Component used: Paging
Version used: 3.0 alpha-04
Devices/Android versions reproduced on: n/a

Today (after many hours of debugging) I seem to have found a bug in latest alpha version of the Paging library. In my app I still have various lists that are not yet migrated to the new v3 API's, but since the documentation states it's backward compatible I figured that was okay.

Those lists use the database + remote pattern. One such list displayed strange behavior, it seemed to call all the boundary callbacks correctly, do the API call, and insert the new items in the database, however the items in the RecyclerView did not change.

The only difference that I could find between that list and other lists that were functioning correctly, is that it used a map function on the Datasource.Factory returned by Room

I think I found the underlying issue. The data source does not seem to be invalidated when using a map function.

DataSource has two variants of addOnValidationCallback. An open function DataSource.kt:366 and a final function that accepts Kotlin functions DataSource.kt:385

The Kotlin version is actually called in LegacyPagingSource

However when using a map/mapByPage function on the factory returned by room, the source is wrapped and in that class (WrappedDataSource, but also WrapperPositionalDataSource for example) only the non Kotlin version of addOnValidationCallback is overridden, and thus if called with the Kotlin version, addOnValidationCallback is never called on the source of the wrapper.

I could see when debugging that indeed onInvalidatedCallbacks is empty.

Hopefully this is actually the cause of the behavior I'm seeing. The Paging code is quite complex so I could be missing something else.