四篇文章帶你快速入門Jetpck（終）之Navigation，Paging

四篇文章帶你快速入門Jetpck（終）之Navigation，Paging

Jetpack

Jetpack 是一個(gè)由多個(gè)庫組成的套件，可幫助開發(fā)者遵循最佳做法，減少樣板代碼并編寫可在各種 Android 版本和設(shè)備中一致運(yùn)行的代碼，讓開發(fā)者精力集中編寫重要的代碼。

Android Architecture Component (AAC)。

image.png

官方推薦架構(gòu)

img

請注意，每個(gè)組件僅依賴于其下一級(jí)的組件。例如，Activity 和 Fragment 僅依賴于視圖模型。存儲(chǔ)區(qū)是唯一依賴于其他多個(gè)類的類；在本例中，存儲(chǔ)區(qū)依賴于持久性數(shù)據(jù)模型和遠(yuǎn)程后端數(shù)據(jù)源。

Navigation

Navigation的誕生

單個(gè)Activity嵌套多個(gè)Fragment的UI架構(gòu)模式,已經(jīng)被大多數(shù)Android工程師所接受和采用。但是，對Fragment的管理一直是一件比較麻煩的事情。

  • 需要在FragmentManager和FragmentTransaction來管理Fragment之間的切換。

  • 頁面的切換通常還包括對應(yīng)用程序App bar管理，F(xiàn)ragment間的切換動(dòng)畫，以及Fragment間的參數(shù)傳遞，

  • Fragment和App bar管理和使用的過程顯得很混亂。

Navigation 的優(yōu)點(diǎn)

  • 可視化的頁面導(dǎo)航圖。
  • 通過destination和action完成頁面間的導(dǎo)航。
  • 方便添加頁面切換動(dòng)畫。
  • 頁面間類型安全的參數(shù)傳遞。
  • 通過NavigationUI類，對菜單，底部導(dǎo)航，抽屜菜單導(dǎo)航進(jìn)行統(tǒng)一的管理。

添加依賴庫

implementation "androidx.navigation:navigation-fragment:2.3.1"
implementation "androidx.navigation:navigation-ui:2.3.1"
implementation 'androidx.navigation:navigation-fragment-ktx:2.3.1'
implementation 'androidx.navigation:navigation-ui-ktx:2.3.1'

創(chuàng)建Navigation

  1. 創(chuàng)建Navigation Graph，nav_graph.xml

  2. 添加NavHostFragment

    <fragment
    android:id="@+id/nav_host_fragment"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:defaultNavHost="true"
    app:navGraph="@navigation/nav_graph" />
    • android:name="androidx.navigation.fragment.NavHostFragment" ，特殊的Fragment。
    • app:defaultNavHost="true"，系統(tǒng)自動(dòng)處理返回鍵。
    • app:navGraph="@navigation/nav_graph"，設(shè)置該Fragment對應(yīng)的導(dǎo)航圖。

  3. 創(chuàng)建destination

  4. 完成Fragment之間的切換

  5. 使用NavController完成導(dǎo)航

    btn_toSettingFragment.setOnClickListener {
 Navigation.findNavController(it).navigate(R.id.action_mainFragment_to_settingFragment)
}

  6. 添加頁面切換東華效果，創(chuàng)建anim文件夾，添加常用東華

    <action
    android:id="@+id/action_mainFragment_to_settingFragment"
    app:destination="@id/settingFragment"
    app:enterAnim="@anim/slide_in_right"
    app:exitAnim="@anim/slide_out_left"
    app:popEnterAnim="@anim/slide_in_left"
    app:popExitAnim="@anim/slide_out_right" />

Fragment傳遞參數(shù)

常見的傳遞參數(shù)的方式

val bundle = Bundle()
bundle.apply {
    putString("user_name", "yaoxin")
    putInt("age", 33)
}
Navigation.findNavController(it).navigate(R.id.action_mainFragment_to_settingFragment, bundle)

使用sate Arg傳遞參數(shù)

