1. Context - React跨组件访问数据的利器

6,318 阅读6分钟

《react-router-dom源码揭秘》系列

2. React-Router的基本使用
3. react-router-dom源码揭秘 - BrowserRouter

Context提供了一种跨组件访问数据的方法。它无需在组件树间逐层传递属性,也可以方便的访问其他组件的数据

在经典的React应用中,数据是父组件通过props向子组件传递的。但是在某些特定场合,有些数据需要在各个组件之间共享。Context为我们提供一种组件之间共享数据的方式,可以避免数据在组件树上逐层传递

使用Context的场合

Context可以在组件树的组件之间共享“全局”数据。例如:登陆的用户信息,用户选择的主题、语言等等。下面的例子中,我们“手动”自上而下传递theme属性,用来设定Button的样式。

class App extends React.Component {
  render() {
    return <Toolbar theme="dark"></Toolbar>;
  }
}

function Toolbar(props) {
  // The Toolbar component must take an extra "theme" prop
  // and pass it to the ThemedButton. This can become painful
  // if every single button in the app needs to know the theme
  // because it would have to be passed through all components.
  return (
    <div>
      <ThemedButton theme={props.theme}></ThemedButton>
    </div>
  );
}

class ThemedButton extends React.Component {
  render() {
    return <Button theme={this.props.theme}></Button>;
  }
}

使用Context,我们可以避免通过多个中间组件传递props

// Context lets us pass a value deep into the component tree
// without explicitly threading it through every component.
// Create a context for the current theme (with "light" as the default).
const ThemeContext = React.createContext('light');

class App extends React.Component {
  render() {
    // Use a Provider to pass the current theme to the tree below.
    // Any component can read it, no matter how deep it is.
    // In this example, we're passing "dark" as the current value.
    return (
      <ThemeContext.Provider value="dark">
        <Toolbar></Toolbar>
      </ThemeContext.Provider>
    );
  }
}

// A component in the middle doesn't have to
// pass the theme down explicitly anymore.
function Toolbar(props) {
  return (
    <div>
      <ThemedButton />
    </div>
  );
}

class ThemedButton extends React.Component {
  // Assign a contextType to read the current theme context.
  // React will find the closest theme Provider above and use its value.
  // In this example, the current theme is "dark".
  static contextType = ThemeContext;
  render() {
    return <Button theme={this.context} />;
  }
}

有时候,有些数据需要被很多组件访问,而且这些组件在组件树的不同层上。Context可以使我们以“广播”的形式,在各个组件中共享数据的改变

Context相关API

React.createContext

const MyContext = React.createContext(defaultValue);

创建一个新的Context对象。当React渲染一个组件,且该组件注册了Context时,它将读取父组件中,距离该组件最近的Provider组件的Context

defaultValue只有在“Consumer”组件找不到Provider组件时,才会被使用。

Context.Provider

<MyContext.Provider value={/* some value */}>

每个Context对象都携带一个名叫Provider的React组件。Provider可以使得“Consumer”组件监听context的变更

通过向Provider的后代Consumer组件传递value的prop,一个Provider可以与多个Consumer组件建立联系。

所有的后代Consumer组件在Provider的value属性更新后,都会被重新渲染。这个更新从Provider到其后代Consumer组件之间传播,但是并不会触发shouldComponentUpdate方法。所以即使Consumer组件的祖先组件没有更新,Consumer组件也会更新

Context使用与Object.is相同的算法来对比value的新、旧值,以判定其value是否被更新了

注意

当向value传递对象时,这种判定value是否改变的方式可能会引起问题。请参加Caveats.

Class.contextType

class MyClass extends React.Component {
  componentDidMount() {
    let value = this.context;
    /* perform a side-effect at mount using the value of MyContext */
  }
  componentDidUpdate() {
    let value = this.context;
    /* ... */
  }
  componentWillUnmount() {
    let value = this.context;
    /* ... */
  }
  render() {
    let value = this.context;
    /* render something based on the value of MyContext */
  }
}
MyClass.contextType = MyContext;

为class的contextTpe属性赋值一个Context对象后,我们可以通过this.context在组件的各个声明周期函数中获取到当前的Context对象的方法

注意:

通过这种方式,每个组件只能注册一个context对象。如果需要读取多个context的value值,参加Consuming Multiple Contexts.

