When swiping this I would have expected that every remember kotlinlang #compose

When swiping this, I would have expected that every “remember page” log is only shown once. However, once swiping or just dragging a page a little left and right, the block gets called again and again. Is that expected behavior? Is there a way to remember something depending on the page?

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HorizontalPager(
    modifier = Modifier.fillMaxSize(),
    pageCount = 42
) { currentPage ->
    remember(currentPage) {
        Log.d("remember", "remember page $currentPage")
        42
    }
    Text(
        modifier = Modifier.fillMaxSize(),
        text = "Page $currentPage"
    )
}

If I make use of rememberSaveable, this works as I want. However the thing I want to remember (a presenter) actually is not saveable.

ephemient

11/01/2023, 11:26 PM

https://newsletter.jorgecastillo.dev/p/movablecontentof-and-movablecontentwithreceivero is one solution

Albert Chang

11/02/2023, 7:39 AM

This is the expected behavior, and I don't think movable content will help here, as

movableContentOf

doesn't prevent the content inside from being disposed when the call to the movable content is disposed. You can just set a appropriate

beyondBoundsPageCount

to make sure the pages are not disposed.

Paul Woitaschek

11/02/2023, 7:41 AM

I tried that but when I touch a pager and drag it by a few pixels left or right it makes my app really laggy as it composes the whole time while dragging and state seems to be discarded

For that specific thing one solution I came up with (and which works) is to have my own memory cache for the objects I’m persisting, but what I really want is some: rememberSaveableInMemoryOnlyAndIgnoreBundlePersistence

ephemient

11/02/2023, 7:45 AM

movableContentOf
will help if you move the content outside of the pager

Albert Chang

11/02/2023, 7:46 AM

No it won't. You can try it yourself.

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val content = remember { movableContentOf { ... } }
content() // When this call is disposed (and not moved to anywhere else), content inside will also be disposed

And to OP, the

rememberSaveableInMemoryOnlyAndIgnoreBundlePersistence

you want is probably just something as simple as

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val map = remember { mutableMapOf<Int, Something>() }
HorizontalPager { currentPage ->
    map.getOrPut(currentCompositeKeyHash) {
        Something()
    }
}

Paul Woitaschek

11/02/2023, 11:00 AM

Yep that’s what I went with 👌