Data: Avoid calling listeners on unchanging state #5339
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aduth
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youknowriad
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Mar 2, 2018
| let lastState = store.getState(); | ||
| store.subscribe( () => { | ||
| const state = store.getState(); | ||
| const hasChanged = state !== lastState; |
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Per curiosity, do we have a lot of use-cases where a dispatch is triggered without any change in the state?
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Many of the actions which incur side-effects often don't do anything to state on their own, e.g.
SETUP_EDITORINSERT_DEFAULT_BLOCKAUTOSAVEFETCH_REUSABLE_BLOCKS
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For some of these, it could make sense to move the effect to be part of the action creator itself. We might not need a standalone INSERT_DEFAULT_BLOCK action type if the insertDefaultBlock action creator just returned the result of insertBlock.
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Related: #5334
This pull request seeks to optimize the behavior of
subscribeto avoid calling a listener except in the case that state has in-fact changed. This is a departure from Redux wheresubscribeis called on every dispatch, whether or not it results in a changed state. It's a departure I'd argue is warranted: listener callbacks don't and shouldn't have access to the dispatched action. That they are expected only to read from state implies that a subscriber react to a change, so an expectation better reflected in these changes.Further, it would be difficult to introduce the desired optimization mentioned in #5334 to avoid running
mapSelectToPropsofwithSelectwithout this change, as we don't know which store the mapping function will select from and to implement would require a shallow equality test on all stores (in some cases more wasteful than to just allow the mapping to be performed).Testing instructions:
Verify unit tests pass: