定義
- Google 推出的一個應用于 Android 平臺的分頁加載庫；
- Paging3和之前版本相差很多，完全可以當成一個新庫去學習
- 之前我們使用ListView和RecyclerView實現分頁功能并不難，那么為啥需要paging3呢？
- 它提供了一套非常合理的分頁架構，我們只需要按照它提供的架構去編寫業務邏輯，就可以輕松實現分頁功能;
- 關聯知識點：協程、Flow、MVVM、RecyclerView、DiffUtil
優點
- 使用內存緩存數據；
- 內置請求去重，更有效率的顯示數據；
- RecyclerView自動加載更多
- 支持Kotlin的協程和Flow，以及LiveData和RxJava2
- 內置狀態處理：刷新，錯誤，加載等
使用流程如下：
需求：
- 展示GitHub上所有Android相關的開源庫，以Star數量排序，每頁返回5條數據；
1. 引入依賴
//paging3
implementation 'androidx.paging:paging-runtime-ktx:3.0.0-beta03'
// 用于測試
testImplementation "androidx.paging:paging-common-ktx:3.0.0-beta03"
// [可選] RxJava 支持
implementation "androidx.paging:paging-rxjava2-ktx:3.0.0-beta03"
//retrofit網絡請求庫
implementation 'com.squareup.retrofit2:retrofit:2.9.0'
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
//下拉刷新
implementation 'androidx.swiperefreshlayout:swiperefreshlayout:1.1.0'
2. 創建數據模型類 RepoResponse
class RepoResponse {
@SerializedName("items") val items:List<Repo> = emptyList()
}
data class Repo(
@SerializedName("id") val id: Int,
@SerializedName("name") val name: String,
@SerializedName("description") val description: String,
@SerializedName("stargazers_count") val starCount: String,
)
3. 定義網絡請求接口 ApiService
interface ApiService {
@GET("search/repositories?sort=stars&q=Android")
suspend fun searRepos(@Query("page") page: Int, @Query("per_page") perPage: Int): RepoResponse
companion object {
private const val BASE_URL = "https://api.github.com/"
fun create(): ApiService {
return Retrofit.Builder()
.baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.build()
.create(ApiService::class.java)
}
}
}
4. 配置數據源
- 自定義一個子類繼承PagingSource，然后重寫 load() 函數，并在這里提供對應當前頁數的數據, 這一步才真正用到了Paging3
- PagingSource的兩個泛型參數，一個是頁數類型，一個是數據item類型
class RepoPagingSource(private val apiService: ApiService) : PagingSource<Int, Repo>() {
override fun getRefreshKey(state: PagingState<Int, Repo>): Int? {
return null
}
override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Repo> {
return try {
val page = params.key ?: 1
val pageSize = params.loadSize
val repoResponse = apiService.searRepos(page, pageSize)
val repoItems = repoResponse.items
val prevKey = if (page > 1) page - 1 else null
val nextKey = if (repoItems.isNotEmpty()) page + 1 else null
LoadResult.Page(repoItems, prevKey, nextKey)
} catch (e: Exception) {
LoadResult.Error(e)
}
}
}
5. 在ViewModel中實現接口請求
- PagingConfig的一個參數prefetchDistance，用于表示距離底部多少條數據開始預加載，設置0則表示滑到底部才加載，默認值為分頁大??；若要讓用戶對加載無感，適當增加預取閾值即可，比如調整到分頁大小的5倍；
- cachedIn() 是 Flow<PagingData> 的擴展方法，用于將服務器返回的數據在viewModelScope這個作用域內進行緩存，假如手機橫豎屏發生了旋轉導致Activity重新創建，Paging 3就可以直接讀取緩存中的數據，而不用重新發起網絡請求了。
//1. Repository中實現網絡請求
object Repository {
private const val PAGE_SIZE = 5
private val gitHubService = ApiService.create()
fun getPagingData(): Flow<PagingData<Repo>> {
// PagingConfig的一個參數prefetchDistance，用于表示距離底部多少條數據開始預加載，
// 設置0則表示滑到底部才加載。默認值為分頁大小。
// 若要讓用戶對加載無感，適當增加預取閾值即可。 比如調整到分頁大小的5倍
