6. Jetpack---Paging你知道怎样上拉加载吗?
作者:Hankkin
链接:https://juejin.im/post/5d63f620f265da03970bc76d
声明:本文已获Hankkin
投稿发表,转发等请联系原作者授权
之前的几篇源码分析我们分别对
Navigation
、Lifecycles
、ViewModel
、LiveData
、进行了分析,也对JetPack有了更深入的了解。但是Jetpack远不止这些组件,今天的主角---Paging,Jetpack中的分页组件,官方是这么形容它的:'’逐步从您的数据源按需加载信息'’
如果你对Jetpack组件有了解或者想对源码有更深入的了解,请看我之前的几篇文章:
5. Jetpack源码解析--ViewModel基本使用及源码解析
3. Jetpack源码解析---用Lifecycles管理生命周期
1. 背景
在我的Jetpack_Note系列中,对每一篇的分析都有相对应的代码片段及使用,我把它做成了一个APP,目前功能还不完善,代码我也上传到了GitHub上,参考了官方的Demo以及目前网上的一些文章,有兴趣的小伙伴可以看一下,别忘了给个Star。
https://github.com/Hankkin/JetPack_Note
今天我们的主角是Paging,介绍之前我们先看一下效果:
2. 简介
2.1 基本介绍
官方定义:
分页库Pagin Library是Jetpack的一部分,它可以妥善的逐步加载数据,帮助您一次加载和显示一部分数据,这样的按需加载可以减少网络贷款和系统资源的使用。分页库支持加载有限以及无限的list,比如一个持续更新的信息源,分页库可以与RecycleView无缝集合,它还可以与LiveData或RxJava集成,观察界面中的数据变化。
2.2 核心组件
1. PagedList
PageList是一个集合类,它以分块的形式异步加载数据,每一块我们称之为页。它继承自AbstractList
,支持所有List的操作,它的内部有五个主要变量:
mMainThreadExecutor 主线程Executor,用于将结果传递到主线程
mBackgroundThreadExecutor 后台线程,执行负载业务逻辑
BoundaryCallback 当界面显示缓存中靠近结尾的数据的时候,它将加载更多的数据
Config PageList从DataSource中加载数据的配置
PagedStorage
用于存储加载到的数据
Config属性:
pageSize:分页加载的数量
prefetchDistance:预加载的数量
initialLoadSizeHint:初始化数据时加载的数量,默认为pageSize*3
enablePlaceholders:当item为null是否使用placeholder显示
PageList会通过DataSource加载数据,通过Config的配置,可以设置一次加载的数量以及预加载的数量。除此之外,PageList还可以想RecycleView.Adapter发送更新的信号,驱动UI的刷新。
2. DataSource
DataSource顾名思义就是数据源,它是一个抽象类,其中Key
对应加载数据的条件信息,Value
对应加载数据的实体类。Paging库中提供了三个子类来让我们在不同场景的情况下使用:
PageKeyedDataSource:如果后端API返回数据是分页之后的,可以使用它;例如:官方Demo中GitHub API中的SearchRespositories就可以返回分页数据,我们在GitHub API的请求中制定查询关键字和想要的哪一页,同时也可以指明每个页面的项数。
ItemKeyedDataSource:如果通过键值请求后端数据;例如我们需要获取在某个特定日期起Github的前100项代码提交记录,该日期将成为DataSource的键,ItemKeyedDataSource允许自定义如何加载初始页;该场景多用于评论信息等类似请求
PositionalDataSource:适用于目标数据总数固定,通过特定的位置加载数据,这里Key是Integer类型的位置信息,T即Value。比如从数据库中的1200条开始加在20条数据。
3. PagedListAdapter
PageListAdapter继承自RecycleView.Adapter,和RecycleView实现方式一样,当数据加载完毕时,通知RecycleView数据加载完毕,RecycleView填充数据;当数据发生变化时,PageListAdapter会接受到通知,交给委托类AsyncPagedListDiffer来处理,AsyncPagedListDiffer是对DiffUtil.ItemCallback持有对象的委托类,AsyncPagedListDiffer使用后台线程来计算PagedList的改变,item是否改变,由DiffUtil.ItemCallback决定。
3.基本使用
3.1 添加依赖包
implementation "androidx.paging:paging-runtime:$paging_version" // For Kotlin use paging-runtime-ktx
implementation "androidx.paging:paging-runtime-ktx:$paging_version" // For Kotlin use paging-runtime-ktx
// alternatively - without Android dependencies for testing
testImplementation "androidx.paging:paging-common:$paging_version" // For Kotlin use paging-common-ktx
// optional - RxJava support
implementation "androidx.paging:paging-rxjava2:$paging_version" // For Kotlin use paging-rxjava2-ktx
3.2 PagingWithRoom使用
新建UserDao
/**
* created by Hankkin
* on 2019-07-19
*/
@Dao
interface UserDao {
@Query("SELECT * FROM User ORDER BY name COLLATE NOCASE ASC")
fun queryUsersByName(): DataSource.Factory<Int, User>
@Insert
fun insert(users: List<User>)
@Insert
fun insert(user: User)
@Delete
fun delete(user: User)
}
创建UserDB数据库
/**
* created by Hankkin
* on 2019-07-19
*/
@Database(entities = arrayOf(User::class), version = 1)
abstract class UserDB : RoomDatabase() {
abstract fun userDao(): UserDao
companion object {
private var instance: UserDB? = null
@Synchronized
fun get(context: Context): UserDB {
if (instance == null) {
instance = Room.databaseBuilder(context.applicationContext,
UserDB::class.java, "UserDatabase")
.addCallback(object : RoomDatabase.Callback() {
override fun onCreate(db: SupportSQLiteDatabase) {
fillInDb(context.applicationContext)
}
}).build()
}
return instance!!
