6. Jetpack---Paging你知道怎樣上拉加載嗎鸵膏？

之前的幾篇源碼分析我們分別對(duì)Navigation租副、Lifecycles、ViewModel较性、LiveData用僧、進(jìn)行了分析，也對(duì)JetPack有了更深入的了解赞咙。但是Jetpack遠(yuǎn)不止這些組件责循，今天的主角---Paging，Jetpack中的分頁(yè)組件攀操，官方是這么形容它的：‘’逐步從您的數(shù)據(jù)源按需加載信息‘’

如果你對(duì)Jetpack組件有了解或者想對(duì)源碼有更深入的了解院仿，請(qǐng)看我之前的幾篇文章：

1. Jetpack源碼解析---看完你就知道Navigation是什么了？

2. Jetpack源碼解析---Navigation為什么切換Fragment會(huì)重繪速和？

3. Jetpack源碼解析---用Lifecycles管理生命周期

4. Jetpack源碼解析—LiveData的使用及工作原理

5. Jetpack源碼解析---ViewModel基本使用及源碼解析

1. 背景

在我的Jetpack_Note系列中歹垫，對(duì)每一篇的分析都有相對(duì)應(yīng)的代碼片段及使用，我把它做成了一個(gè)APP颠放，目前功能還不完善排惨，代碼我也上傳到了GitHub上，參考了官方的Demo以及目前網(wǎng)上的一些文章碰凶，有興趣的小伙伴可以看一下暮芭，別忘了給個(gè)Star。

https://github.com/Hankkin/JetPack_Note

今天我們的主角是Paging欲低，介紹之前我們先看一下效果：

19-08-10-11-18-59.2019-08-10 11_33_51

2. 簡(jiǎn)介

2.1 基本介紹

官方定義：

分頁(yè)庫(kù)Pagin Library是Jetpack的一部分辕宏，它可以妥善的逐步加載數(shù)據(jù)，幫助您一次加載和顯示一部分?jǐn)?shù)據(jù)砾莱，這樣的按需加載可以減少網(wǎng)絡(luò)貸款和系統(tǒng)資源的使用瑞筐。分頁(yè)庫(kù)支持加載有限以及無(wú)限的list，比如一個(gè)持續(xù)更新的信息源腊瑟，分頁(yè)庫(kù)可以與RecycleView無(wú)縫集合聚假，它還可以與LiveData或RxJava集成块蚌，觀察界面中的數(shù)據(jù)變化。

image

2.2 核心組件

1. PagedList

PageList是一個(gè)集合類(lèi)魔策，它以分塊的形式異步加載數(shù)據(jù)，每一塊我們稱(chēng)之為頁(yè)河胎。它繼承自AbstractList,支持所有List的操作闯袒，它的內(nèi)部有五個(gè)主要變量：

mMainThreadExecutor 主線程Executor，用于將結(jié)果傳遞到主線程
mBackgroundThreadExecutor 后臺(tái)線程游岳，執(zhí)行負(fù)載業(yè)務(wù)邏輯
BoundaryCallback 當(dāng)界面顯示緩存中靠近結(jié)尾的數(shù)據(jù)的時(shí)候政敢，它將加載更多的數(shù)據(jù)
Config PageList從DataSource中加載數(shù)據(jù)的配置
PagedStorage<T> 用于存儲(chǔ)加載到的數(shù)據(jù)

Config屬性：

pageSize：分頁(yè)加載的數(shù)量
prefetchDistance：預(yù)加載的數(shù)量
initialLoadSizeHint：初始化數(shù)據(jù)時(shí)加載的數(shù)量，默認(rèn)為pageSize*3
enablePlaceholders：當(dāng)item為null是否使用placeholder顯示

PageList會(huì)通過(guò)DataSource加載數(shù)據(jù)胚迫，通過(guò)Config的配置喷户，可以設(shè)置一次加載的數(shù)量以及預(yù)加載的數(shù)量。除此之外访锻，PageList還可以想RecycleView.Adapter發(fā)送更新的信號(hào)褪尝，驅(qū)動(dòng)UI的刷新。

2. DataSource

DataSource<Key,Value> 顧名思義就是數(shù)據(jù)源期犬，它是一個(gè)抽象類(lèi)河哑，其中Key對(duì)應(yīng)加載數(shù)據(jù)的條件信息，Value對(duì)應(yīng)加載數(shù)據(jù)的實(shí)體類(lèi)龟虎。Paging庫(kù)中提供了三個(gè)子類(lèi)來(lái)讓我們?cè)诓煌瑘?chǎng)景的情況下使用：

PageKeyedDataSource：如果后端API返回?cái)?shù)據(jù)是分頁(yè)之后的璃谨，可以使用它；例如：官方Demo中GitHub API中的SearchRespositories就可以返回分頁(yè)數(shù)據(jù)鲤妥，我們?cè)贕itHub API的請(qǐng)求中制定查詢(xún)關(guān)鍵字和想要的哪一頁(yè)佳吞，同時(shí)也可以指明每個(gè)頁(yè)面的項(xiàng)數(shù)。
ItemKeyedDataSource：如果通過(guò)鍵值請(qǐng)求后端數(shù)據(jù)棉安；例如我們需要獲取在某個(gè)特定日期起Github的前100項(xiàng)代碼提交記錄底扳，該日期將成為DataSource的鍵,ItemKeyedDataSource允許自定義如何加載初始頁(yè)；該場(chǎng)景多用于評(píng)論信息等類(lèi)似請(qǐng)求
PositionalDataSource：適用于目標(biāo)數(shù)據(jù)總數(shù)固定贡耽，通過(guò)特定的位置加載數(shù)據(jù)花盐，這里Key是Integer類(lèi)型的位置信息，T即Value菇爪。比如從數(shù)據(jù)庫(kù)中的1200條開(kāi)始加在20條數(shù)據(jù)算芯。

3. PagedListAdapter

PageListAdapter繼承自RecycleView.Adapter,和RecycleView實(shí)現(xiàn)方式一樣，當(dāng)數(shù)據(jù)加載完畢時(shí)凳宙，通知RecycleView數(shù)據(jù)加載完畢熙揍，RecycleView填充數(shù)據(jù)；當(dāng)數(shù)據(jù)發(fā)生變化時(shí)氏涩，PageListAdapter會(huì)接受到通知届囚，交給委托類(lèi)AsyncPagedListDiffer來(lái)處理有梆，AsyncPagedListDiffer是對(duì)DiffUtil.ItemCallback<T>持有對(duì)象的委托類(lèi),AsyncPagedListDiffer使用后臺(tái)線程來(lái)計(jì)算PagedList的改變,item是否改變,由DiffUtil.ItemCallback<T>決定。

3.基本使用

3.1 添加依賴(lài)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)

}

創(chuàng)建UserDB數(shù)據(jù)庫(kù)

/**
 * 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) })
            }
        }
    }
}

創(chuàng)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承載數(shù)據(jù)

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中觀察到數(shù)據(jù)源的變化后意系，會(huì)通知Adapter自動(dòng)更新數(shù)據(jù)

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

上面我們通過(guò)Room進(jìn)行了數(shù)據(jù)庫(kù)加載數(shù)據(jù)泥耀，下面看一下通過(guò)網(wǎng)絡(luò)請(qǐng)求記載列表數(shù)據(jù)：

和上面不同的就是Respository數(shù)據(jù)源的加載，之前我們是通過(guò)Room加載DB數(shù)據(jù)蛔添，現(xiàn)在我們要通過(guò)網(wǎng)絡(luò)獲取數(shù)據(jù)：

GankRespository 干貨數(shù)據(jù)源倉(cāng)庫(kù)

/**
 * 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<Gank>,那么Listing是個(gè)什么呢痰催？