  1. 添加插件外層build.gradle
dependencies {
    classpath "androidx.navigation:navigation-safe-args-gradle-plugin:2.3.1"
}
  1. 添加依賴內(nèi)層build.gradle
apply plugin: 'androidx.navigation.safeargs'
  1. 傳遞數(shù)據(jù)
val bundle = MainFragmentArgs.Builder().setUserName("yaoxin1").setAge(12).build().toBundle()
Navigation.findNavController(it).navigate(R.id.action_mainFragment_to_settingFragment, bundle)
  1. 獲取數(shù)據(jù)
Log.d("SettingFragment", " username : ${arguments?.let { MainFragmentArgs.fromBundle(it).userName }}")
Log.d("SettingFragment", " age : ${arguments?.let { MainFragmentArgs.fromBundle(it).age }}")

NacvigationUI的使用

actionBar

  1. 添加menu.xml。

  2. 在Acitivity中重寫onCreateOptionsMenu實(shí)例化菜單。

  3. 初始化，AppBarConfiguration，NavController類并使用NacvigationUI完成配置。

  4. 重寫onOptionsItemSelected，onSupportNavigateUp方法，實(shí)現(xiàn)點(diǎn)擊，頁面切換。

  5. 實(shí)現(xiàn)addOnDestinationChangedListener接口，監(jiān)聽頁面變化。

    navController.addOnDestinationChangedListener { _, _, _ ->
 Log.d("NavigationActivity", "頁面發(fā)生了改變")
}

DrawLayout + NavigationView 抽屜菜單

  1. 添加DrawerLayout布局

    <androidx.drawerlayout.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/dl_drawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

  1. 添加NavigationView控件

    <com.google.android.material.navigation.NavigationView
    android:id="@+id/nv_NavigationView"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    app:menu="@menu/menu_settings" />

  2. 使用NacvigationUI完成配置

    appBarConfiguration = AppBarConfiguration.Builder(navController.graph).setDrawerLayout(dl_drawerLayout).build()
NavigationUI.setupWithNavController(nv_navigationView, navController)

ButtomNavigationView 底部菜單

  1. 添加BottomNavigationView控件

    <com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bnv_bottomNavigationView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:background="@color/design_default_color_primary"
    app:itemTextColor="#FFFFFF"
    app:menu="@menu/menu_setting" />

  2. 使用NacvigationUI完成配置

    NavigationUI.setupWithNavController(bnv_bottomNavigationView, navController)

示例

NavigationActivity

class NavigationActivity : AppCompatActivity() {
    lateinit var appBarConfiguration: AppBarConfiguration
    lateinit var navController: NavController

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_navigation)

        navController = Navigation.findNavController(this, R.id.nav_host_fragment)
        appBarConfiguration = AppBarConfiguration.Builder(navController.graph).build()
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration)

        navController.addOnDestinationChangedListener { _, _, _ ->
            Log.d("NavigationActivity", "頁面發(fā)生了改變")
        }

        appBarConfiguration = AppBarConfiguration.Builder(navController.graph).setDrawerLayout(dl_drawerLayout).build()
        NavigationUI.setupWithNavController(nv_navigationView, navController)


        NavigationUI.setupWithNavController(bnv_bottomNavigationView, navController)

    }

    override fun onCreateOptionsMenu(menu: Menu?): Boolean {
        super.onCreateOptionsMenu(menu)
        menuInflater.inflate(R.menu.menu_setting, menu)
        return true
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        return super.onOptionsItemSelected(item) || NavigationUI.onNavDestinationSelected(item, navController)
    }

    override fun onSupportNavigateUp(): Boolean {
        return super.onSupportNavigateUp() || NavigationUI.navigateUp(navController, appBarConfiguration)
    }
}

activity_navigation.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/dl_drawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".seventh.NavigationActivity">

        <fragment
            android:id="@+id/nav_host_fragment"
            android:name="androidx.navigation.fragment.NavHostFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:defaultNavHost="true"
            app:navGraph="@navigation/nav_graph" />

        <com.google.android.material.bottomnavigation.BottomNavigationView
            android:id="@+id/bnv_bottomNavigationView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:background="@color/design_default_color_primary"
            app:itemTextColor="#FFFFFF"
            app:menu="@menu/menu_setting" />
    </RelativeLayout>

    <com.google.android.material.navigation.NavigationView
        android:id="@+id/nv_navigationView"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        app:menu="@menu/menu_setting" />

</androidx.drawerlayout.widget.DrawerLayout>

MainFragment

class MainFragment : Fragment() {


    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_main, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        btn_toSettingFragment.setOnClickListener {
            val bundle = Bundle()
            bundle.apply {
                putString("user_name", "yaoxin")
                putInt("age", 33)
            }
            val bundle1 = MainFragmentArgs.Builder().setUserName("yaoxin1").setAge(12).build().toBundle()


