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Android Paging 分页加载库详解与实践

news 2025/7/26 11:36:14

前言

在现代移动应用开发中，处理大量数据并实现流畅的用户体验是一个常见需求。Android Paging 库正是为解决这一问题而生，它帮助开发者轻松实现数据的分页加载和显示。本文将深入探讨 Paging 库的核心概念、架构组件以及实际应用。

一、Paging 库概述

Android Paging 库是 Jetpack 组件的一部分，它提供了一套完整的解决方案来处理大型数据集的分页加载。主要优势包括：

内存高效：按需加载数据，减少内存消耗
无缝体验：支持列表的平滑滚动
可配置性：支持自定义数据源和加载策略
与RecyclerView深度集成：简化列表展示逻辑
支持协程和RxJava：与现代异步编程范式完美结合

二、Paging 3 核心组件

Paging 3 是当前最新版本，相比之前版本有显著改进，主要包含以下核心组件：

1. PagingSource

PagingSource 是数据加载的核心抽象类，负责定义如何按页获取数据：

class MyPagingSource(private val apiService: ApiService) : PagingSource<Int, User>() {override suspend fun load(params: LoadParams<Int>): LoadResult<Int, User> {return try {val page = params.key ?: 1val response = apiService.getUsers(page, params.loadSize)LoadResult.Page(data = response.users,prevKey = if (page == 1) null else page - 1,nextKey = if (response.isLastPage) null else page + 1)} catch (e: Exception) {LoadResult.Error(e)}}
}

2. Pager

Pager 是生成 PagingData 流的类，配置了如何获取分页数据：

val pager = Pager(config = PagingConfig(pageSize = 20,enablePlaceholders = false,initialLoadSize = 40),pagingSourceFactory = { MyPagingSource(apiService) }
)

3. PagingData

PagingData 是一个容器，持有分页加载的数据流，可以与 UI 层进行交互。

4. PagingDataAdapter

专为 RecyclerView 设计的适配器，用于显示分页数据：

class UserAdapter : PagingDataAdapter<User, UserViewHolder>(USER_COMPARATOR) {override fun onBindViewHolder(holder: UserViewHolder, position: Int) {val user = getItem(position)user?.let { holder.bind(it) }}override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {return UserViewHolder.create(parent)}companion object {private val USER_COMPARATOR = object : DiffUtil.ItemCallback<User>() {override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {return oldItem.id == newItem.id}override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {return oldItem == newItem}}}
}

三、Paging 库的完整实现流程

1. 添加依赖

首先在 build.gradle 中添加依赖：

dependencies {def paging_version = "3.1.1"implementation "androidx.paging:paging-runtime:$paging_version"// 可选 - RxJava支持implementation "androidx.paging:paging-rxjava2:$paging_version"// 可选 - Guava ListenableFuture支持implementation "androidx.paging:paging-guava:$paging_version"// 协程支持implementation "androidx.paging:paging-compose:1.0.0-alpha18"
}

2. 数据层实现

// 定义数据源
class UserPagingSource(private val apiService: ApiService) : PagingSource<Int, User>() {override fun getRefreshKey(state: PagingState<Int, User>): 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, User> {return try {val page = params.key ?: 1val response = apiService.getUsers(page, params.loadSize)LoadResult.Page(data = response.users,prevKey = if (page == 1) null else page - 1,nextKey = if (response.isLastPage) null else page + 1)} catch (e: Exception) {LoadResult.Error(e)}}
}// 定义Repository
class UserRepository(private val apiService: ApiService) {fun getUsers() = Pager(config = PagingConfig(pageSize = 20,enablePlaceholders = false,initialLoadSize = 40),pagingSourceFactory = { UserPagingSource(apiService) }).flow
}

3. ViewModel 层实现

class UserViewModel(private val repository: UserRepository) : ViewModel() {val users = repository.getUsers().cachedIn(viewModelScope)
}

4. UI 层实现

class UserActivity : AppCompatActivity() {private lateinit var binding: ActivityUserBindingprivate lateinit var viewModel: UserViewModelprivate val adapter = UserAdapter()override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)binding = ActivityUserBinding.inflate(layoutInflater)setContentView(binding.root)viewModel = ViewModelProvider(this).get(UserViewModel::class.java)binding.recyclerView.layoutManager = LinearLayoutManager(this)binding.recyclerView.adapter = adapterlifecycleScope.launch {repeatOnLifecycle(Lifecycle.State.STARTED) {viewModel.users.collectLatest {adapter.submitData(it)}}}}
}

四、高级功能与最佳实践

1. 添加加载状态监听

lifecycleScope.launch {adapter.loadStateFlow.collectLatest { loadStates ->binding.swipeRefresh.isRefreshing = loadStates.refresh is LoadState.Loadingwhen (val refresh = loadStates.refresh) {is LoadState.Error -> {// 显示错误showError(refresh.error)}// 其他状态处理}when (val append = loadStates.append) {is LoadState.Error -> {// 显示加载更多错误showLoadMoreError(append.error)}// 其他状态处理}}
}

2. 实现下拉刷新

binding.swipeRefresh.setOnRefreshListener {adapter.refresh()
}

3. 添加分隔符和加载更多指示器

binding.recyclerView.addItemDecoration(DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
)binding.recyclerView.adapter = adapter.withLoadStateHeaderAndFooter(header = LoadStateAdapter { adapter.retry() },footer = LoadStateAdapter { adapter.retry() }
)

4. 数据库与网络结合 (RemoteMediator)

@ExperimentalPagingApi
class UserRemoteMediator(private val database: AppDatabase,private val apiService: ApiService
) : RemoteMediator<Int, User>() {override suspend fun load(loadType: LoadType,state: PagingState<Int, User>): MediatorResult {return try {val loadKey = when (loadType) {LoadType.REFRESH -> nullLoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)LoadType.APPEND -> {val lastItem = state.lastItemOrNull()if (lastItem == null) {return MediatorResult.Success(endOfPaginationReached = true)}lastItem.id}}val response = apiService.getUsers(loadKey, state.config.pageSize)database.withTransaction {if (loadType == LoadType.REFRESH) {database.userDao().clearAll()}database.userDao().insertAll(response.users)}MediatorResult.Success(endOfPaginationReached = response.isLastPage)} catch (e: Exception) {MediatorResult.Error(e)}}
}