/**
 * Data class that is necessary for a UI to show a listing and interact w/ the rest of the system
 * 封裝需要監(jiān)聽(tīng)的對(duì)象和執(zhí)行的操作，用于上拉下拉操作
 * pagedList ： 數(shù)據(jù)列表
 * networkState ： 網(wǎng)絡(luò)狀態(tài)
 * refreshState ： 刷新?tīng)顟B(tài)
 * refresh ： 刷新操作
 * retry ： 重試操作
 */
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)

Listing是我們封裝的一個(gè)數(shù)據(jù)類(lèi)迎瞧，將數(shù)據(jù)源夸溶、網(wǎng)絡(luò)狀態(tài)、刷新?tīng)顟B(tài)凶硅、下拉刷新操作以及重試操作都封裝進(jìn)去了缝裁。那么我們的數(shù)據(jù)源從哪里獲取呢，可以看到Listing的第一個(gè)參數(shù)pageList = livePageList,livePageList通過(guò)LivePagedListBuilder創(chuàng)建足绅，LivePagedListBuilder需要兩個(gè)參數(shù)(DataSource,PagedList.Config)：

GankSourceFactory

/**
 * created by Hankkin
 * on 2019-07-30
 */
class GankSourceFactory(private val api: Api = Injection.provideApi()) : DataSource.Factory<Int, Gank>(){

    val sourceLiveData = MutableLiveData<GankDataSource>()

    override fun create(): DataSource<Int, Gank> {
        val source = GankDataSource(api)
        sourceLiveData.postValue(source)
        return  source
    }
}

GankDataSource


/**
 * created by Hankkin
 * on 2019-07-30
 */
class GankDataSource(private val api: Api = Injection.provideApi()) : PageKeyedDataSource<Int, Gank>() {

    private var retry: (() -> Any)? = null
    val netWorkState = MutableLiveData<NetworkState>()
    val initialLoad = MutableLiveData<NetworkState>()

    fun retryAllFailed() {
        val prevRetry = retry
        retry = null
        prevRetry?.also { it.invoke() }
    }

    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<Int, Gank>) {
        initialLoad.postValue(NetworkState.LOADED)
        netWorkState.postValue(NetworkState.HIDDEN)
        api.getGank(params.requestedLoadSize, 1)
            .enqueue(object : Callback<GankResponse> {
                override fun onFailure(call: Call<GankResponse>, t: Throwable) {
                    retry = {
                        loadInitial(params, callback)
                    }
                    initialLoad.postValue(NetworkState.FAILED)
                }

                override fun onResponse(call: Call<GankResponse>, response: Response<GankResponse>) {
                    if (response.isSuccessful) {
                        retry = null
                        callback.onResult(
                            response.body()?.results ?: emptyList(),
                            null,
                            2
                        )
                        initialLoad.postValue(NetworkState.LOADED)
                    } else {
                        retry = {
                            loadInitial(params, callback)
                        }
                        initialLoad.postValue(NetworkState.FAILED)
                    }
                }

            })
    }

    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<Int, Gank>) {
        netWorkState.postValue(NetworkState.LOADING)
        api.getGank(params.requestedLoadSize, params.key)
            .enqueue(object : Callback<GankResponse> {
                override fun onFailure(call: Call<GankResponse>, t: Throwable) {
                    retry = {
                        loadAfter(params, callback)
                    }
                    netWorkState.postValue(NetworkState.FAILED)
                }

                override fun onResponse(call: Call<GankResponse>, response: Response<GankResponse>) {
                    if (response.isSuccessful) {
                        retry = null
                        callback.onResult(
                            response.body()?.results ?: emptyList(),
                            params.key + 1
                        )
                        netWorkState.postValue(NetworkState.LOADED)
                    } else {
                        retry = {
                            loadAfter(params, callback)
                        }
                        netWorkState.postValue(NetworkState.FAILED)
                    }
                }

            })
    }

    override fun loadBefore(params: LoadParams<Int>, callback: LoadCallback<Int, Gank>) {
    }