            Navigation.findNavController(it).navigate(R.id.action_mainFragment_to_settingFragment, bundle1)
        }
    }
}

SettingFragment

class SettingFragment : Fragment() {
    
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_setting, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val bundle = arguments
        if (bundle != null) {
            Log.d("SettingFragment", " username : ${bundle.getString("user_name")}")
            Log.d("SettingFragment", " age : ${bundle.getInt("age")}")

            Log.d("SettingFragment", " username : ${arguments?.let { MainFragmentArgs.fromBundle(it).userName }}")
            Log.d("SettingFragment", " age : ${arguments?.let { MainFragmentArgs.fromBundle(it).age }}")
        }
    }
}

menu_setting.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/mainFragment"
        android:title="主頁" />
    <item
        android:id="@+id/settingFragment"
        android:title="設(shè)置" />
</menu>

nav_graph.xml

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/nav_graph"
    app:startDestination="@id/mainFragment">

    <fragment
        android:id="@+id/mainFragment"
        android:name="com.yx.androidseniorprepare.seventh.MainFragment"
        android:label="fragment_main"
        tools:layout="@layout/fragment_main">
        <action
            android:id="@+id/action_mainFragment_to_settingFragment"
            app:destination="@id/settingFragment"
            app:enterAnim="@anim/slide_in_right"
            app:exitAnim="@anim/slide_out_left"
            app:popEnterAnim="@anim/slide_in_left"
            app:popExitAnim="@anim/slide_out_right" />

        <argument
            android:name="user_name"
            android:defaultValue="unknown"
            app:argType="string" />
        <argument
            android:name="age"
            android:defaultValue="0"
            app:argType="integer" />
    </fragment>
    <fragment
        android:id="@+id/settingFragment"
        android:name="com.yx.androidseniorprepare.seventh.SettingFragment"
        android:label="fragment_setting"
        tools:layout="@layout/fragment_setting" />
</navigation>

Paging

Paging組件的意義

分頁加載是在應(yīng)用程序開發(fā)過程中十分常見的需求，我們經(jīng)常需要以列表的形式加載大量的數(shù)據(jù)，這些數(shù)據(jù)通常來自網(wǎng)絡(luò)或本地?cái)?shù)據(jù)庫。若一次加載大量的數(shù)據(jù)，這些數(shù)據(jù)消耗大量的時(shí)間和數(shù)據(jù)流量。

[圖片上傳失敗...(image-19ef07-1610060544137)]

然而用戶實(shí)際需要的可能只是部分?jǐn)?shù)據(jù)，因此有了分頁加載，分業(yè)加載按需加載，在不影響用戶體驗(yàn)的同時(shí)，還能節(jié)省數(shù)據(jù)流量，提升應(yīng)用性能。

數(shù)據(jù)來源

[圖片上傳失敗...(image-8f3c85-1610060544137)]

工作原理

img
  1. 在RecyclerView的滑動(dòng)過程中，會(huì)觸發(fā)PagedListAdapter類中的onBindViewHolder()方法。數(shù)據(jù)與RecycleView中Item布局的UI控件正是在該方法中進(jìn)行綁定的。
  2. 當(dāng)RecyclerView滑動(dòng)到底部時(shí)，在onBindViewHolder()方法中所調(diào)用的getItem()方法會(huì)通知PagedList,當(dāng)前需要載入更多數(shù)據(jù)。
  3. 接著，PagedList會(huì)根據(jù)PageList.Config中的配置通知DataSource執(zhí)行具體的數(shù)據(jù)獲取工作。
  4. DataSource從網(wǎng)絡(luò)/本地?cái)?shù)據(jù)庫取得數(shù)據(jù)后，交給PagedList,PagedList將持有這些數(shù)據(jù)。
  5. PagedList將數(shù)據(jù)交給PagedListAdapter中的DiffUtil進(jìn)行比對和處理。
  6. 數(shù)據(jù)在經(jīng)過處理后，交由RecyclerView進(jìn)行展示。

