useContext 和 useReducer

useContext 和 useReducer 传递state + dispatch, 模拟redux

useContext 用法

// App.tsx
const UserContext = React.createContext({ name: '' })
function App() {return (<UserContext.Provider value={{ name: 'jack' }}><div><p>欢迎学习React后台课程</p><Child1 /></div></UserContext.Provider>)
}function Child1() {return (<div><p><span>Child1</span></p><p><Child2 /></p></div>)
}function Child2() {const { name } = useContext(UserContext)return <span>Child2{name}</span>
}

用 createContext 创建上下文对象，传递全局变量，在底层任意一个子组件可使用 useContext 获取该上下文对象，同时获取里面定义的全局变量值，进行渲染。

解决问题

当组件嵌套层级过多时，传递属性不方便。

useReducer 用法

const [state, dispatch] = useReducer(reducer, initialArg);

useReducer 案例

const initialState = {count: 0};function reducer(state, action) {switch (action.type) {case 'increment':return {count: state.count + 1};case 'decrement':return {count: state.count - 1};default:throw new Error();}
}function Counter() {const [state, dispatch] = useReducer(reducer, initialState);return (<>Count: {state.count}<button onClick={() => dispatch({type: 'decrement'})}>-</button><button onClick={() => dispatch({type: 'increment'})}>+</button></>);
}