渲染和提交 | Render and Commit

Imagine that your components are cooks in the kitchen, assembling tasty dishes from ingredients. In this scenario, React is the waiter who puts in requests from customers and brings them their orders. This process of requesting and serving UI has three steps: 想象一下，你的组件是厨房里的厨师，把食材烹制成美味的菜肴。在这种场景下，React 就是一名服务员，他会帮客户们下单并为他们送来所点的菜品。这种请求和提供 UI 的过程总共包括三个步骤：

1. Triggering a render (delivering the guest’s order to the kitchen)
2. 触发 一次渲染（把客人的点单分发到厨房）
3. Rendering the component (preparing the order in the kitchen)
4. 渲染 组件（在厨房准备订单）
5. Committing to the DOM (placing the order on the table)
6. 提交 到 DOM（将菜品放在桌子上）

步骤 1: 触发一次渲染 | Step 1: Trigger a render

There are two reasons for a component to render: 有两种原因会导致组件的渲染:

1. It’s the component’s initial render.
2. 组件的 初次渲染。
3. The component’s (or one of its ancestors’) state has been updated.
4. 组件（或者其祖先之一）的 状态发生了改变。

初次渲染 | Initial render

When your app starts, you need to trigger the initial render. Frameworks and sandboxes sometimes hide this code, but it’s done by calling createRoot with the target DOM node, and then calling its render method with your component: 当应用启动时，会触发初次渲染。框架和沙箱有时会隐藏这部分代码，但它是通过调用 createRoot 方法并传入目标 DOM 节点，然后用你的组件调用 render 函数完成的：

import Image from './Image.js';
import { createRoot } from 'react-dom/client';

const root = createRoot(document.getElementById('root'))
root.render(<Image />);

Try commenting out the root.render() call and see the component disappear! 试着注释掉 root.render()，然后你将会看到组件消失。

状态更新时重新渲染 | Re-renders when state updates

Once the component has been initially rendered, you can trigger further renders by updating its state with the set function. Updating your component’s state automatically queues a render. (You can imagine these as a restaurant guest ordering tea, dessert, and all sorts of things after putting in their first order, depending on the state of their thirst or hunger.) 一旦组件被初次渲染，你就可以通过使用 set 函数 更新其状态来触发之后的渲染。更新组件的状态会自动将一次渲染送入队列。（你可以把这种情况想象成餐厅客人在第一次下单之后又点了茶、点心和各种东西，具体取决于他们的胃口。）

步骤 2: React 渲染你的组件 | Step 2: React renders your components

After you trigger a render, React calls your components to figure out what to display on screen. “Rendering” is React calling your components. 在你触发渲染后，React 会调用你的组件来确定要在屏幕上显示的内容。“渲染中” 即 React 在调用你的组件。

• On initial render, React will call the root component.
• 在进行初次渲染时, React 会调用根组件。
• For subsequent renders, React will call the function component whose state update triggered the render.
• 对于后续的渲染, React 会调用内部状态更新触发了渲染的函数组件。

This process is recursive: if the updated component returns some other component, React will render that component next, and if that component also returns something, it will render that component next, and so on. The process will continue until there are no more nested components and React knows exactly what should be displayed on screen. 这个过程是递归的：如果更新后的组件会返回某个另外的组件，那么 React 接下来就会渲染 那个 组件，而如果那个组件又返回了某个组件，那么 React 接下来就会渲染 那个 组件，以此类推。这个过程会持续下去，直到没有更多的嵌套组件并且 React 确切知道哪些东西应该显示到屏幕上为止。

In the following example, React will call Gallery() and Image() several times: 在接下来的例子中，React 将会调用 Gallery() 和 Image() 若干次：

export default function Gallery() {
  return (
    <section>
      <h1>鼓舞人心的雕塑</h1>
      <Image />
      <Image />
      <Image />
    </section>
  );
}

function Image() {
  return (
    <img
      src="https://i.imgur.com/ZF6s192.jpg"
      alt="'Floralis Genérica' by Eduardo Catalano: a gigantic metallic flower sculpture with reflective petals"
    />
  );
}

