We are trying to use paging together with sticky h...

# compose

[Feri Nagy]

We are trying to use paging together with sticky headers. But now instead of using items(pagingItems) {} , we have to iterate through the items manually, do pagingItems.peek(index) and add single item {} one by one:

@Composable
fun PlainPaging(pagingItems: LazyPagingItems<Item>) {
    LazyColumn {
        items(pagingItems) { item ->
            when (item) {
                is Item.Header -> Text(text = item.title)
                is Item.Data -> Text(text = item.text)
                null -> Text("placeholder")
            }
        }
    }
}

// vs

@OptIn(ExperimentalFoundationApi::class)
@Composable
fun StickyPaging(pagingItems: LazyPagingItems<Item>) {
    LazyColumn {
        for (index in 0 until pagingItems.itemCount) {
            when (val item = pagingItems.peek(index)) {
                is Item.Header -> stickyHeader { Text(text = item.title) }
                is Item.Data -> item { Text(text = item.text) }
                null -> item { Text("placeholder") }
            }
        }
    }
}

Is this going to have some performance implications? Is there a better way to do it? Thanks

[Feri Nagy]

Or a more general question, is it a big deal to use items(list) {...} vs list.forEach { item {...} } ?

[Andrey Kulikov]

using items instead of forEach is a bit more efficient. when we lookup the index to composable mapping we will have to use a binary search in order to find a correct item as there are could be any amount of item items calls. but for Paging use case there is no better solution for now, so probably it is ok

[Feri Nagy]

Oops, I just realised that I simplified the sample a bit too much - there is no call to pagingItems.get(index) , so it would not trigger loading. 😬

What about using items(count) {} ? Would it be worth pre-processing the list so it could be used? Something along:

val sectionIndices = (0 until pagingItems.itemCount).mapNotNull {
    it.takeIf { pagingItems.peek(it) is Item.Header }
}

val countBefore = sectionIndices.firstOrNull() ?: pagingItems.itemCount
items(countBefore) {
    //...
}

sectionIndices.forEachIndexed { index, section ->
    stickyHeader { Text(text = (pagingItems[section] as Item.Header).title) }

    val end = if (index == sectionIndices.lastIndex) pagingItems.itemCount else sectionIndices[index + 1]
    items(end - section) {
        // ...
    }
}

[Filip Wiesner]

For future reference, there is issue for this https://issuetracker.google.com/issues/193785330