RootTag

RootTag 是用于标记 React Native 原生根视图层的不透明标识符（opaque identifier）。具体来说就是 ReactRootView(android) 或是 RCTRootView(iOS) 的实例 ID.

什么时候使用 RootTag?

对于绝大多数 React Native 开发者来说，一般你并不需要用到RootTag。

RootTags are useful for when an app renders multiple React Native root views and you need to handle native API calls differently depending on the surface. An example of this is when an app is using native navigation and each screen is a separate React Native root view.

In native navigation, every React Native root view is rendered in a platform’s navigation view (e.g., Activity for Android, UINavigationViewController for iOS). By this, you are able to leverage the navigation paradigms of the platform such as native look and feel and navigation transitions. The functionality to interact with the native navigation APIs can be exposed to React Native via a native module.

For example, to update the title bar of a screen, you would call the navigation module’s API setTitle("Updated Title"), but it would need to know which screen in the stack to update. A RootTag is necessary here to identify the root view and its hosting container.

Another use case for RootTag is when your app needs to attribute a certain JavaScript call to native based on its originating root view. A RootTag is necessary to differentiate the source of the call from different surfaces.

如何使用 RootTag （如果你确实要用的话）

In versions 0.65 and below, RootTag is accessed via a legacy context. To prepare React Native for Concurrent features coming in React 18 and beyond, we are migrating to the latest Context API via RootTagContext in 0.66. Version 0.65 supports both the legacy context and the recommended RootTagContext to allow developers time to migrate their call-sites. See the breaking changes summary.

How to access RootTag via the RootTagContext.

import { RootTagContext } from 'react-native';
import NativeAnalytics from 'native-analytics';
import NativeNavigation from 'native-navigation';

function ScreenA() {
  const rootTag = useContext(RootTagContext);

  const updateTitle = (title) => {
    NativeNavigation.setTitle(rootTag, title);
  };

  const handleOneEvent = () => {
    NativeAnalytics.logEvent(rootTag, 'one_event');
  };

  // ...
}

class ScreenB extends React.Component {
  static contextType: typeof RootTagContext = RootTagContext;

  updateTitle(title) {
    NativeNavigation.setTitle(this.context, title);
  }

  handleOneEvent() {
    NativeAnalytics.logEvent(this.context, 'one_event');
  }

  // ...
}

Learn more about the Context API for classes and hooks from the React docs.

0.65 版本中的不兼容变更

RootTagContext was formerly named unstable_RootTagContext and changed to RootTagContext in 0.65. Please update any usages of unstable_RootTagContext in your codebase.

0.66 版本中的不兼容变更

The legacy context access to RootTag will be removed and replaced by RootTagContext. Beginning in 0.65, we encourage developers to proactively migrate RootTag accesses to RootTagContext.

未来的计划

With the new React Native architecture progressing, there will be future iterations to RootTag, with the intention to keep the RootTag type opaque and prevent thrash in React Native codebases. Please do not rely on the fact that RootTag currently aliases to a number! If your app relies on RootTags, keep an eye on our version change logs, which you can find here.