• During the initial render, React will create the DOM nodes for <section>, <h1>, and three <img> tags.
• 在初次渲染中， React 将会为<section>、<h1> 和三个 <img> 标签 创建 DOM 节点。
• During a re-render, React will calculate which of their properties, if any, have changed since the previous render. It won’t do anything with that information until the next step, the commit phase.
• 在一次重渲染过程中, React 将计算它们的哪些属性（如果有的话）自上次渲染以来已更改。在下一步（提交阶段）之前，它不会对这些信息执行任何操作。

深入探讨

性能优化 | Optimizing performance

显示更多

The default behavior of rendering all components nested within the updated component is not optimal for performance if the updated component is very high in the tree. If you run into a performance issue, there are several opt-in ways to solve it described in the Performance section. Don’t optimize prematurely! 如果更新的组件在树中的位置非常高，渲染更新后的组件内部所有嵌套组件的默认行为将不会获得最佳性能。如果你遇到了性能问题，性能 章节描述了几种可选的解决方案 。不要过早进行优化！

步骤 3: React 把更改提交到 DOM 上 | Step 3: React commits changes to the DOM

After rendering (calling) your components, React will modify the DOM. 在渲染（调用）你的组件之后，React 将会修改 DOM。

• For the initial render, React will use the appendChild() DOM API to put all the DOM nodes it has created on screen.
• 对于初次渲染， React 会使用 appendChild() DOM API 将其创建的所有 DOM 节点放在屏幕上。
• For re-renders, React will apply the minimal necessary operations (calculated while rendering!) to make the DOM match the latest rendering output.
• 对于重渲染， React 将应用最少的必要操作（在渲染时计算！），以使得 DOM 与最新的渲染输出相互匹配。

React only changes the DOM nodes if there’s a difference between renders. For example, here is a component that re-renders with different props passed from its parent every second. Notice how you can add some text into the <input>, updating its value, but the text doesn’t disappear when the component re-renders: React 仅在渲染之间存在差异时才会更改 DOM 节点。 例如，有一个组件，它每秒使用从父组件传递下来的不同属性重新渲染一次。注意，你可以添加一些文本到 <input> 标签，更新它的 value，但是文本不会在组件重渲染时消失：

export default function Clock({ time }) {
  return (
    <>
      <h1>{time}</h1>
      <input />
    </>
  );
}

This works because during this last step, React only updates the content of <h1> with the new time. It sees that the <input> appears in the JSX in the same place as last time, so React doesn’t touch the <input>—or its value! 这个例子之所以会正常运行，是因为在最后一步中，React 只会使用最新的 time 更新 <h1> 标签的内容。它看到 <input> 标签出现在 JSX 中与上次相同的位置，因此 React 不会修改 <input> 标签或它的 value！

尾声：浏览器绘制 | Epilogue: Browser paint

After rendering is done and React updated the DOM, the browser will repaint the screen. Although this process is known as “browser rendering”, we’ll refer to it as “painting” to avoid confusion throughout the docs. 在渲染完成并且 React 更新 DOM 之后，浏览器就会重新绘制屏幕。尽管这个过程被称为“浏览器渲染”（“browser rendering”），但我们还是将它称为“绘制”（“painting”），以避免在这些文档的其余部分中出现混淆。

摘要

• Any screen update in a React app happens in three steps:
• 在一个 React 应用中一次屏幕更新都会发生以下三个步骤：
  1. Trigger
  • 触发
  2. Render
  • 渲染
  3. Commit
  • 提交
• You can use Strict Mode to find mistakes in your components
• 你可以使用严格模式去找到组件中的错误
• React does not touch the DOM if the rendering result is the same as last time
• 如果渲染结果与上次一样，那么 React 将不会修改 DOM