Android Paging3 不调用 PageDataSource 并且始终将实时数据获取为 null

Question

我使用 Android Jetpack Paging3 库进行分页，我遵循了 google code lab 和一些媒体博客以供引用。我已经按照他们的建议实现了分页，但它不起作用，viewmodel 总是返回 null 值。当我调试应用程序时，我知道 PageDataSource 没有调用。我不明白我按照文章关注的问题在哪里。以下是我的项目的代码 fragment 。

我在 RepositoryImpl 类中调用 Pager 的代码。

override suspend fun getSearchResult(searchKey: String): LiveData<PagingData<MyDataModel>> {
    return Pager(
        config = PagingConfig(NETWORK_PAGE_SIZE, enablePlaceholders = false),
        pagingSourceFactory = {
            MyPageDataSource(searchKey)
        }).liveData
}

这是我的 View 模型类代码

viewModelScope.launch{
        searchResult.value = Resource.loading(null)
        val response = repo.getSearchResult(searchKey)
        searchResult.value = Resource.success(response.value)
    }

这是 MyPageDataSource 类

 override fun getRefreshKey(state: PagingState<Int, MyDataModel>): Int? {
    return state.anchorPosition?.let { anchorPosition ->
        state.closestPageToPosition(anchorPosition)?.prevKey?.plus(1)
            ?: state.closestPageToPosition(anchorPosition)?.nextKey?.minus(1)
    }
}

override suspend fun load(params: LoadParams<Int>): LoadResult<Int, MyDataModel> {
    // Start refresh at position 1 if undefined.
    val page = params.key ?: INITIAL_LOAD_SIZE
    val offset = params.loadSize
    return try {
        val response = searchRequest(searchQuery, page, offset) // its just api calling function which get result from API.
        val nextKey: Int?
        when (response) {
            is SearchResult.Success -> {
                nextKey = if (response.result.searchRes.isEmpty()) {
                    null
                } else {
                    // initial load size = 3 * NETWORK_PAGE_SIZE
                    // ensure we're not requesting duplicating items, at the 2nd request
                    page + (params.loadSize / NETWORK_PAGE_SIZE)
                }
                LoadResult.Page(
                    data = response.result.searchRes,
                    prevKey = if (page == INITIAL_LOAD_SIZE) null else page - 1,// Only paging forward.
                    // assume that if a full page is not loaded, that means the end of the data
                    nextKey = nextKey
                )
            }
            is SearchResult.NoResultFound -> {
                nextKey = null
                LoadResult.Page(
                    data = emptyList(),
                    prevKey = if (page == INITIAL_LOAD_SIZE) null else page - 1,// Only paging forward.
                    // assume that if a full page is not loaded, that means the end of the data
                    nextKey = nextKey
                )
            }

            is SearchResult.ServerError -> {
                val exception = Exception(response.message)
                LoadResult.Error(exception)
            }

        }

    } catch (e: Exception) {
        e.printStackTrace()
        LoadResult.Error(e)
    }
}

Answer 1

我尝试了不同的方法来进行分页，最终通过使用函数返回类型作为LiveData来实现结果。在加载函数触发之后，一切就都解决了。

以下是 ViewModel 代码

 suspend fun getSearchResult(searchKey:String):LiveData<PagingData<MyDataModel>> =repo.getSearchResult(searchKey)

在 fragment 中我可以像这样访问它

 lifecycleScope.launch { viewmodel.getSearchResult(searchKey)..observe(viewLifecycleOwner) {
     result ->  //TODO}

我不知道为什么当我使用livedata变量访问时它不起作用。如果还有其他解决方案请留言。