为了复制这个。setState类组件的合并行为，
React文档建议使用useState的函数形式来扩展对象，不需要useReducer:
setState(prevState => {
return {...prevState, loading, data};
});
这两个状态现在合并为一个，这将为您节省一个呈现周期。
使用一个状态对象还有另一个优点:加载和数据是相互依赖的状态。无效的状态变化变得更加明显，当把状态放在一起:
setState({ loading: true, data }); // ups... loading, but we already set data
你甚至可以通过以下方法更好地确保状态的一致性:1.)在你的状态中显式地显示状态——加载、成功、错误等;2.)使用useReducer在一个reducer中封装状态逻辑:
const useData = () => {
const [state, dispatch] = useReducer(reducer, /*...*/);
useEffect(() => {
api.get('/people').then(test => {
if (test.ok) dispatch(["success", test.data.results]);
});
}, []);
};
const reducer = (state, [status, payload]) => {
if (status === "success") return { ...state, data: payload, status };
// keep state consistent, e.g. reset data, if loading
else if (status === "loading") return { ...state, data: undefined, status };
return state;
};
const App = () => {
const { data, status } = useData();
return status === "loading" ? <div> Loading... </div> : (
// success, display data
)
}
const useData = () => {
const [state, dispatch] = useReducer(reducer, {
data: undefined,
status: "loading"
});
useEffect(() => {
fetchData_fakeApi().then(test => {
if (test.ok) dispatch(["success", test.data.results]);
});
}, []);
return state;
};
const reducer = (state, [status, payload]) => {
if (status === "success") return { ...state, data: payload, status };
// e.g. make sure to reset data, when loading.
else if (status === "loading") return { ...state, data: undefined, status };
else return state;
};
const App = () => {
const { data, status } = useData();
const count = useRenderCount();
const countStr = `Re-rendered ${count.current} times`;
return status === "loading" ? (
<div> Loading (3 sec)... {countStr} </div>
) : (
<div>
Finished. Data: {JSON.stringify(data)}, {countStr}
</div>
);
}
//
// helpers
//
const useRenderCount = () => {
const renderCount = useRef(0);
useEffect(() => {
renderCount.current += 1;
});
return renderCount;
};
const fetchData_fakeApi = () =>
new Promise(resolve =>
setTimeout(() => resolve({ ok: true, data: { results: [1, 2, 3] } }), 3000)
);
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>var { useReducer, useEffect, useState, useRef } = React</script>
PS:请确保在自定义Hooks前加上use (useData而不是getData)。同时传递给useEffect的回调不能是异步的。