}
/**
* fill database with list of cheeses
*/
private fun fillInDb(context: Context) {
// inserts in Room are executed on the current thread, so we insert in the background
ioThread {
get(context).userDao().insert(
CHEESE_DATA.map { User(id = 0, name = it) })
}
}
}
}
创建PageListAdapter
/**
* created by Hankkin
* on 2019-07-19
*/
class PagingDemoAdapter : PagedListAdapter<User, PagingDemoAdapter.ViewHolder>(diffCallback) {
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) =
ViewHolder(AdapterPagingItemBinding.inflate(LayoutInflater.from(parent.context), parent, false))
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val item = getItem(position)
holder.apply {
bind(createOnClickListener(item), item)
itemView.tag = item
}
}
private fun createOnClickListener(item: User?): View.OnClickListener {
return View.OnClickListener {
Toast.makeText(it.context, item?.name, Toast.LENGTH_SHORT).show()
}
}
class ViewHolder(private val binding: AdapterPagingItemBinding) : RecyclerView.ViewHolder(binding.root) {
fun bind(listener: View.OnClickListener, item: User?) {
binding.apply {
clickListener = listener
user = item
executePendingBindings()
}
}
}
companion object {
/**
* This diff callback informs the PagedListAdapter how to compute list differences when new
* PagedLists arrive.
* <p>
* When you add a Cheese with the 'Add' button, the PagedListAdapter uses diffCallback to
* detect there's only a single item difference from before, so it only needs to animate and
* rebind a single view.
*
* @see android.support.v7.util.DiffUtil
*/
private val diffCallback = object : DiffUtil.ItemCallback<User>() {
override fun areItemsTheSame(oldItem: User, newItem: User): Boolean =
oldItem.id == newItem.id
/**
* Note that in kotlin, == checking on data classes compares all contents, but in Java,
* typically you'll implement Object#equals, and use it to compare object contents.
*/
override fun areContentsTheSame(oldItem: User, newItem: User): Boolean =
oldItem == newItem
}
}
}
ViewModel承载数据
class PagingWithDaoViewModel internal constructor(private val pagingRespository: PagingRespository) : ViewModel() {
val allUsers = pagingRespository.getAllUsers()
fun insert(text: CharSequence) {
pagingRespository.insert(text)
}
fun remove(user: User) {
pagingRespository.remove(user)
}
}
Activity中观察到数据源的变化后,会通知Adapter自动更新数据
class PagingWithDaoActivity : AppCompatActivity() {
private lateinit var viewModel: PagingWithDaoViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_paging_with_dao)
setLightMode()
setupToolBar(toolbar) {
title = resources.getString(R.string.paging_with_dao)
setDisplayHomeAsUpEnabled(true)
}
viewModel = obtainViewModel(PagingWithDaoViewModel::class.java)
val adapter = PagingDemoAdapter()
rv_paging.adapter = adapter
viewModel.allUsers.observe(this, Observer(adapter::submitList))
}
override fun onOptionsItemSelected(item: MenuItem?): Boolean {
when (item?.itemId) {
android.R.id.home -> finish()
}
return super.onOptionsItemSelected(item)
}
}
3.3 PagingWithNetWork 使用
上面我们通过Room进行了数据库加载数据,下面看一下通过网络请求记载列表数据:
和上面不同的就是Respository数据源的加载,之前我们是通过Room加载DB数据,现在我们要通过网络获取数据:
GankRespository 干货数据源仓库
/**
* created by Hankkin
* on 2019-07-30
*/
class GankRespository {
companion object {
private const val PAGE_SIZE = 20
@Volatile
private var instance: GankRespository? = null
fun getInstance() =
instance ?: synchronized(this) {
instance
?: GankRespository().also { instance = it }
}
}
fun getGank(): Listing<Gank> {
val sourceFactory = GankSourceFactory()
val config = PagedList.Config.Builder()
.setPageSize(PAGE_SIZE)
.setInitialLoadSizeHint(PAGE_SIZE * 2)
.setEnablePlaceholders(false)
.build()
val livePageList = LivePagedListBuilder<Int, Gank>(sourceFactory, config).build()
val refreshState = Transformations.switchMap(sourceFactory.sourceLiveData) { it.initialLoad }
return Listing(
pagedList = livePageList,
networkState = Transformations.switchMap(sourceFactory.sourceLiveData) { it.netWorkState },
retry = { sourceFactory.sourceLiveData.value?.retryAllFailed() },
refresh = { sourceFactory.sourceLiveData.value?.invalidate() },
refreshState = refreshState
)
}
}
可以看到getGank()方法返回了Listing,那么Listing
是个什么呢?
/**
* Data class that is necessary for a UI to show a listing and interact w/ the rest of the system
* 封装需要监听的对象和执行的操作,用于上拉下拉操作
* pagedList : 数据列表
* networkState : 网络状态
* refreshState : 刷新状态
* refresh : 刷新操作
* retry : 重试操作
*/
data class Listing<T>(
// the LiveData of paged lists for the UI to observe
val pagedList: LiveData<PagedList<T>>,
// represents the network request status to show to the user
val networkState: LiveData<NetworkState>,
// represents the refresh status to show to the user. Separate from networkState, this
// value is importantly only when refresh is requested.
val refreshState: LiveData<NetworkState>,
// refreshes the whole data and fetches it from scratch.
val refresh: () -> Unit,
// retries any failed requests.
val retry: () -> Unit)
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