Paging的3個(gè)核心類

  1. PagedListAdapter:
  • 適配器，RecyclerView的適配器，通過分析數(shù)據(jù)是否發(fā)生了變化，負(fù)責(zé)處理UI展示的邏輯(增加/刪除/替換等)。
  1. PageList:
  • 負(fù)責(zé)通知DataSource何時(shí)獲取數(shù)據(jù)，以及如何獲取數(shù)據(jù)。例如，何時(shí)加載第一頁/下一頁、第一頁加載的數(shù)量、提前多少條數(shù)據(jù)開始執(zhí)行預(yù)加載等。需要注意的是，從DataSource獲取的數(shù)據(jù)將存儲(chǔ)在PagedList中。
  1. DataSource
  • 數(shù)據(jù)源，執(zhí)行具體的數(shù)據(jù)載入工作。注意：數(shù)據(jù)的載入需要在工作線程中執(zhí)行。數(shù)據(jù)可以來自網(wǎng)絡(luò)，也可以來自本地?cái)?shù)據(jù)庫，如Room.根據(jù)分頁機(jī)制的不同，Paging為我們提供了3種DataSource。

添加依賴權(quán)限

implementation 'com.squareup.retrofit2:retrofit:2.9.0'
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
implementation 'com.squareup.picasso:picasso:2.71828'
implementation "android.arch.lifecycle:extensions:1.1.1"
implementation 'androidx.paging:paging-runtime-ktx:2.1.2'

 <uses-permission android:name="android.permission.INTERNET" />

3種DataSource

PositionalDataSource

  • 適用于可通過任意位置加載數(shù)據(jù)，且目標(biāo)數(shù)據(jù)源數(shù)量固定的情況。例如，若請求時(shí)攜帶的參數(shù)為start=2&count=5,則表示向服務(wù)端請求從第2條數(shù)據(jù)開始往后的5條數(shù)據(jù)。
    [圖片上傳失敗...(image-afa103-1610060544137)]

  • 接口：https://gank.io/api/v2/data/category/Girl/type/Girl/page/1/count/10

PageKeyedDataSource

  • 適用于數(shù)據(jù)源以"頁"的方式進(jìn)行請求的情況。例如，若請求時(shí)攜帶的參數(shù)為page=2&pageSize=5,則表示數(shù)據(jù)源以5條數(shù)據(jù)為一頁，當(dāng)前返回第二頁的5條數(shù)據(jù)。
    [圖片上傳失敗...(image-ab156b-1610060544137)]
  • 接口：https://api.stackexchange.com/2.2/users?page=1&pagesize=6&site=stackoverflow

ItemKeyedDataSource

  • 適用于當(dāng)目標(biāo)數(shù)據(jù)的下一頁需要依賴于上一頁數(shù)據(jù)中最后一個(gè)對象中的某個(gè)字段作為key的情況。此類分頁形式常見于評(píng)論功能的實(shí)現(xiàn)。例如，若上一頁數(shù)據(jù)中最后一個(gè)對象的key為9527,那么在請求下一頁時(shí)，需要攜帶參數(shù)since=9527&pageSize=5,則服務(wù)器會(huì)返回key=9527之后的5條數(shù)據(jù)
    [圖片上傳失敗...(image-9c9ff7-1610060544137)]

  • 接口： https://api.github.com/users?since=0&per_page=5

PositionalDataSource 使用

  1. 創(chuàng)建實(shí)體類

    data class Users(var page: Int, var page_count: Int, var total_counts: Int, var data: List<User>)

data class User(var _id: String, var author: String, var createdAt: String, var url: String)

  2. 創(chuàng)建API接口

    interface Api {
    @GET("page/{page}/count/{count}")
    fun getUsers(@Path("page") page:Int,@Path("count") pageSize:Int):Call<Users>
}

  3. 創(chuàng)建網(wǎng)絡(luò)請求框架

    object RetrofitClient {
    private const val BASE_URL = "https://gank.io/api/v2/data/category/Girl/type/Girl/"
    private var retrofit: Retrofit? = null

    init {
        retrofit = Retrofit.Builder().baseUrl(BASE_URL).addConverterFactory(GsonConverterFactory.create()).build()
    }

    fun getApi(): Api? {
        return retrofit?.create(Api::class.java)
    }
}

  4. 創(chuàng)建UserDataSource繼承PositionalDataSource

    class UserDataSource : PositionalDataSource<User>() {

    companion object {
        const val PER_PAGE = 10
        var startPosition = 1
    }

    override fun loadInitial(params: LoadInitialParams, callback: LoadInitialCallback<User>) {

