There is no out of the box functionality to support insertio

Question

There is no out of the box functionality to support insertion deletion of elements in large lazy containers such as `LazyColumn` right Background Demos such as `compose foundation demos LazyColumnDragAndDropDemo kt` replace the entire list when adding removing an element This is less than ideal for really large lists which are backed by a fully synchronized data source i e no such thing as refresh Suggested solution in thread

Oliver.O · Answer

To deal with large editable lists I m considering the following Create lazy items like so vertical scenario from top to bottom Insert a single placeholder Composable with a height calculated from the sum of all element placeholders above the visible range of elements reserving scroll space Insert as many items as necessary for the range of visible elements Insert a single placeholder Composable like above but this time for all items below the visible range Update as indicated by `LazyListState` Rebuild the above on insertions deletions the number of visible items is low basically replacing the small item list Does that make sense Better ideas

Arkadii Ivanov · Answer

How big is the list I had an app with ~1000 items in an immutable list sometimes updating 60 fps What about using `mutableStateListOf`

Oliver.O · Answer

Oh my use case is a very generic scenario so the list is potentially arbitrarily big Could be hundreds of thousands or millions of elements

Oliver.O · Answer

Composing a large number of invisible placeholders would probably put a pretty high load on the slot table

Arkadii Ivanov · Answer

The docs say that only visible elements are composed <https developer android com jetpack compose lists lazy|https developer android com jetpack compose lists lazy> But I will stay subscribed to hear more solutions grinning

Oliver.O · Answer

Interesting Surely elements not seen yet in a top to bottom scenario those at the bottom won t be composed once the lazy container detects that they are outside of the viewport Not so sure regarding elements above the viewport At least they d have to be measured to properly size the scrollbar And my list can be really large yet interaction could start at the bottom displaying only the last couple of elements with potentially millions of invisible elements above upside down face

ste · Answer

I don t really understand your problem The demo updates the source while you are dragging the item I guess you want to update the source only when the drag is ended

Oliver.O · Answer

Doesn t really matter whether in flight updates are included or not It s not just about drag amp drop It is more generally about inserting deleting elements in a really large list

Tobias Suchalla · Answer

Items above the viewport are not needed for the scrollbar Instead the average item size of the currently shown items is used to approximate the scrollbar size The LazyColumn really only measures and draws the visible items relevant ancient issue regarding the scrollbars <https github com JetBrains compose jb issues 181>

Oliver.O · Answer

That s interesting I still need to check that behavior I ll report back here

Oliver.O · Answer

< Tobias Suchalla> It seems like you are entirely correct Compose s lazy lists can perfectly deal with a small viewport into large lists composing only the visible items and apparently guessing measurements for items outside the viewport That s exactly what I need and much more than I expected Yay rocket compose multiplatform tada

Dominaezzz · Answer

What do you mean by guessing

Oliver.O · Answer

If you do not measure the elements outside the viewport you have to guess their size to properly size the scrollbar vertically height of visible elements height of visible+invisible elements

Dominaezzz · Answer

Ahhhh I see fair enough