反应多个上下文

Question

我正在使用通过上下文传递的函数。

ChildComponent.contextType = SomeContext;

现在，我使用this.context.someFunction();。这行得通。

如果我需要来自两个不同父级组件的函数怎么办？

Answer 1

您仍可以通过16.3上下文API使用子级功能消费者节点，这就是React documentation suggests doing：

// 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>
  );
}

要在组件的上下文中使用函数，通常将组件包装在HOC中，以便将上下文作为props传递：

export const withThemeContext = Component => (
  props => (
    <ThemeContext.Consumer>
      {context => <Component themeContext={context} {...props} />}
    </ThemeContext.Consumer>
  )
)

const YourComponent = ({ themeContext, ...props }) => {
  themeContext.someFunction()
  return (<div>Hi Mom!</div>)
}

export default withThemeContext(YourComponent)

Answer 2

另一种解决方案是创建一个单独的上下文，以提供其他上下文：

import React, { createContext, memo, useContext } from "react";
import isEqual from "react-fast-compare";

export const MultiContext = createContext(null);
MultiContext.displayName = "MultiContext";

export const MultiContextProvider = memo(
  function({ map, children }) {
    const contextMap = {};
    for (const i in map) {
      contextMap[i] = useContext(map[i]);
    }

    return (
      <MultiContext.Provider value={contextMap}>
        {children}
      </MultiContext.Provider>
    );
  },
  (prevProps, nextProps) => isEqual(prevProps.children, nextProps.children)
);

MultiContextProvider.displayName = "MultiContextProvider";

用法示例：

class DemoConsumer extends React.Component {
  static contextType = MultiContext;

  render() {
    return JSON.stringify({
      someValue: this.context.SomeContext.someValue,
      otherValue: this.context.OtherContext.otherValue,
    });
  }
}

function App() {
  return (
    <MultiContextProvider map={{ SomeContext, OtherContext }}>
      <MultiContextDemoClassConsumer />
    </MultiContextProvider>
  );
}

---

演示：

const {
  createContext,
  memo,
  useContext,
  useState,
  useEffect,
} = React;

const MultiContext = createContext(null);
MultiContext.displayName = "MultiContext";

const MultiContextProvider = memo(
  function({ map, children }) {
    console.log("render provider");
    const contextMap = {};
    for (const i in map) {
      contextMap[i] = useContext(map[i]);
    }

    return (
      <MultiContext.Provider value={contextMap}>
        {children}
      </MultiContext.Provider>
    );
  },
  (prevProps, nextProps) => isEqual(prevProps.children, nextProps.children)
);
MultiContextProvider.displayName = "MultiContextProvider";

const initialMinutes = new Date().getMinutes();
const MinutesContext = createContext(initialMinutes);
MinutesContext.displayName = "MinutesContext";

const IncrementContext = createContext(0);
IncrementContext.displayName = "IncrementContext";

class MultiContextDemoClassConsumer extends React.Component {
  static contextType = MultiContext;

  render() {
    return JSON.stringify(this.context);
  }
}

const multiContextMap = { MinutesContext, IncrementContext };
function App() {
  const forceUpdate = useForceUpdate();

  const [minutes, setMinutes] = useState(initialMinutes);
  useEffect(() => {
    const timeoutId = setInterval(() => {
      // console.log('set minutes')
      setMinutes(new Date().getMinutes());
    }, 1000);
    return () => {
      clearInterval(timeoutId);
    };
  }, [setMinutes]);

  const [increment, setIncrement] = useState(0);

  console.log("render app");

  return (
    <MinutesContext.Provider value={minutes}>
      <IncrementContext.Provider value={increment}>
        <MultiContextProvider map={multiContextMap}>
          <MultiContextDemoClassConsumer />
        </MultiContextProvider>
        <button onClick={() => setIncrement(i => i + 1)}>Increment</button>
        <button onClick={forceUpdate}>Force Update</button>
      </IncrementContext.Provider>
    </MinutesContext.Provider>
  );
}

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

<script type="module">
  import React from 'https://dev.jspm.io/react@16';
  import ReactDOM from 'https://dev.jspm.io/react-dom@16';
  import useForceUpdate from 'https://dev.jspm.io/use-force-update@1.0.7';
  import isEqual from 'https://dev.jspm.io/react-fast-compare@3.0.1';
  window.React = React;
  window.ReactDOM = ReactDOM;
  window.useForceUpdate = useForceUpdate.default;
  window.isEqual = isEqual;
</script>
<div id="root"></div>

Answer 3

您还可以将所有上下文简单地合并为一个上下文：

const AppContext = React.createContext({
  user: { name: 'Guest' },
  theme: 'light',
})

ChildComponent.contextType = AppContext;

完成。如果您在应用的某些部分具有不同的上下文（例如，不同的主题或用户），则只需合并新值即可。