        RetrofitClient
            .getApi()?.getUsers(startPosition, PER_PAGE)
            ?.enqueue(object : Callback<Users?> {
                override fun onResponse(call: Call<Users?>, response: Response<Users?>) {
                    if (response.body() != null) {
                        Log.e(
                            "loadInitial()",
                            "startPosition:" + params.requestedStartPosition + " response:" + response.body()
                        )
                        callback.onResult(
                            response.body()!!.data,
                            response.body()!!.page,
                            response.body()!!.total_counts
                        )
                    }
                }

                override fun onFailure(call: Call<Users?>, t: Throwable) {
                    Log.e("loadInitial()", call.toString())
                }
            })
    }

    override fun loadRange(params: LoadRangeParams, callback: LoadRangeCallback<User>) {
        RetrofitClient
            .getApi()?.getUsers(startPosition + 1, PER_PAGE)
            ?.enqueue(object : Callback<Users?> {
                override fun onResponse(call: Call<Users?>, response: Response<Users?>) {
                    if (response.body() != null) {
                        startPosition += 1
                        Log.e(
                            "loadRange()",
                            "startPosition:" + params.startPosition + " response:" + response.body()
                        )
                        callback.onResult(response.body()!!.data)
                    }
                }

                override fun onFailure(call: Call<Users?>, t: Throwable) {
                    Log.e("loadRange()", call.toString())
                }
            })
    }

}

  5. 創(chuàng)建UserDataSourceFactory負(fù)責(zé)創(chuàng)建UserDataSource，并使用LiveData包裝UserDataSource講其暴露給UserViewModel

    class UserDataSourceFactory : DataSource.Factory<Int, User>() {
    private val liveDataSource = MutableLiveData<UserDataSource>()
    override fun create(): DataSource<Int, User> {
        val dataSource = UserDataSource()
        liveDataSource.postValue(dataSource)
        return dataSource
    }
}

  6. 創(chuàng)建ViewModel

    class UserViewModel : ViewModel() {
    var userPagedList: LiveData<PagedList<User>>? = null

    init {
        val config =
            PagedList.Config.Builder() // 是否顯示占位。默認(rèn)為true。當(dāng)被設(shè)置為true時(shí)，要求在DataSource中提供數(shù)據(jù)源的總量，否則會(huì)報(bào)錯(cuò)。
                // 這是因?yàn)镽ecyclerView需要知道數(shù)據(jù)總量，為這些數(shù)據(jù)預(yù)留位置
                .setEnablePlaceholders(true)
                //設(shè)置每頁加載的數(shù)量
                .setPageSize(UserDataSource.PER_PAGE)
                //設(shè)置距離底部還有多少條數(shù)據(jù)時(shí)加載下一頁數(shù)據(jù)
                .setPrefetchDistance(0)
                //重點(diǎn)，這里需要設(shè)置預(yù)獲取的值>0，否則getKey()和loadBefore()以及l(fā)oadAfter()不會(huì)被調(diào)用
                //首次初始化加載的數(shù)量，需是分頁加載數(shù)量的倍數(shù)。若沒有設(shè)置，則默認(rèn)是PER_PAGE的三倍
                .setInitialLoadSizeHint(UserDataSource.PER_PAGE * 4)
                //好像沒有效果
                //設(shè)置PagedList所能承受的最大數(shù)量，一般來說是PER_PAGE的許多許多倍，超過可能會(huì)出現(xiàn)異常。
                .setMaxSize(65536 * UserDataSource.PER_PAGE).build()

        // 通過LivePagedListBuilder創(chuàng)建PagedList
        userPagedList = LivePagedListBuilder(UserDataSourceFactory(), config).build()
    }
    
}

  7. 創(chuàng)建PagedListAdapter

    class UserAdapter(context: Context) : PagedListAdapter<User, UserAdapter.UserViewHolder>(DIFF_CALLBACK) {
    private var context: Context? = null

    init {
        this.context = context
    }

    companion object {
        /**
         * 用于計(jì)算列表中兩個(gè)非空項(xiàng)之間的差異的回調(diào)。
         *
         * 之前數(shù)據(jù)更新了，需要通過notifyDataSetChanged()通知整個(gè)RecyclerView，效率不高
         * 使用DiffUtil只會(huì)更新需要更新的Item，不需要刷新整個(gè)RecyclerView，并且可以在Item刪除的時(shí)候加上動(dòng)畫效果
         */
        private val DIFF_CALLBACK: DiffUtil.ItemCallback<User> = object : DiffUtil.ItemCallback<User>() {
            /**
             * 當(dāng)DiffUtil想要檢測兩個(gè)對象是否代表同一個(gè)Item時(shí)，調(diào)用該方法進(jìn)行判斷
             */
            override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
                return oldItem._id == newItem._id
            }