如果编码中使用了ES实验中的语法,那么可以使用类的静态(static)成员来初始化contextTYpe.代码如下:

class MyClass extends React.Component {
 static contextType = MyContext;
 render() {
   let value = this.context;
   /* render something based on the value */
 }
}

Context.Consumer

<MyContext.Consumer>
  {value => /* render something based on the context value */}
</MyContext.Consumer>

Consumer是一个监听context变化的React组件。它使得我们可以在一个函数组件中,监听contxt的改变。

Consumer组件要求其子元素为一个函数。该函数的参数接收当前的context的value值,要求返回一个React节点(node) 传递给该函数的参数value等于距离此Consumner最近的外层Provider组件的context值。如果没有外层的Provider组件,则等于调用createContext()时传递的参数值(context的默认值)。

注意

更多关于“子元素为一个函数”的信息,请参加render props

栗子

在嵌套组件中更新Context

开发中,我们经常需要在某些嵌套结构很深的组件上更新context的value值。此时,我们可以向下传递一个函数,用它来更新context的value。代码如下:

theme-context.js

// Make sure the shape of the default value passed to
// createContext matches the shape that the consumers expect!
export const ThemeContext = React.createContext({
  theme: themes.dark,
  toggleTheme: () => {},
});

theme-toggler-button.js

import {ThemeContext} from './theme-context';

function ThemeTogglerButton() {
  // The Theme Toggler Button receives not only the theme
  // but also a toggleTheme function from the context
  return (
    <ThemeContext.Consumer>
      {({theme, toggleTheme}) => (
        <button
          onClick={toggleTheme}
          style={{backgroundColor: theme.background}}>
          Toggle Theme
        </button>
      )}
    </ThemeContext.Consumer>
  );
}

export default ThemeTogglerButton;

app.js

import {ThemeContext, themes} from './theme-context';
import ThemeTogglerButton from './theme-toggler-button';

class App extends React.Component {
  constructor(props) {
    super(props);

    this.toggleTheme = () => {
      this.setState(state => ({
        theme:
          state.theme === themes.dark
            ? themes.light
            : themes.dark,
      }));
    };

    // State also contains the updater function so it will
    // be passed down into the context provider
    this.state = {
      theme: themes.light,
      toggleTheme: this.toggleTheme,
    };
  }

  render() {
    // The entire state is passed to the provider
    return (
      <ThemeContext.Provider value={this.state}>
        <Content />
      </ThemeContext.Provider>
    );
  }
}

function Content() {
  return (
    <div>
      <ThemeTogglerButton />
    </div>
  );
}

ReactDOM.render(<App />, document.root);

使用多个Contexts

为了保持React的快速渲染,我们需要将每个consumer组件编写成一个独立的组件节点(node)

// Theme context, default to light theme
const ThemeContext = React.createContext('light');

// Signed-in user context
const UserContext = React.createContext({
  name: 'Guest',
});

class App extends React.Component {
  render() {
    const {signedInUser, theme} = this.props;

    // App component that provides initial context values
    return (
      <ThemeContext.Provider value={theme}>
        <UserContext.Provider value={signedInUser}>
          <Layout />
        </UserContext.Provider>
      </ThemeContext.Provider>
    );
  }
}

function Layout() {
  return (
    <div>
      <Sidebar />
      <Content />
    </div>
  );
}

// A component may consume multiple contexts
function Content() {
  return (
    <ThemeContext.Consumer>
      {theme => (
        <UserContext.Consumer>
          {user => (
            <ProfilePage user={user} theme={theme} />
          )}
        </UserContext.Consumer>
      )}
    </ThemeContext.Consumer>
  );
}

如果有两个以上的context经常一起使用,我们需要考虑创建一个render prop component一并提供两个Context

注意

因为context使用引用标示符(reference identity)来判断何时需要重新渲染,所以有些情况下,当provider的父元素重新渲染时,会触发consumer的非内部渲染。例如下面代码,在每次Provider重新渲染时,会重新渲染所有的consumer组件。因为会一直创建一个新的对象赋值给value(value一直在变)

class App extends React.Component {
  render() {
    return (
      <Provider value={{something: 'something'}}>
        <Toolbar />
      </Provider>
    );
  }
}

为了避免这个问题,可以将value放在组件的state中

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: {something: 'something'},
    };
  }

  render() {
    return (
      <Provider value={this.state.value}>
        <Toolbar />
      </Provider>
    );
  }
}