return Pager(config = PagingConfig(pageSize = PAGE_SIZE, prefetchDistance = PAGE_SIZE * 5),
pagingSourceFactory = { RepoPagingSource(gitHubService) }).flow
}
}
//2. ViewModel中調用Repository
class Paging3ViewModel : ViewModel() {
fun getPagingData(): Flow<PagingData<Repo>> {
return Repository.getPagingData().cachedIn(viewModelScope)
}
}
6. 實現RecyclerView的Adapter
class RepoAdapter : PagingDataAdapter<Repo, RepoAdapter.ViewHolder>(COMPARATOR) {
companion object {
//因為Paging 3在內部會使用DiffUtil來管理數據變化，所以這個COMPARATOR是必須的
private val COMPARATOR = object : DiffUtil.ItemCallback<Repo>() {
override fun areItemsTheSame(oldItem: Repo, newItem: Repo): Boolean {
return oldItem.id == newItem.id
}
override fun areContentsTheSame(oldItem: Repo, newItem: Repo): Boolean {
return oldItem == newItem
}
}
}
class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView){
val binding: LayoutRepoItemBinding? =DataBindingUtil.bind(itemView)
}
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
holder.binding?.repo=getItem(position)
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
val view=LayoutInflater.from(parent.context).inflate(R.layout.layout_repo_item,parent,false)
return ViewHolder(view)
}
}
7. FooterAdapter的實現
- 用于實現加載更多，必須繼承自LoadStateAdapter，
- retry():使用Kotlin的高階函數來給重試按鈕注冊點擊事件
class FooterAdapter(val retry: () -> Unit) : LoadStateAdapter<FooterAdapter.ViewHolder>() {
class ViewHolder(val binding: ViewDataBinding) : RecyclerView.ViewHolder(binding.root)
override fun onBindViewHolder(holder: ViewHolder, loadState: LoadState) {
val binding=holder.binding as LayoutFooterItemBinding
when (loadState) {
is LoadState.Error -> {
binding.progressBar.visibility = View.GONE
binding.retryButton.visibility = View.VISIBLE
binding.retryButton.text = "Load Failed, Tap Retry"
binding.retryButton.setOnClickListener {
retry()
}
}
is LoadState.Loading -> {
binding.progressBar.visibility = View.VISIBLE
binding.retryButton.visibility = View.VISIBLE
binding.retryButton.text = "Loading"
}
is LoadState.NotLoading -> {
binding.progressBar.visibility = View.GONE
binding.retryButton.visibility = View.GONE
}
}
}
override fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): ViewHolder {
val binding: LayoutFooterItemBinding =
LayoutFooterItemBinding.inflate(
LayoutInflater.from(parent.context), parent, false
)
return ViewHolder(binding)
}
}
8. 在Activity中使用
- mAdapter.submitData()是觸發Paging 3分頁功能的核心; 它接收一個PagingData參數，這個參數我們需要調用ViewModel中返回的Flow對象的collect()函數才能獲取到，collect()函數有點類似于Rxjava中的subscribe()函數，總之就是訂閱了之后，消息就會源源不斷往這里傳。不過由于collect()函數是一個掛起函數，只有在協程作用域中才能調用它，因此這里又調用了lifecycleScope.launch()函數來啟動一個協程。
- 加載更多：通過mAdapter.withLoadStateFooter實現;
- 下拉刷新：這里下來刷新是配合SwipeRefreshLayout使用，在其OnRefreshListener中調用mAdapter.refresh(),并在mAdapter.addLoadStateListener中處理下拉刷新的UI邏輯；
- 雖然有withLoadStateHeader，但它并不是用于實現刷新，而是加載上一頁，需要當前起始頁>1時才生效
class Paging3Activity : AppCompatActivity() {
