React中的Context应用场景分析

const MyContext = createContex(defaultValue);

<MyContext.Provider value={}>

const value = useContext(MyContext);

<MyContext.Consumer>
  {value => }
</MyContext.Consumer>

//App.js

// ....

export default function App() {
    //...
    
    // 
    const {contextValue, setContextValue} = React.useState(initialValue);

    // function to update the context value
    function updateContext(newValue) {
        // ...

        // 更新contextValue, ConsumerComponent1, ConsumerComponent2, ConsumerComponent3, ConsumerComponent11都会触发重新渲染。
        setContextValue(newValue)
    }

    ...
    return (
        <App>
            <MyContext.Provider value={contextValue}>
                <ConsumerComponent1>
                    <ConsumerComponent11>
    					// ....
                    </ComsumerComponent11>
                </ConsumerComponent1>

                <ConsumerComponent2 />
                <ConsumerComponent3 />
            </MyContext.Provider>
        </App>
    );
    
}

// app.js

export default function App() {
    ...
    const {contextValue, setContextValue} = React.useState(initialValue);

    // function to update the context value
    function updateContext(newValue) {
        // ...

        // 更新contextValue, ConsumerComponent1, ConsumerComponent2, ConsumerComponent3, ConsumerComponent11都会触发重新渲染。
        setContextValue(newValue)
    }

    ...
    return (
        <App>
            <MyContext.Provider value={contextValue}>
                <ConsumerComponent1>
                    <ConsumerComponent11 updateValue={updateContext}> // 通过回调形式的props, 在ConsumerComponent11中更新contextValue, 因为contextValue属于最顶层的Provider的值，所以也会触发ConsumerComponent1, ConsumerComponent2, ConsumerComponent3重新渲染。
                    </ComsumerComponent11>
                </ConsumerComponent1>

                <ConsumerComponent2 />
                <ConsumerComponent3 />
            </MyContext.Provider>
        </App>
    );
}

...
const {contextValue, setContextValue} = React.useState(initialValue);

// function to update the context value
function updateContext(newValue) {
    // ...
    
    // 更新contextValue, ConsumerComponent1, ConsumerComponent2, ConsumerComponent3, ConsumerComponent11都会触发重新渲染。
    setContextValue(newValue)
}

...
return (
	<App>
        <MyContext.Provider value={contextValue}>
            <ConsumerComponent1>
                <ConsumerComponent11 />
            </ConsumerComponent1>

            <ConsumerComponent2>
                ...
                // 如果只希望更新ComsumerComponent21, ComsumerComponent22中的值
                
                const localContextValue = useContext(MyContext); // 从上一层Provider中获取当前值

				const {tempContextValue, setTempContextValue} = React.useState(localContextValue);

				function updateTempContext(newValue) {
                    // 这里更新以后只会触发ConsumerComponent21和ConsumerComponent22的重新渲染
                    setTempContextValue(newValue); 
                }

				// 这里新建Provider，在ConsumerComponent21和ConsumerComponent22之间共享数据。
                <MyContext.Provider value={tempValue}>
                    <ConsumerComponent21>
                    	// 在ConsumerComponent21中通过useContext(MyContext)订阅
                    	// 获取到的值为离自身最近的那个匹配的Provider中读取到的Context值,即tempValue
                    </ConsumerComponent21>
                    <ConsumerComponent22>
                    </ConsumerComponent22>
				</MyContext.Provider value={contextValue}>
            </ConsumerComponent2>
            <ConsumerComponent3 />
        </MyContext.Provider>
    </App>
);