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new feature to support smooth bi-directional content loading
Summary: == Problem / Background == Most lists paginate in a single direction (standard infinite list), but some paginate in both directions. Most common example is a chat thread where new messages show up on the bottom, and old content can be loaded by scrolling up. Comment threads are another example. Right now, adding content to the bottom of a scroll view is smooth - the content doesn't jump. But when adding to the top of the scrollview, the content gets pushed down, which is jarring (note this may appear reversed because of inverting the list which is common for chat applications). == Approach == The basic idea is simple - we set a flag in JS, then for every uimanager transaction, we record which is the first eligible and visible view in the ScrollView, and compare it's new origin to the old one. If it has changed, we update the contentOffset of the ScrollView to compensate. This is done by observing `willPerformMounting` directly (only from scrollviews that have this new property set), and then observing the prev state with prependUIBlock and making the update synchronously in addUIBlock to avoid any flicker. There is also a way to skip views that we don't care about, like a spinner at the top of the view that we don't want to stay in place - we actually want it to get pushed up by the new content, replaced visually in the viewport. == Notes == Most chat applications will probably want to do a scrollToTop when new content comes in and the user is already scrolled at or near the bottom. This is glitchy if visible children are re-ordered, which could be fixed with additional logic, but it doesn't come up in the type of applications we're targetting here so punting on that. == Test Plan == https://youtu.be/4GcqDGz9eOE Reviewed By: shergin Differential Revision: D6696921 fbshipit-source-id: 822e7dfcb207006cd1ba098356324ea81f619428
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