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1. 为什么须要 useCallback
- 在应用 useCallback 和 useMemo 钩子之前,如果每次批改 parent 的 count,都会导致 child 从新渲染
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,个别 useCallback 和 useMemo 都是搭配应用,模仿 componentShouldUpdate 钩子,防止不必要的渲染。优化之后,咱们就发现,在 parent 中,一直的 setCount 批改状态,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。然而当点击子组件的 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. 解决形式有两种:两种形式都是将 callback 保留在一个援用对象中,区别就在于,形式 2 不须要去批改子组件,然而也减少了了解的老本
- 放弃 useCallback, 应用 useRef 钩子,用 ref.current 去援用箭头函数。应用 useRef 还须要对子组件回调的调用批改一下。
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,应用 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>
)
}
正文完