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.
useRef to read future valueCurrently, 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.
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 */
You have access to current state from setter function, so you could make it:
const handleResize = () => {
setActivePoint(activePoint => {
console.log(activePoint);
return activePoint;
})
};
const [activePoint, setActivePoint] = useState(null); // initial is null
const handleResize = () => {
setActivePoint(currentActivePoint => { // call set method to get the value
console.log(currentActivePoint);
return currentActivePoint; // set the same value, so nothing will change
// or a different value, depends on your use case
});
};