keep reducers pure 是一个很好的做法。它将使useReducer 更具可预测性并简化可测试性。随后的方法都将异步操作与纯 reducer 结合起来：
1。取dispatch之前的数据（简单）
用asyncDispatch包裹原来的dispatch，让上下文向下传递这个函数：
const AppContextProvider = ({ children }) => {
const [state, dispatch] = useReducer(reducer, initState);
const asyncDispatch = () => { // adjust args to your needs
dispatch({ type: "loading" });
fetchData().then(data => {
dispatch({ type: "finished", payload: data });
});
};
return (
<AppContext.Provider value={{ state, dispatch: asyncDispatch }}>
{children}
</AppContext.Provider>
);
// Note: memoize the context value, if Provider gets re-rendered more often
};
const reducer = (state, { type, payload }) => {
if (type === "loading") return { status: "loading" };
if (type === "finished") return { status: "finished", data: payload };
return state;
};
const initState = {
status: "idle"
};
const AppContext = React.createContext();
const AppContextProvider = ({ children }) => {
const [state, dispatch] = React.useReducer(reducer, initState);
const asyncDispatch = () => { // adjust args to your needs
dispatch({ type: "loading" });
fetchData().then(data => {
dispatch({ type: "finished", payload: data });
});
};
return (
<AppContext.Provider value={{ state, dispatch: asyncDispatch }}>
{children}
</AppContext.Provider>
);
};
function App() {
return (
<AppContextProvider>
<Child />
</AppContextProvider>
);
}
const Child = () => {
const val = React.useContext(AppContext);
const {
state: { status, data },
dispatch
} = val;
return (
<div>
<p>Status: {status}</p>
<p>Data: {data || "-"}</p>
<button onClick={dispatch}>Fetch data</button>
</div>
);
};
function fetchData() {
return new Promise(resolve => {
setTimeout(() => {
resolve(42);
}, 2000);
});
}
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js" integrity="sha256-32Gmw5rBDXyMjg/73FgpukoTZdMrxuYW7tj8adbN8z4=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js" integrity="sha256-bjQ42ac3EN0GqK40pC9gGi/YixvKyZ24qMP/9HiGW7w=" crossorigin="anonymous"></script>
<div id="root"></div>
2。为dispatch 使用中间件（通用）
dispatch 可以通过middlewares 增强，例如redux-thunk、redux-observable、redux-saga，以获得更大的灵活性和可重用性。或write your own 一个。
假设我们想要 1.) 使用 redux-thunk 获取异步数据 2.) 做一些日志记录 3.) 调用 dispatch 并得到最终结果。首先定义中间件：
import thunk from "redux-thunk";
const middlewares = [thunk, logger]; // logger is our own implementation
然后编写一个自定义的useMiddlewareReducer Hook，您可以在这里看到useReducer 捆绑了额外的中间件，类似于 Redux applyMiddleware：
const [state, dispatch] = useMiddlewareReducer(middlewares, reducer, initState);
中间件作为第一个参数传递，否则 API 与 useReducer 相同。对于实现，我们将 applyMiddleware source code 带到 React Hooks 中。
const middlewares = [ReduxThunk, logger];
const reducer = (state, { type, payload }) => {
if (type === "loading") return { ...state, status: "loading" };
if (type === "finished") return { status: "finished", data: payload };
return state;
};
const initState = {
status: "idle"
};
const AppContext = React.createContext();
const AppContextProvider = ({ children }) => {
const [state, dispatch] = useMiddlewareReducer(
middlewares,
reducer,
initState
);
return (
<AppContext.Provider value={{ state, dispatch }}>
{children}
</AppContext.Provider>
);
};
function App() {
return (
<AppContextProvider>
<Child />
</AppContextProvider>
);
}
const Child = () => {
const val = React.useContext(AppContext);
const {
state: { status, data },
dispatch
} = val;
return (
<div>
<p>Status: {status}</p>
<p>Data: {data || "-"}</p>
<button onClick={() => dispatch(fetchData())}>Fetch data</button>
</div>
);
};
function fetchData() {
return (dispatch, getState) => {
dispatch({ type: "loading" });
setTimeout(() => {
// fake async loading
dispatch({ type: "finished", payload: (getState().data || 0) + 42 });
}, 2000);
};
}
function logger({ getState }) {
return next => action => {
console.log("state:", JSON.stringify(getState()), "action:", JSON.stringify(action));
return next(action);
};
}
// same API as useReducer, with middlewares as first argument
function useMiddlewareReducer(
middlewares,
reducer,
initState,
initializer = s => s
) {
const [state, setState] = React.useState(initializer(initState));
const stateRef = React.useRef(state); // stores most recent state
const dispatch = React.useMemo(
() =>
enhanceDispatch({
getState: () => stateRef.current, // access most recent state
stateDispatch: action => {
stateRef.current = reducer(stateRef.current, action); // makes getState() possible
