【发布时间】：2020-11-12 01:03:45
【问题描述】：
我正在使用 Room 数据库按照此 Codelab 教程实现带有缓存的 Paging：-https://codelabs.developers.google.com/codelabs/android-paging/#13
然而，在实现缓存部分时，我在recyclerview 中附加下一页时遇到了一些奇怪的行为。
RemoteMediator 仅在连续无限循环中获取第二页数据。它不是为下一页获取数据，而是为第二页连续获取数据。
我发现这是由于表的@PrimaryKey 而发生的。我在表格中有Int 作为@PrimaryKey，它有5 个字符长。
在 codelab 教程中也实现了相同的功能，他们也将Int 作为@PrimaryKey。但它们至少有 7 个字符长的随机主键。它工作得很好。为了验证这一点，我通过添加使我的 @PrimaryKey 长度约为 15 个字符
id = id + System.currentTimeMillis()
并且在这样做之后它完美地工作。但在实际场景中我无法修改 id。
那么，Room 表的@PrimayKey 是否有任何限制（即主键必须至少有 7 个字符）？还是 @PrimaryKey 必须在 Room 中随机化？还是我在这里做错了什么？
这是代码和 JSON。
JSON 格式
[
{
"id": 24087,
"date": "2020-07-15T11:20:00",
"link": "https://www.somesite.com/24534-eu-covid-19-stimulus-negotiations-and-its-historic-backdrop-2020-07-21/",
"title": {
"rendered": "EU Covid-19 Stimulus Negotiations And Its Historic Backdrop"
},
"excerpt": {
"rendered": "After a long debate between the opposing factions, the European Union agreed on a recovery fund to aid the economy amidst the Covid-19 pandemic..."
}
},
{..},
{..}
]
Story.kt
@Entity(tableName = "stories")
data class Story(
@PrimaryKey @field:SerializedName("id") var storyId: Long,
@field:SerializedName("date") val date: String,
@field:SerializedName("link") val link: String,
@Embedded(prefix = "title_") @field:SerializedName("title") val title: Title,
@Embedded(prefix = "excerpt_") @field:SerializedName("excerpt") val excerpt: Excerpt
)
data class Title(
val rendered: String
)
data class Excerpt(
val rendered: String
)
RemoteMediator.kt
@OptIn(ExperimentalPagingApi::class)
class StoryRemoteMediator(
private val category: Int,
private val ocapiDatabase: OCAPIDatabase
) : RemoteMediator<Int, Story>() {
private val storyService = StoryService.create()
override suspend fun load(
loadType: LoadType,
state: PagingState<Int, Story>
): MediatorResult {
val page = when (loadType) {
LoadType.REFRESH -> {
val remoteKeys = getRemoteKeysClosestToCurrentPosition(state)
remoteKeys?.nextKey?.minus(1) ?: STORY_STARTING_PAGE_INDEX
}
LoadType.PREPEND -> {
val remoteKeys = getRemoteKeysForFirstItem(state) ?:
throw InvalidObjectException("Remote key and the prevKey should not be null")
remoteKeys.prevKey ?: return MediatorResult.Success(endOfPaginationReached = true)
}
LoadType.APPEND -> {
val remoteKeys = getRemoteKeysForLastItem(state) ?:
throw InvalidObjectException("Remote key should not be null for $loadType")
remoteKeys.nextKey ?: return MediatorResult.Success(endOfPaginationReached = true)
}
}
try {
val stories = storyService.getStories(page = page, category = category)
val endOfPaginationReached = stories.isEmpty()
ocapiDatabase.withTransaction {
if (loadType == LoadType.REFRESH && stories.isNotEmpty()) {
ocapiDatabase.storyDao().deleteAll()
ocapiDatabase.storyRemoteKeysDao().deleteAll()
}
val prevKey = if (page == STORY_STARTING_PAGE_INDEX) null else page - 1
val nextKey = if (endOfPaginationReached) null else page + 1
val keys = stories.map {
StoryRemoteKeys(
storyId = it.storyId,
prevKey = prevKey,
nextKey = nextKey
)
}
ocapiDatabase.storyDao().insertAll(stories)
ocapiDatabase.storyRemoteKeysDao().insertAll(keys)
}
return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
} catch (exception: IOException) {
return MediatorResult.Error(exception)
} catch (exception: HttpException) {
return MediatorResult.Error(exception)
}
}
private suspend fun getRemoteKeysClosestToCurrentPosition(state: PagingState<Int, Story>): StoryRemoteKeys? {
return state.anchorPosition?.let { position ->
state.closestItemToPosition(position)?.storyId?.let { storyId ->
ocapiDatabase.storyRemoteKeysDao().getRemoteKeysById(storyId)
}
}
}
private suspend fun getRemoteKeysForFirstItem(state: PagingState<Int, Story>): StoryRemoteKeys? {
return state.pages.firstOrNull { it.data.isNotEmpty() }?.data?.firstOrNull()
?.let { story ->
ocapiDatabase.storyRemoteKeysDao().getRemoteKeysById(story.storyId)
}
}
private suspend fun getRemoteKeysForLastItem(state: PagingState<Int, Story>): StoryRemoteKeys? {
return state.pages.lastOrNull { it.data.isNotEmpty() }?.data?.lastOrNull()
?.let { story ->
ocapiDatabase.storyRemoteKeysDao().getRemoteKeysById(story.storyId)
}
}
}
}
为了更清楚，这里是打印当前页码以从网络获取的日志。
2020-07-22 18:21:35.767 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:36.599 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:37.517 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:38.456 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:39.723 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:40.459 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:41.275 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:42.191 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:43.095 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:44.010 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
2020-07-22 18:21:44.903 3510-3510/com.milan.ocapi I/STORY_PAGE: 2
如您所见，它不会获取下一个（即 3、4、5）页面。它进入循环。 为了测试，我还将@PrimaryKey 更改为链接（这是字符串类型的值），它也在这种情况下工作。
所以我很确定@PrimaryKey 长度有问题。
【问题讨论】：
标签： android kotlin android-recyclerview android-room android-paging