我仍然围绕着反应钩,但努力想看看我在这里做错了什么。我有一个用于调整面板大小的组件,onmousedown的一个边缘我更新了一个状态值然后有一个使用这个值的mousemove的事件处理程序,但它似乎在值更改后似乎没有更新。
这是我的代码:
export default memo(() => {
const [activePoint, setActivePoint] = useState(null); // initial is null
const handleResize = () => {
console.log(activePoint); // is null but should be 'top|bottom|left|right'
};
const resizerMouseDown = (e, point) => {
setActivePoint(point); // setting state as 'top|bottom|left|right'
window.addEventListener('mousemove', handleResize);
window.addEventListener('mouseup', cleanup); // removed for clarity
};
return (
<div className="interfaceResizeHandler">
{resizePoints.map(point => (
<div
key={ point }
className={ `interfaceResizeHandler__resizer interfaceResizeHandler__resizer--${ point }` }
onMouseDown={ e => resizerMouseDown(e, point) }
/>
))}
</div>
);
});
问题是handleResize
函数,这应该使用最新版本的activePoint
,这将是一个字符串top|left|bottom|right
,而是null
。
useRef
to read future value目前,您的问题是您正在阅读过去的值。当你定义handleResize
它属于那个渲染时,因此,当你重新渲染时,事件监听器没有任何反应,所以它仍然从它的渲染中读取旧值。
要解决这个问题,你应该使用useRef
的ref,你会不断更新,以便你可以读取当前值。
const [activePoint, _setActivePoint] = React.useState(null);
// define a ref
const activePointRef = React.useRef(activePoint);
// in place of original `setActivePoint`
const setActivePoint = x => {
activePointRef.current = x; // keep updated
_setActivePoint(x);
};
const handleResize = () => {
// now when reading `activePointRef.current` you'll
// have access to the current state
console.log(activePointRef.current);
};
const resizerMouseDown = /* still the same */;
return /* return is still the same */