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Joshua Gross
b161241db2
Summary: FlatList relies heavily on onScroll events + the measure API. In Fabric, usage of `measure` relies on C++ having an accurate view of the current scroll position of the ScrollView. We already have a mechanism for updating the scroll position in C++ using UpdateState. But, it is only used currently at the /beginning/ and /end/ of scrolling, and UpdateState is not called /during/ scrolling at all. This means that we will see a series of events like this while scrolling: ``` Scrolling begins UPDATE C++ STATE: scrollLeft = 0, scrollTop = 0 JS event: onScroll x=0, y=0 JS event: onScroll x=0, y=100 JS event: onScroll x=0, y=200 ... JS event: onScroll x=0, y=1000 UPDATE C++ STATE: scrollLeft = 0, scrollTop = 1000 ``` Notably, not many C++ state updates are queued; and the last one is queued AFTER the JS event is sent. The last JS event and UpdateState will race, which means that sometimes the C++ update will /lose/ and C++ will have an inaccurate view of the world when FlatList receives the onScroll event and calls `measure`. My proposed solution, gated behind a feature flag, is to delay /some/ onScroll events until the C++ UpdateState has made its way back to Java, and send UpdateStates more frequently. The balance here is that UpdateState is a relatively expensive operation, so we probably still want to call it /less/ than we call onScroll. This means that `measure` will still return some incorrect results but will return correct results more frequently. Win? Changelog: [Internal[ Reviewed By: mdvacca Differential Revision: D28558380 fbshipit-source-id: 11c7cd714fae67ee5a94c4413be988481413ec03
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