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Paging 分页加载

问题​

Paging 3 的核心架构是什么？如何实现网络 + 数据库的分页？

答案​

架构概览​

基本使用（纯网络）​

// 1. 定义 PagingSource
class ArticlePagingSource(
    private val api: ArticleApi
) : PagingSource<Int, Article>() {
    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Article> {
        val page = params.key ?: 1
        return try {
            val response = api.getArticles(page, params.loadSize)
            LoadResult.Page(
                data = response.data,
                prevKey = if (page == 1) null else page - 1,
                nextKey = if (response.data.isEmpty()) null else page + 1
            )
        } catch (e: Exception) {
            LoadResult.Error(e)
        }
    }

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

// 2. 在 ViewModel 中创建 Pager
class ArticleViewModel(private val api: ArticleApi) : ViewModel() {
    val articles: Flow<PagingData<Article>> = Pager(
        config = PagingConfig(
            pageSize = 20,
            prefetchDistance = 5,       // 距离底部 5 个 item 时预加载
            enablePlaceholders = false
        ),
        pagingSourceFactory = { ArticlePagingSource(api) }
    ).flow.cachedIn(viewModelScope)
}

// 3. 在 UI 中使用 PagingDataAdapter
class ArticleAdapter : PagingDataAdapter<Article, ArticleViewHolder>(DIFF_CALLBACK) {
    override fun onBindViewHolder(holder: ArticleViewHolder, position: Int) {
        getItem(position)?.let { holder.bind(it) }
    }
}

// 提交数据
lifecycleScope.launch {
    viewModel.articles.collectLatest { pagingData ->
        adapter.submitData(pagingData)
    }
}

RemoteMediator（网络 + 数据库）​

@OptIn(ExperimentalPagingApi::class)
class ArticleRemoteMediator(
    private val api: ArticleApi,
    private val db: AppDatabase
) : RemoteMediator<Int, Article>() {

    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, Article>
    ): MediatorResult {
        val page = when (loadType) {
            LoadType.REFRESH -> 1
            LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)
            LoadType.APPEND -> getNextPage(state)
        }

        val response = api.getArticles(page, state.config.pageSize)
        db.withTransaction {
            if (loadType == LoadType.REFRESH) db.articleDao().clearAll()
            db.articleDao().insertAll(response.data)
        }
        return MediatorResult.Success(endOfPaginationReached = response.data.isEmpty())
    }
}

加载状态处理​

// 在 Adapter 底部添加加载指示器
val footer = ArticleLoadStateAdapter { adapter.retry() }
recyclerView.adapter = adapter.withLoadStateFooter(footer)

// 监听加载状态
adapter.addLoadStateListener { loadStates ->
    binding.progressBar.isVisible = loadStates.refresh is LoadState.Loading
    binding.errorView.isVisible = loadStates.refresh is LoadState.Error
}

常见面试问题​

Q1: PagingSource 和 RemoteMediator 的区别？​

答案：

特性PagingSourceRemoteMediator
数据来源单一来源（网络或数据库）网络 + 数据库双层
离线支持❌✅（数据库缓存）
典型用途纯网络分页列表需离线可用

Q2: cachedIn(viewModelScope) 的作用？​

答案：

将 PagingData 缓存在 ViewModel 中，配置变更（旋转）后恢复数据而无需重新加载。如果不调用 cachedIn，每次订阅都会重新从首页加载。

相关链接​