Component used: Paging Library
Version used:all versions
Devices/Android versions reproduced on: all versions

Hi,
thanks for the great work.

I use a slightly modified version of the PagingSample. Instead of just adding one item when tapping on the "Add" button, I add 20 copies of the same item:

fun insert(text: CharSequence) = ioThread {
dao.insert(listOf(
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString()),
Cheese(id = 0, name = text.toString())))
}

When adding items which will be pre-pended to the list, the PositionalDataSource fails to re-load the visible items. This is a general problem.

My guess:

The PositionalDatasource doesn't know if newly inserted items will be pre-pended by the query or appended. Let's say we have the following scenario:

- initially loading items from position 20
- pre-pending 50 new items
- results in datasource invalidation and new initial load from position 20
- now there are completely different items at position 20
- async diff sees changes and triggers animations

Somehow this happens to be related to the page size used. When increasing the page size, it happens after more inserts. When decreasing the page size, this behavior happen after just the first inserts.

My question is:

How to handle pre-pending inserts with the PositionalDataSource?