我const foo: Array<ObjectExt> = useSelector(fooSelector);在功能组件中有一个变量。我想要这个变量的副本，从第一次加载组件开始就不会改变foo。在使用类组件时，我可以简单地拥有const fooCopy = foo.slice();，但这在这里不起作用，因为组件每次都会重新加载并fooCopy发生变化。我如何在功能组件中实现这一点？
4 回答
沧海一幻觉
TA贡献1824条经验 获得超5个赞
只是useState将初始值作为 的副本foo。
const foo : Array<ObjectExt> = useSelector(fooSelector);
const [origFoo] = useState(foo.slice());
一旦origFoo被初始化，它就不会在重新渲染时被重新初始化。如果以后需要更新它的值，可以解构 setter：
const [origFoo, setOrigFoo] = useState(foo);
// ...
if(someCondition) setOrigFoo(foo.slice())
const {useState} = React;
function App() {
const foo = [new Date().getTime()];
const [origFoo] = useState(foo.slice());
// Just so we have a way to force a rerender
const [count, setCount] = React.useState(0);
return (
<div>
<p>{JSON.stringify(foo)} </p>
<p>{JSON.stringify(origFoo)}</p>
<button onClick={() => setCount(count + 1)}>Update</button>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(
<App />,
rootElement
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="root"></div>
慕娘9325324
TA贡献1783条经验 获得超4个赞
我想要这个变量的副本，从第一次加载组件开始，当 foo 加载时它不会改变。
当您使用useSelector(selector)react-redux 时，selector每次状态更改时都会运行，如果返回值与上次运行时不同，则 react-redux 将重新渲染组件。
最简单的方法是使用一个只返回第一次调用时获得的值的选择器：
const { Provider, useDispatch, useSelector } = ReactRedux;
const { createStore, applyMiddleware, compose } = Redux;
const initialState = { count: 0 };
//action types
const ADD = 'ADD';
//action creators
const add = () => ({
type: ADD,
});
const reducer = (state, { type, payload }) => {
if (type === ADD) {
return { ...state, count: state.count + 1 };
}
return state;
};
//selectors
const selectCount = (state) => state.count;
//function returning a function
const createSelectInitialCount = () => {
//initialize NONE when createSelectInitialCount is called
const NONE = {};
let last = NONE;
//return the selector
return (state) => {
//check if last value was set
if (last === NONE) {
//set last value (only when called the first time)
last = selectCount(state);
}
return last;
};
};
//creating store with redux dev tools
const composeEnhancers =
window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose;
const store = createStore(
reducer,
initialState,
composeEnhancers(
applyMiddleware(() => (next) => (action) =>
next(action)
)
)
);
const InitialFoo = React.memo(function InitialFoo(props) {
const selectInitialCount = React.useMemo(//can also use useCallback
createSelectInitialCount,//createSelectInitialCount() if you use useCallback
[]
);
const foo = useSelector(selectInitialCount);
return (
<div>
<h3>initialfoo</h3>
<pre>
{JSON.stringify({ ...props, foo }, undefined, 2)}
</pre>
</div>
);
});
const App = () => {
const foo = useSelector(selectCount);
const dispatch = useDispatch();
const [other, setOther] = React.useState(0);
const [showFoo, setShowFoo] = React.useState(true);
const remountFoo = () => {
setShowFoo(false);
Promise.resolve().then(() => setShowFoo(true));
};
return (
<div>
<button onClick={() => dispatch(add())}>
foo:{foo}
</button>
<button onClick={() => setOther((o) => o + 1)}>
other{other}
</button>
<button onClick={remountFoo}>remount Foo</button>
{showFoo && <InitialFoo other={other} />}
</div>
);
};
ReactDOM.render(
<Provider store={store}>
<App />
</Provider>,
document.getElementById('root')
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/redux/4.0.5/redux.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-redux/7.2.0/react-redux.min.js"></script>
<div id="root"></div>
另一种方法是使用 React.memo 和忽略 foo 的自定义比较函数来创建纯组件：
const { Provider, useDispatch, useSelector } = ReactRedux;
const { createStore, applyMiddleware, compose } = Redux;
const initialState = { count: 0 };
//action types
const ADD = 'ADD';
//action creators
const add = () => ({
type: ADD,
});
const reducer = (state, { type, payload }) => {
if (type === ADD) {
return { ...state, count: state.count + 1 };
}
return state;
};
//selectors
const selectCount = (state) => state.count;
//creating store with redux dev tools
const composeEnhancers =
window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose;
const store = createStore(
reducer,
initialState,
composeEnhancers(
applyMiddleware(() => (next) => (action) =>
next(action)
)
)
);
const InitialFoo = React.memo(
function InitialFoo(props) {
return (
<div>
<h3>initialfoo</h3>
<pre>{JSON.stringify(props, undefined, 2)}</pre>
</div>
);
},
//custom compare function when returning false
// component will re render
({ other }, { other: newOther }) => {
return other === newOther;
}
);
const InitialFooContainer = (props) => {
const foo = useSelector(selectCount);
//store foo in ref, will never change after mount
const fooRef = React.useRef(foo);
const newProps = { ...props, foo: fooRef.current };
return <InitialFoo {...newProps} />;
};
const App = () => {
const foo = useSelector(selectCount);
const dispatch = useDispatch();
const [other, setOther] = React.useState(0);
const [showFoo, setShowFoo] = React.useState(true);
const remountFoo = () => {
setShowFoo(false);
Promise.resolve().then(() => setShowFoo(true));
};
return (
<div>
<button onClick={() => dispatch(add())}>
foo:{foo}
</button>
<button onClick={() => setOther((o) => o + 1)}>
other{other}
</button>
<button onClick={remountFoo}>remount Foo</button>
{showFoo && (
<InitialFooContainer foo={foo} other={other} />
)}
</div>
);
};
ReactDOM.render(
<Provider store={store}>
<App />
</Provider>,
document.getElementById('root')
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/redux/4.0.5/redux.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-redux/7.2.0/react-redux.min.js"></script>
<div id="root"></div>
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