.</p>
  </div>
  <div className="Row">
    <p>This is the second item.</p>
  </div>
  <div className="Row">
    <p>This is the third item.</p>
  </div>
</div>

Children.map 与 使用 map() 转换数组。 类似，区别在于 children 数据结构被视为不透明。这意味着即使它有时是一个数组，你也不应假设它是数组或任何其他特定数据类型。这就是为什么如果需要转换它，应该使用 Children.map。

¥Children.map is similar to to transforming arrays with map(). The difference is that the children data structure is considered opaque. This means that even if it’s sometimes an array, you should not assume it’s an array or any other particular data type. This is why you should use Children.map if you need to transform it.

import RowList from './RowList.js';

export default function App() {
  return (
    <RowList>
      <p>This is the first item.</p>
      <MoreRows />
    </RowList>
  );
}

function MoreRows() {
  return (
    <>
      <p>This is the second item.</p>
      <p>This is the third item.</p>
    </>
  );
}

为每个子组件运行部分代码

¥Running some code for each child

调用 Children.forEach 迭代 children 数据结构中的每个子组件。它不返回任何值，类似于 数组 forEach 方法。。你可以使用它来运行自定义逻辑，例如构建你自己的数组。

¥Call Children.forEach to iterate over each child in the children data structure. It does not return any value and is similar to the array forEach method. You can use it to run custom logic like constructing your own array.

计数子元素

¥Counting children

调用 Children.count(children) 来计算子组件的数量。

¥Call Children.count(children) to calculate the number of children.

将子元素转换为数组

¥Converting children to an array

调用 Children.toArray(children) 将 children 数据结构转换为常规 JavaScript 数组。这允许你使用内置数组方法（例如 filter、sort 或 reverse。）来操作数组。

¥Call Children.toArray(children) to turn the children data structure into a regular JavaScript array. This lets you manipulate the array with built-in array methods like filter, sort, or reverse.

备选方案

¥Alternatives

import { Children } from 'react';

公开多个组件

¥Exposing multiple components

使用 Children 方法操作子组件通常会导致代码脆弱。当你在 JSX 中将子组件传递给组件时，你通常不会期望组件操作或转换单个子组件。

¥Manipulating children with the Children methods often leads to fragile code. When you pass children to a component in JSX, you don’t usually expect the component to manipulate or transform the individual children.

如果可以，请尽量避免使用 Children 方法。例如，如果你想要将 RowList 的每个子组件都封装在 <div className="Row"> 中，可以导出一个 Row 组件，并手动将每一行封装到其中，如下所示：

¥When you can, try to avoid using the Children methods. For example, if you want every child of RowList to be wrapped in <div className="Row">, export a Row component, and manually wrap every row into it like this:

与使用 Children.map 不同，此方法不会自动封装每个子组件。然而，与 之前使用 Children.map 的示例 相比，这种方法有一个显著的优势，因为即使你不断提取更多组件，它也能正常工作。例如，即使你提取自己的 MoreRows 组件，它仍然有效：

¥Unlike using Children.map, this approach does not wrap every child automatically. However, this approach has a significant benefit compared to the earlier example with Children.map because it works even if you keep extracting more components. For example, it still works if you extract your own MoreRows component:

这不适用于 Children.map，因为它会将 “看到” 和 <MoreRows /> 视为单个子元素（和单个行）。

¥This wouldn’t work with Children.map because it would “see” <MoreRows /> as a single child (and a single row).

接受对象数组作为 prop

¥Accepting an array of objects as a prop

你还可以显式传递数组作为 prop。例如，此 RowList 接受 rows 数组作为 prop：

¥You can also explicitly pass an array as a prop. For example, this RowList accepts a rows array as a prop:

由于 rows 是一个常规的 JavaScript 数组，因此 RowList 组件可以对其使用内置数组方法，例如 map。

¥Since rows is a regular JavaScript array, the RowList component can use built-in array methods like map on it.

当你希望能够将更多信息作为结构化数据与子元素一起传递时，此模式尤其有用。在下面的例子中，TabSwitcher 组件接收一个对象数组作为 tabs 属性：

¥This pattern is especially useful when you want to be able to pass more information as structured data together with children. In the below example, the TabSwitcher component receives an array of objects as the tabs prop:

与将子组件作为 JSX 传递不同，此方法允许你将一些额外的数据（例如 header）与每个组件关联。由于你直接使用 tabs，并且它是一个数组，因此你不需要 Children 方法。

¥Unlike passing the children as JSX, this approach lets you associate some extra data like header with each item. Because you are working with the tabs directly, and it is an array, you do not need the Children methods.

调用渲染属性自定义渲染

¥Calling a render prop to customize rendering

除了为每个项目生成 JSX 代码之外，你还可以传递一个返回 JSX 代码的函数，并在必要时调用该函数。在此示例中，App 组件将 renderContent 函数传递给 TabSwitcher 组件。TabSwitcher 组件仅为选定的选项卡调用 renderContent：

¥Instead of producing JSX for every single item, you can also pass a function that returns JSX, and call that function when necessary. In this example, the App component passes a renderContent function to the TabSwitcher component. The TabSwitcher component calls renderContent only for the selected tab:

像 renderContent 这样的属性被称为渲染 prop，因为它指定了如何渲染用户界面的一部分。然而，这并没有什么特别之处：它是一个常规的 prop，恰好是一个函数。

¥A prop like renderContent is called a render prop because it is a prop that specifies how to render a piece of the user interface. However, there is nothing special about it: it is a regular prop which happens to be a function.

渲染属性是函数，因此你可以向其传递信息。例如，此 RowList 组件将每行的 id 和 index 传递给 renderRow 渲染 prop，后者使用 index 高亮偶数行：

¥Render props are functions, so you can pass information to them. For example, this RowList component passes the id and the index of each row to the renderRow render prop, which uses index to highlight even rows:

这是另一个父子组件如何在不操作子组件的情况下进行协作的示例。

¥This is another example of how parent and child components can cooperate without manipulating the children.

故障排除

¥Troubleshooting

我传递了一个自定义组件，但 Children 方法没有显示其渲染结果

¥I pass a custom component, but the Children methods don’t show its render result

假设你将两个子元素传递给 RowList，如下所示：

¥Suppose you pass two children to RowList like this:

<RowList>
  <p>First item</p>
  <MoreRows />
</RowList>

如果在 RowList 中执行 Children.count(children)，则会得到 2。即使 MoreRows 渲染了 10 个不同的项目，或者它返回了 null，Children.count(children) 仍然会是 2。从 RowList 的角度来看，它只会 “看到” 接收到的 JSX。它不会 “看到” MoreRows 组件的内部。

¥If you do Children.count(children) inside RowList, you will get 2. Even if MoreRows renders 10 different items, or if it returns null, Children.count(children) will still be 2. From the RowList’s perspective, it only “sees” the JSX it has received. It does not “see” the internals of the MoreRows component.

此限制使得提取组件变得困难。这就是为什么 alternatives 优于 Children 的原因。

¥The limitation makes it hard to extract a component. This is why alternatives are preferred to using Children.