:wave: asking in case someone has run into the sa...

# compose

[Jorge Martín]

👋 asking in case someone has run into the same issue or will run into this issue in the future and might find a solution: when using HorizontalPager, we have this weird issue:

val pagerState = rememberPagerState(state.currentIndex, 0f) {
    state.listData.size
}

HorizontalPager(
    state = pagerState,
    // Pre-load previous and next pages
    beyondViewportPageCount = 1,
    key = { index -> state.listData[index].pagerKey },
) { page ->
    // Pager item needs to know when it's the 'selected/visible' item
    PagerItem(isDisplayed = page == pagerState.settledPage)
}

With this code, we were seeing an issue when reloading the list that acts as a data source, and having it change from something like [A, B, C] (A being the visible item), to something like [D, A, B, C] would:

1. First, re-create the pages inside the HorizontalPager lambda, so we'd have pagerState.pageCount == 4 now.
2. Then, the pagerState.settledPage /`pagerState.currentPage` changes from 0 to 1 since the HorizontalPager is capable of keeping the same item focused.
However, this means for 1 recomposition we have the index of A change from 0 to 1 while the settledPage is still 0, so isDisplayed will be false, then on the next recomposition, the pager state's currentPage and settledPage values will change from 0 to 1 too, and isDisplayed will have the right value again. This is particularly important for us because isDisplayed dictates whether some media in the pager item should play or pause, so we have some brief stuttering.

So what I've ended up doing is doing val isDisplayed by remember(pagerState.settledPage) { page == pagerState.settledPage } so the value is only recomputed when either we need a new item (not re-composing an existing one) or the pager state actually changes into the update value and the value will be computed properly. So while this seems to work, I'm wondering: is this behaviour normal, is there some better way to avoid this desynchronization between the pager item provider and the current page value in the pager state?