[HOLD][HIGH] Migrate the main chat list to FlashList

[roryabraham]

HOLD on:

• Comment linking to be 100% complete
• The new architecture to be enabled (this is not a hard requirement but I have a hunch that this should wait on that too)

Problem

The main chat list is one of the most, if not the most important component in our app. If you scroll far or fast on this list, you may see frames drop. Furthermore, there are some known performance issues with this list (example). We have already migrated almost every other virtualized list in our app from FlatList to FlashList, because its performance is much better.

Solution

Let's build support for bidirectional pagination in FlashList, then enable it on the main chat list in E/App.

[melvin-bot]

Triggered auto assignment to @garrettmknight (NewFeature), see https://stackoverflowteams.com/c/expensify/questions/14418#:~:text=BugZero%20process%20steps%20for%20feature%20requests for more details.

[roryabraham]

I expect this to be on HOLD until sometime in February, if not later, so I'm moving this back to monthly

[roryabraham]

cc @muttmuure adding this to the New Expensify performance improvements project. I acknowledge that the problem statement is a bit of a reverse-solution but I think it's a good bet that this will provide substantial performance improvements, as FlashList is just much better for performance than FlatList. We've established that elsewhere where we've migrated to FlashList

[roryabraham]

Still on HOLD

[roryabraham]

on HOLD for comment linking

[roryabraham]

HOLD, but could come off HOLD soon. Unclear if we'll prioritize

[garrettmknight]

@roryabraham we good to take this off hold? If so, where do you think it might fit in a wave, if any? I'm not 100% on the significance of this migration beyond general, better performance.

[roryabraham]

Good question. As far as I know there's no significance beyond general, better performance of chat screens.

[muttmuure]

@janicduplessis how is this going?

[janicduplessis]

I've been working on some other tasks, but should be good to come back on this soon.

[mountiny]

@janicduplessis @hannojg any updates here?

[garrettmknight]

@janicduplessis @hannojg same question! any updates here?

[hannojg]

Current status update to using FlashList

We are still in the process of trying to fix maintainVisibleScrollPosition for FlashList.

FlashList is based on recyclerlistview which was not designed in a way that makes it easy to maintain scroll position.
Our current work is split between these branches:

• https://github.com/janicduplessis/Expensify-App/tree/%40janic/chat-flashlist
• https://github.com/janicduplessis/flash-list
• https://github.com/janicduplessis/recyclerlistview

To summarise:

• We are still working on adding the features to FlashList needed / fixing bugs to make it work well for expensify.
• When we have all those changes ready we'd need to make a big patch for expensify + open upstream PRs
  • Given how slow the bi-directional support PR is coming along in FlashList its questionable how fast our changes will go upstream.
  • It might makes more sense to add the packages as custom modules to expensify and maintain from the code on our own (to be discussed)

---

Something else we'd like to discuss is trying out a new list library called legend-list. It supports maintainVisibleScrollPosition inherently, is JS only but said to be much better performing than FlatList.
It might be that his library as a "faster FlatList" alternative that already includes the features we need is a better bet than going down the road with our "custom FlashList fork". I will open a separate ticket for investigating this if you agree?

[mountiny]

I will open a separate ticket for investigating this if you agree?

I think this is tricky as I bet there is many alternatives to explore. I think we should aim to look at the most popular and supported options.

Feel free to check it out and if its really promising, consider doing some MVP and posting a proposal, but lets try not to spend too much time on it

[garrettmknight]

@hannojg @mountiny any thoughts on this one?

[mountiny]

@hannojg @janicduplessis can you please post another update? thanks!

[hannojg]

We haven't progressed with that task yet. Janic unfortunately become unavailable for working on this.
I am currently trying to reschedule this task in our team!
Will keep you posted!

[mallenexpensify]

Posted in #expert-contributors

We're looking for someone to take this on.
@vit @Hanno Gödecke
, any ideas who might be the best fit?

[hannojg]

We re-evaluated this ticket internally and think that it's probably not worth the effort.
FlashList isn't really build with bi-directional scrolling in mind and forcing it to do it will only gain mediocre result that wouldn't justify the high amount of time we'd need to invest.

We still want to work on making the chat list as fast as it can be. We want to build a few PoC (where we keep bidirectional scrolling in mind) and evaluate those. This will probably be a mix of:

• Write our own implementation
• Legend list
• Other solutions we might come across

We might want to either:

• close this ticket and open a new one
• convert this ticket to be a general list performance investigation ticket

[mallenexpensify]

Thanks for the update @hannojg , I put the issue on hold for now. There's no big rush, I'm just going through HIGH and CRITICAL issues on the #quality project to ensure we're properly aligned and prioritized.

[garrettmknight]

Still on hold.