commit | 6bd2347ff259c28b34789c8569b7a298d2a8fd8d | [log] [tgz] |
---|---|---|
author | Clara Fok <clarafok@google.com> | Fri Jul 09 12:53:22 2021 -0700 |
committer | Clara Fok <clarafok@google.com> | Tue Jul 13 09:13:53 2021 -0700 |
tree | 6c793563056c24a7854b6ab5d6b0403676cd209d | |
parent | dd93dae8127f914390b4063c8da6136e5da8e645 [diff] |
Implement Invalid return type in PagingSource Added third return type for LoadResult sealed class in PagingSource. It was added as a support feature to handle race scenarios between data loads and database invalidations, such as if Room's InvalidationTracker does not propagate invalidation signal to the PagingSource in time and the PagingSource continues querying an updated database. This can result in data being loaded onto a stale PageFetcherSnapshot causing duplicated or missing data. Related bug regarding race scenario in Room b/191806126. For example, if data was deleted and items shifted positions, positionally-keyed paging sources can end up loading duplicated items. Paging handles this return type by discarding the loaded data and invalidating the paging source. This will trigger a new paging source to be generated. This return type is also supported for Paging2 APIs leveraging PagingSource such as LivePagedList and RxPagedList. Aside from handling invalid data due to races, this return type can be leveraged in general where the database or network returns potentially invalid or stale data that needs to be discarded. Bug: 192013267 Test: ./gradlew :paging:paging-common:test Relnote: "A third LoadResult return type LoadResult.Invalid is added to PagingSource. When a PagingSource.load returns LoadResult.Invalid, paging will discard the loaded data and invalidate the PagingSource. This return type is designed to handle potentially invalid or stale data that can be returned from the database or network. For example, if the underlying database gets written into but the PagingSource does not invalidate in time, it may return inconsistent results if its implementation depends on the immutability of the backing dataset it loads from (e.g., LIMIT OFFSET style db implementations). In this scenario, it is recommended to check for invalidation after loading and to return LoadResult.Invalid, which causes Paging to discard any pending or future load requests to this PagingSource and invalidate it. This return type is also supported by Paging2 API that leverages LivePagedList or RxPagedList. When using a PagingSource with Paging2's PagedList APIs, the PagedList is immediately detached, stopping further attempts to load data on this PagedList and triggers invalidation on the PagingSource. LoadResult is a sealed class, which means this is a source-incomptabile change such that use cases directly using PagingSource.load results will have to handle LoadResult.Invalid at compile time. For example, Kotlin users leveraging exhaustive-when to check return type will have to add a check for Invalid type." Change-Id: Id6bd3f2544f1ba97a95a0d590353438a95fedf2a
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