how does useSelector cache without re-running the selector ?

[ethan-cao]

Hi 👋

From the doc, A cached result may be returned by the hook without re-running the selector if it's the same function reference as on a previous render of the component.

I have tried supplying useSelecor with the same selecor instance during re-render, however the selector still got called.

Here is the codesandbox
Whenever I click the Add one button, selector gets called. However, I was expecting it not got called based on the doc.

Did I misunderstand something?

[markerikson]

It'll also re-run if the state changed as well:

react-redux/src/hooks/useSelector.js

Lines 31 to 48 in 9306158

	if (
	selector !== latestSelector.current ||
	storeState !== latestStoreState.current ||
	latestSubscriptionCallbackError.current
	) {
	const newSelectedState = selector(storeState)
	// ensure latest selected state is reused so that a custom equality function can result in identical references
	if (
	latestSelectedState.current === undefined ||
	!equalityFn(newSelectedState, latestSelectedState.current)
	) {
	selectedState = newSelectedState
	} else {
	selectedState = latestSelectedState.current
	}
	} else {
	selectedState = latestSelectedState.current
	}

[ethan-cao]

Thanks @markerikson!

I tried dispatching an random aciton which does not change the state, then the selector is indeed skiped.

[ethan-cao]

In the doc, we say useSelector() will also subscribe to the Redux store, and run your selector whenever an action is dispatched.

Does whenever an action is dispatched actually means whenever the state changes?

[markerikson]

Roughly, yeah.

The Redux store itself always runs all subscribers after every dispatch, regardless of whether or not any state was updated.

But, useSelector tries to optimize behavior - if the rootState hasn't changed, then it skips running the selector.

Most of the time we can shorten that and assume that "if an action was dispatched, some state got updated". But, it is very possible to dispatch actions that result in no state updates.