kopia lustrzana https://github.com/ryukoposting/Signal-Android
95978f16e9
Shoutout to @fumiakiy for the excellent research here! Sometimes we get thrown to the bottom of the list (or other list locations) when reading content in the middle of the list. Most often, this happens when you have a lot of unread messages and you open the conversation. FixedSizePagingController#onDataNeededAroundIndex() can be called very fast in rapid succession, and we use the DataStatus class for bookkeeping to know which requests are in-flight. We then make those requests in LIFO order in order to make sure that the data visible on screen now gets the highest priority. ...But in practice, that LIFO ordering can make things a little screwy. Imagine we called onDataNeedAroundIndex() 50 times in rapid succession (1, 2..., 50). Each time it's called, we generate a range and mark that range as being fetched in DataStatus. That could mean that the latest request for index 50 might only have, like, 1 item in it, because a previously-enqueued fetch already got assigned most of it's data. BUT we execute the nearly-empty request for index 50 first because of the LIFO ordering. We give that data to RecyclerView first, and it doesn't like that at all, and it jumps to weird places because we gave it mostly null values, which are rendered as placeholder values (which are smaller than real cells). So then, when we give it the real data right after, its position is all off. I switched to a serial executor. That prevents us from giving back weird lists. The consequence is that if you scroll super fast, you run the risk of the executor getting 'backed up' fetching data that's offscreen. However, in practice, I couldn't trigger this. We'll see how it goes. I think the true solution is a smarter way of fetching and ordering requests, but that gets to be really tricky from a threading perspective, and I'd rather keep things simple. |
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