private val viewModel by lazy {
ViewModelProvider(this).get(Paging3ViewModel::class.java)
}
private val mAdapter:RepoAdapter = RepoAdapter()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
//在Activity中使用
val binding: ActivityPaging3Binding =
DataBindingUtil.setContentView(this, R.layout.activity_paging3)
binding.lifecycleOwner = this
//下拉刷新
binding.refreshlayout.setOnRefreshListener {
mAdapter.refresh()
}
binding.recyclerView.layoutManager = LinearLayoutManager(this)
//添加footer
binding.recyclerView.adapter = mAdapter.withLoadStateFooter(FooterAdapter {
mAdapter.retry()
})
// binding.recyclerView.adapter = repoAdapter.withLoadStateHeaderAndFooter(
// header = HeaderAdapter { repoAdapter.retry() },
// footer = FooterAdapter { repoAdapter.retry() }
// )
lifecycleScope.launch {
viewModel.getPagingData().collect {
mAdapter.submitData(it)
}
}
//監聽加載狀態
mAdapter.addLoadStateListener {
//比如處理下拉刷新邏輯
when (it.refresh) {
is LoadState.NotLoading -> {
binding.recyclerView.visibility = View.VISIBLE
binding.refreshlayout.isRefreshing = false
}
is LoadState.Loading -> {
binding.refreshlayout.isRefreshing = true
binding.recyclerView.visibility = View.VISIBLE
}
is LoadState.Error -> {
val state = it.refresh as LoadState.Error
binding.refreshlayout.isRefreshing = false
Toast.makeText(this, "Load Error: ${state.error.message}", Toast.LENGTH_SHORT)
.show()
}
}
}
}
}
9. RemoteMediator
RemoteMediator 和 PagingSource 的區別：
- PagingSource：實現單一數據源以及如何從該數據源中查找數據，推薦用于加載有限的數據集（本地數據庫），例如 Room，數據源的變動會直接映射到 UI 上；
- RemoteMediator：實現加載網絡分頁數據并更新到數據庫中，但是數據源的變動不能直接映射到 UI 上；
- 可以使用 RemoteMediator 實現從網絡加載分頁數據更新到數據庫中，使用 PagingSource 從數據庫中查找數據并顯示在 UI 上
RemoteMediator的使用
- 定義數據源
// 本地數據庫存儲使用的Room，Room使用相關的之后會在另一篇文章中詳細介紹，這里直接貼代碼了
//1. 定義實體類，并添加@Entity注釋
@Entity
data class RepoEntity(
@PrimaryKey val id: Int,
@ColumnInfo(name = "name") val name: String,
@ColumnInfo(name = "description") val description: String,
@ColumnInfo(name = "star_count") val starCount: String,
@ColumnInfo(name = "page") val page: Int ,
)
//2. 定義數據訪問對象RepoDao
@Dao
interface RepoDao {
@Insert(onConflict = OnConflictStrategy.REPLACE)
suspend fun insert(pokemonList: List<RepoEntity>)
@Query("SELECT * FROM RepoEntity")
fun get(): PagingSource<Int, RepoEntity>
@Query("DELETE FROM RepoEntity")
suspend fun clear()
@Delete
fun delete(repo: RepoEntity)
@Update
fun update(repo: RepoEntity)
}
//3. 定義Database
@Database(entities = [RepoEntity::class], version = Constants.DB_VERSION)
abstract class AppDatabase : RoomDatabase() {
abstract fun repoDao(): RepoDao
companion object {
val instance = AppDatabaseHolder.db
}
private object AppDatabaseHolder {
val db: AppDatabase = Room
.databaseBuilder(
AppHelper.mContext,
AppDatabase::class.java,
Constants.DB_NAME
)
.allowMainThreadQueries() //允許在主線程中查詢
.build()
}
}
//4. 數據庫常量管理
interface Constants {
/**
* 數據庫名稱
*/
String DB_NAME = "JetpackDemoDataBase.db";