setState(stateRef.current); // trigger re-render
return action;
}
})(...middlewares),
[middlewares, reducer]
);
return [state, dispatch];
}
// | dispatch fn |
// A middleware has type (dispatch, getState) => nextMw => action => action
function enhanceDispatch({ getState, stateDispatch }) {
return (...middlewares) => {
let dispatch;
const middlewareAPI = {
getState,
dispatch: action => dispatch(action)
};
dispatch = middlewares
.map(m => m(middlewareAPI))
.reduceRight((next, mw) => mw(next), stateDispatch);
return dispatch;
};
}
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js" integrity="sha256-32Gmw5rBDXyMjg/73FgpukoTZdMrxuYW7tj8adbN8z4=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js" integrity="sha256-bjQ42ac3EN0GqK40pC9gGi/YixvKyZ24qMP/9HiGW7w=" crossorigin="anonymous"></script>
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/redux-thunk/2.3.0/redux-thunk.min.js" integrity="sha256-2xw5MpPcdu82/nmW2XQ6Ise9hKxziLWV2GupkS9knuw=" crossorigin="anonymous"></script>
<script>var ReduxThunk = window.ReduxThunk.default</script>
注意：我们将中间状态存储在 mutable refs - stateRef.current = reducer(...) 中，因此每个中间件都可以在使用 getState 调用时访问当前的最新状态。
要将确切 API 设为useReducer，您可以动态创建 Hook：
const useMiddlewareReducer = createUseMiddlewareReducer(middlewares); //init Hook
const MyComp = () => { // later on in several components
// ...
const [state, dispatch] = useMiddlewareReducer(reducer, initState);
}
const middlewares = [ReduxThunk, logger];
const reducer = (state, { type, payload }) => {
if (type === "loading") return { ...state, status: "loading" };
if (type === "finished") return { status: "finished", data: payload };
return state;
};
const initState = {
status: "idle"
};
const AppContext = React.createContext();
const useMiddlewareReducer = createUseMiddlewareReducer(middlewares);
const AppContextProvider = ({ children }) => {
const [state, dispatch] = useMiddlewareReducer(
reducer,
initState
);
return (
<AppContext.Provider value={{ state, dispatch }}>
{children}
</AppContext.Provider>
);
};
function App() {
return (
<AppContextProvider>
<Child />
</AppContextProvider>
);
}
const Child = () => {
const val = React.useContext(AppContext);
const {
state: { status, data },
dispatch
} = val;
return (
<div>
<p>Status: {status}</p>
<p>Data: {data || "-"}</p>
<button onClick={() => dispatch(fetchData())}>Fetch data</button>
</div>
);
};
function fetchData() {
return (dispatch, getState) => {
dispatch({ type: "loading" });
setTimeout(() => {
// fake async loading
dispatch({ type: "finished", payload: (getState().data || 0) + 42 });
}, 2000);
};
}
function logger({ getState }) {
return next => action => {
console.log("state:", JSON.stringify(getState()), "action:", JSON.stringify(action));
return next(action);
};
}
function createUseMiddlewareReducer(middlewares) {
return (reducer, initState, initializer = s => s) => {
const [state, setState] = React.useState(initializer(initState));
const stateRef = React.useRef(state); // stores most recent state
const dispatch = React.useMemo(
() =>
enhanceDispatch({
getState: () => stateRef.current, // access most recent state
stateDispatch: action => {
stateRef.current = reducer(stateRef.current, action); // makes getState() possible
setState(stateRef.current); // trigger re-render
return action;
}
})(...middlewares),
[middlewares, reducer]
);
return [state, dispatch];
}
}
// | dispatch fn |
// A middleware has type (dispatch, getState) => nextMw => action => action
function enhanceDispatch({ getState, stateDispatch }) {
return (...middlewares) => {
let dispatch;
const middlewareAPI = {
getState,
dispatch: action => dispatch(action)
};
dispatch = middlewares
.map(m => m(middlewareAPI))
.reduceRight((next, mw) => mw(next), stateDispatch);
return dispatch;
};
}
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js" integrity="sha256-32Gmw5rBDXyMjg/73FgpukoTZdMrxuYW7tj8adbN8z4=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js" integrity="sha256-bjQ42ac3EN0GqK40pC9gGi/YixvKyZ24qMP/9HiGW7w=" crossorigin="anonymous"></script>
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/redux-thunk/2.3.0/redux-thunk.min.js" integrity="sha256-2xw5MpPcdu82/nmW2XQ6Ise9hKxziLWV2GupkS9knuw=" crossorigin="anonymous"></script>
<script>var ReduxThunk = window.ReduxThunk.default</script>
更多信息 - 外部库： react-use、react-hooks-global-state、react-enhanced-reducer-hook