使用初始状态来反应useState钩子事件处理程序

Question

我仍然不停地做出反应，但仍在努力查看我在做什么错。我有一个用于调整面板大小的组件，边缘的onmousedown会更新状态的值，然后有一个用于mousemove的事件处理程序，该事件处理程序使用此值，但是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>
  );
});

问题出在handleResize函数上，应该使用的activePoint是字符串的最新版本，top|left|bottom|right而实际上是null。

Answer 1

当前，您的问题是您正在读取过去的价值。当您定义handleResize它属于该渲染时，因此，在重新渲染时，事件侦听器没有任何反应，因此它仍从其渲染中读取旧值。

要解决此问题，您应该使用一个ref并useRef保持更新，以便您可以读取当前值。

示例（链接到jsfiddle）：

  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 */