/**
* 數據庫版本
*/
int DB_VERSION = 1;
}
- 實現 RemoteMediator
// 1. RemoteMediator 目前是實驗性的 API ，所有實現 RemoteMediator 的類
//都需要添加 @OptIn(ExperimentalPagingApi::class) 注解,
//使用 OptIn 注解，要App的build.gradle中配置
android {
kotlinOptions {
freeCompilerArgs += ["-Xopt-in=kotlin.RequiresOptIn"]
}
}
//2. 自定義RepoMediator，繼承RemoteMediator
//RemoteMediator 和 PagingSource 相似，都需要覆蓋 load() 方法，但是其參數不同
@OptIn(ExperimentalPagingApi::class)
class RepoMediator(
val api: ApiService,
val db: AppDatabase
) : RemoteMediator<Int, RepoEntity>() {
override suspend fun load(
loadType: LoadType,
state: PagingState<Int, RepoEntity>
): MediatorResult {
val repoDao = db.repoDao()
val pageKey = when (loadType) {
//首次訪問 或者調用 PagingDataAdapter.refresh()時
LoadType.REFRESH -> null
//在當前加載的數據集的開頭加載數據時
LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)
//下拉加載更多時
LoadType.APPEND -> {
val lastItem = state.lastItemOrNull()
if (lastItem == null) {
return MediatorResult.Success(
endOfPaginationReached = true
)
}
lastItem.page
}
}
//無網絡則加載本地數據
if (!AppHelper.mContext.isConnectedNetwork()) {
return MediatorResult.Success(endOfPaginationReached = true)
}
//請求網絡分頁數據
val page = pageKey ?: 0
val pageSize = Repository.PAGE_SIZE
val result = api.searRepos(page, pageSize).items
val endOfPaginationReached = result.isEmpty()
val items = result.map {
RepoEntity(
id = it.id,
name = it.name,
description = it.description,
starCount = it.starCount,
page=page + 1
)
}
//插入數據庫
db.withTransaction {
if (loadType==LoadType.REFRESH){
repoDao.clear()
}
repoDao.insert(items)
}
return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
}
}
- 在 Repository 中構建 Pager
object Repository {
const val PAGE_SIZE = 5
private val gitHubService = ApiService.create()
private val db = AppDatabase.instance
private val pagingConfig = PagingConfig(
// 每頁顯示的數據的大小
pageSize = PAGE_SIZE,
// 開啟占位符
enablePlaceholders = true,
// 預刷新的距離，距離最后一個 item 多遠時加載數據
// 默認為 pageSize
prefetchDistance = PAGE_SIZE,
// 初始化加載數量，默認為 pageSize * 3
initialLoadSize = PAGE_SIZE
)
@OptIn(ExperimentalPagingApi::class)
fun getPagingData2(): Flow<PagingData<Repo>> {
return Pager(
config = pagingConfig,
remoteMediator = RepoMediator(gitHubService, db)
) {
db.repoDao().get()
}.flow.map { pagingData ->
pagingData.map { RepoEntity2RepoMapper().map(it) }
}
}
}
class RepoEntity2RepoMapper : Mapper<RepoEntity, Repo> {
override fun map(input: RepoEntity): Repo = Repo(
id = input.id,
name = input.name,
description = input.description,
starCount = input.starCount
)
}
- 在 ViewModel 獲取數據
class Paging3ViewModel : ViewModel() {
fun getPagingData2(): LiveData<PagingData<Repo>> =
Repository.getPagingData2().cachedIn(viewModelScope).asLiveData()
}
- 在Activity中注冊觀察者
viewModel.getPagingData2().observe(this, {
mAdapter.submitData(lifecycle, it)
})
- 到此打完收工，跑一下代碼，發現無網絡情況下就會加載數據庫中的數據，有網絡就會從網絡請求數據更新數據庫并刷新UI界面
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