javascript - React useState hook event handler using initial state

Question

I'm still getting my head around react hooks but struggling to see what I'm doing wrong here. I have a component for resizing panels, onmousedown of an edge I update a value on state then have an event handler for mousemove which uses this value however it doesn't seem to be updating after the value has changed.

Here is my code:

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>
  );
});

The problem is with the handleResize function, this should be using the latest version of activePoint which would be a string top|left|bottom|right but instead is null.

Answer 1

useRef to read future value

Currently, your issue is that you're reading a value from the past. When you define handleResize it belongs to that render, therefore, when you rerender, nothing happens to the event listener so it still reads the old value from its render.

To fix this, you should use a ref via useRef that you keep updated so that you can read the current value.

Example (link to 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 */