useSelect: improve transition from async to sync mode #40680
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youknowriad
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Apr 28, 2022
| // initial render + rerender with isAsync=false + store state update | ||
| expect( TestComponent ).toHaveBeenCalledTimes( 3 ); | ||
| // initial render + rerender with isAsync=false | ||
| expect( TestComponent ).toHaveBeenCalledTimes( 2 ); |
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This sounds like a potential performance improvement.
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Unfortunately this happens only in a very rare condition where there actually was a pending store update when the component was switching from async to sync mode.
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Size Change: -6 B (0%) Total Size: 1.23 MB
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youknowriad
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Apr 28, 2022
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This PR refactors
useSelectcode that makes sure that when the component is switching from async to sync mode, then the state updates scheduled while still in async mode won't be forgotten.The old code achieved that by flushing the render queue during the transition, which means synchronously calling
onStoreChangeand reading the new state from stores.The new code reads the new state from stores, too, but does it by doing a
hasLeftAsyncModecheck and callingmapSelectif the check is true. It's an addition to the set of existing checks that trigger call ofmapSelect: change of registry, change of themapSelectfunction itself (and its dependencies), and failure on previous call.Why is this change for the better? Because it removes a
renderQueue.flushcall, and concentrates allrenderQueuereferences into the effect that maintains store subscriptions and calls theonStoreChangecallback. That makes it easier to extract that effect to a separate hook, i.e., finishing the work in #40093.How to test:
Covered by unit tests, namely the "catches updates while switching from async to sync" one. See also #19286 which describes the conditions where doing the async->sync transition incorrectly can lead to UI bugs.