react useCallback 闭包陷阱

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· 阅读数 4
1. 为什么需要useCallback
- 在使用useCallback 和 useMemo钩子之前,如果每次修改parent的count,都会导致child重新渲染
const Child = (props) => {
console.log('Child render');
return (
<div>
<button onClick={props.clickCallback}>Child click</button>
</div>
)
}
const parent = () => {
const [count, setCount] = useState(1);
const cb = () => {
console.log(count);
};
return (
<div>
parent
<button onClick={() => setCount()}>click</button>
<Child clickCallback={cb} />
</div>
)
}
2. 场景复现
- 函数组件中,一般 useCallback 和 useMemo都是搭配使用, 模拟componentShouldUpdate钩子,避免不必要的渲染。优化之后,我们就发现,在parent中,不断的setCount修改状态,也不会触发Child组件的更新。彷佛解决了 父函数组件更新状态,导致子组件的不必要更新问题。但是当点击子组件的button时,就会发现,打印的是 count的初始值 1,这也就是 useCallback的闭包陷阱,不能拿到最新的state。
const Child = (props) => {
console.log('Child render');
return (
<div>
<button onClick={props.clickCallback}>Child click</button>
</div>
)
}
const parent = () => {
const [count, setCount] = useState(1);
const cb = React.useCallback(() => {
console.log(count);
}, []);
const ChildMemo = React.useMemo(() => <Child clickCallback={cb} />, [cb]);
return (
<div style={{
width: '100vw', height: '100vh',
justifyContent: 'center', alignItems: 'center',
display: 'flex',
flexDirection: 'column' }}
>
parent count : {count}
<button onClick={() => setCount(count + 1)}>click</button>
{ChildMemo}
</div>
)
}
3. 解决方式有两种: 两种方式都是将callback保存在一个引用对象中,区别就在于,方式2不需要去修改子组件,但是也增加了理解的成本
- 放弃useCallback, 使用 useRef钩子,用ref.current去引用箭头函数。使用useRef还需要对子组件回调的调用修改一下。
const Child = (props) => {
console.log('Child render');
return (
<div>
<button onClick={() => {
props.callbackRef.current();
}}
>Child click</button>
</div>
)
}
const parent = () => {
const [count, setCount] = useState(1);
const cbRef = React.useRef(null);
cbRef.current = () => {
console.log(count);
};
const ChildMemo = React.useMemo(
() => <Child callbackRef={cbRef} />,
[]
);
return (
<div style={{
width: '100vw', height: '100vh',
justifyContent: 'center', alignItems: 'center',
display: 'flex',
flexDirection: 'column' }}
>
parent count : {count}
<button onClick={() => setCount(count + 1)}>click</button>
{ChildMemo}
</div>
)
}
- 自定义一个hook,使用 useCallback搭配useRef
const useImmediateValue = (cb) => {
const ref = useRef(cb);
ref.current = cb;
const val = useCallback(() => {
ref.current();
}, []);
return val;
}
const Child = (props) => {
console.log('Child render');
return (
<div>
<button onClick={props.clickCallback}>Child click</button>
</div>
)
}
const parent = () => {
const [count, setCount] = useState(1);
const cb = useImmediateValue(() => {
console.log(count);
})
const ChildMemo = React.useMemo(
() => <Child clickCallback={cb} />,
[]
);
return (
<div style={{
width: '100vw', height: '100vh',
justifyContent: 'center', alignItems: 'center',
display: 'flex',
flexDirection: 'column' }}
>
parent count : {count}
<button onClick={() => setCount(count + 1)}>click</button>
{ChildMemo}
</div>
)
}