android - Room Table 的 @PrimaryKey 在 Android Paging 中产生意外行为

问题描述

我正在按照这个codelab教程Paging使用数据库来实现缓存： - https://codelabs.developers.google.com/codelabs/android-paging/#13Room

但是，在实现缓存部分时，我在将下一页附加到recyclerview.

RemoteMediator唯一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Int在表中有@PrimaryKey，它有 5 个字符长。

在 codelab 教程中也实现了相同的功能，它们也具有Int@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 长约 15 个字符

id = id + System.currentTimeMillis()

并且在这样做之后它完美地工作。但在实际情况下，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？或者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..."
        }
    },
    {..},
    {..}
]

故事.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
)

远程调解器.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（这是字符串类型值），它也在这种情况下工作。

所以我很确定@PrimaryKey 长度有问题。

标签： androidkotlinandroid-recyclerviewandroid-roomandroid-paging

android - Room Table 的 @PrimaryKey 在 Android Paging 中产生意外行为

问题描述

解决方案

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