}

網(wǎng)絡(luò)請(qǐng)求的核心代碼在GankDataSource中捷绑，因?yàn)槲覀兊恼?qǐng)求是分頁(yè)請(qǐng)求，所以這里的GankDataSource我們繼承自PageKeyedDataSource,它實(shí)現(xiàn)了三個(gè)方法:

loadInitial: 初始化加載氢妈，初始加載的數(shù)據(jù) 也就是我們直接能看見(jiàn)的數(shù)據(jù)

loadAfter: 下一頁(yè)加載胎食，每次傳遞的第二個(gè)參數(shù) 就是你加載數(shù)據(jù)依賴(lài)的key

loadBefore: 往上滑加載的數(shù)據(jù)

可以看到我們?cè)?code>loadInitial中設(shè)置了initialLoad和netWorkState的狀態(tài)值，同時(shí)通過(guò)RetrofitApi獲取網(wǎng)絡(luò)數(shù)據(jù)允懂，并在成功和失敗的回調(diào)中對(duì)數(shù)據(jù)和網(wǎng)絡(luò)狀態(tài)值以及加載初始化做了相關(guān)的設(shè)置厕怜，具體就不介紹了，可看代碼蕾总。loadAfter同理粥航，只不過(guò)我們?cè)诩虞d數(shù)據(jù)后對(duì)key也就是我們的page進(jìn)行了+1操作。

Config參數(shù)就是我們對(duì)分頁(yè)加載的一些配置：

val config = PagedList.Config.Builder()
            .setPageSize(PAGE_SIZE)
            .setInitialLoadSizeHint(PAGE_SIZE * 2)
            .setEnablePlaceholders(false)
            .build()

下面看我們?cè)贏ctivity中怎樣使用：

PagingWithNetWorkActivity

class PagingWithNetWorkActivity : AppCompatActivity() {

    private lateinit var mViewModel: PagingWithNetWorkViewModel
    private lateinit var mDataBinding: ActivityPagingWithNetWorkBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mDataBinding = DataBindingUtil.setContentView(this,R.layout.activity_paging_with_net_work)
        setLightMode()
        setupToolBar(toolbar) {
            title = resources.getString(R.string.paging_with_network)
            setDisplayHomeAsUpEnabled(true)
        }
        mViewModel = obtainViewModel(PagingWithNetWorkViewModel::class.java)
        mDataBinding.vm = mViewModel
        mDataBinding.lifecycleOwner = this

        val adapter = PagingWithNetWorkAdapter()
        mDataBinding.rvPagingWithNetwork.adapter = adapter
        mDataBinding.vm?.gankList?.observe(this, Observer { adapter.submitList(it) })
        mDataBinding.vm?.refreshState?.observe(this, Observer {
            mDataBinding.rvPagingWithNetwork.post {
                mDataBinding.swipeRefresh.isRefreshing = it == NetworkState.LOADING
            }
        })

        mDataBinding.vm?.netWorkState?.observe(this, Observer {
            adapter.setNetworkState(it)
        })
    }

    override fun onOptionsItemSelected(item: MenuItem?): Boolean {
        when (item?.itemId) {
            android.R.id.home -> finish()
        }
        return super.onOptionsItemSelected(item)
    }
}

ViewModel中的gankList是一個(gè)LiveData生百，所以我們?cè)谶@里給它設(shè)置一個(gè)觀察递雀，當(dāng)數(shù)據(jù)變動(dòng)是調(diào)用adapter.submitList(it),刷新數(shù)據(jù)，這個(gè)方法是PagedListAdapter中的蚀浆，里面回去檢查新數(shù)據(jù)和舊數(shù)據(jù)是否相同缀程，也就是上面我們提到的AsyncPagedListDiffer來(lái)實(shí)現(xiàn)的。到這里整個(gè)流程就已經(jīng)結(jié)束了市俊，想看源碼可以到Github上杨凑。

4. 總結(jié)

我們先看下官網(wǎng)給出的gif圖：