            /**
             * 當(dāng)DiffUtil想要檢測兩個(gè)Item是否有一樣的數(shù)據(jù)時(shí)，調(diào)用該方法進(jìn)行判斷
             *
             * 內(nèi)容如果更新了，展示給用戶看的東西可能也需要更新，所以需要這個(gè)判斷
             */
            override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {
                return oldItem == newItem
            }
        }
    }
    
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {
        val view = LayoutInflater.from(context).inflate(R.layout.item_user, parent, false)
        return UserViewHolder(view)
    }

    override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
        val user = getItem(position)
        if (user != null) {
            Picasso.get().load(user.url).placeholder(R.drawable.ic_launcher_background).error(R.drawable.ic_launcher_background).into(holder.ivAvatar)
            holder.tvName.text = user.author + " " + user.createdAt
        } else {
            holder.ivAvatar.setImageResource(R.drawable.ic_launcher_background)
            holder.tvName.text = ""
        }
    }

    class UserViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        var ivAvatar: ImageView = itemView.findViewById(R.id.iv_Avatar)
        var tvName: TextView = itemView.findViewById(R.id.tv_Name)
    }

}

    xml

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="12dp">

    <ImageView
        android:id="@+id/ivAvatar"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:layout_alignParentLeft="true" />

    <TextView
        android:id="@+id/tvName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:gravity="left"
        android:layout_marginLeft="12dp"
        android:layout_toRightOf="@+id/ivAvatar"
        android:textSize="24sp" />

</RelativeLayout>

  8. 配置Activity

    class PositionalDataSourceActivity : AppCompatActivity() {
    val TAG = this.javaClass.simpleName
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_positional_data_source)
        rv_recycler.layoutManager = LinearLayoutManager(this)
        rv_recycler.setHasFixedSize(true)
        val userAdapter = UserAdapter(this)
        val movieViewModel = ViewModelProviders.of(this)[UserViewModel::class.java]
        movieViewModel.userPagedList!!.observe(this, {
            Log.d(TAG, "onChange()->$it")
            userAdapter.submitList(it)
        })
        rv_recycler.adapter = userAdapter
        title = "PositionalDataSourceActivity"
    }
}

  9. 效果圖
    [圖片上傳失敗...(image-896f07-1610060544137)]

PageKeyedDataSource使用

  1. 創(chuàng)建實(shí)體類

    data class User(var account_id: Int, var display_name: String?, var profile_image: String?)
data class UserResponse(var items: List<User>?, var has_more: Boolean?)

  2. 創(chuàng)建API接口

    interface Api {
    @GET("users")
    fun getUsers(@Query("page") page: Int, @Query("pagesize") pageSize: Int, @Query("site") site: String): Call<UserResponse>
}

  3. 創(chuàng)建網(wǎng)絡(luò)請求框架

    object RetrofitClient {
    private const val BASE_URL = "https://api.stackexchange.com/2.2/"
    private var retrofit: Retrofit? = null

    init {
        retrofit = Retrofit.Builder().baseUrl(BASE_URL).addConverterFactory(GsonConverterFactory.create()).build()
    }

    fun getApi(): Api? {
        return retrofit?.create(Api::class.java)
    }
}

  4. 創(chuàng)建UserDataSource繼承PageKeyedDataSource

    class UserDataSource : PageKeyedDataSource<Int, User>() {

    companion object {
        const val FIRST_PAGE = 1
        const val PER_PAGE = 10
        const val SITE = "stackoverflow"
    }

    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<Int, User>) {
        RetrofitClient.getApi()?.getUsers(FIRST_PAGE, PER_PAGE, SITE)?.enqueue(object : Callback<UserResponse> {
            override fun onResponse(call: Call<UserResponse>, response: Response<UserResponse>) {
                if (response.body() != null) {
                    Log.e("loadInitial()", " response:" + response.body())
                    response.body()!!.items?.toMutableList()?.let { callback.onResult(it, null, FIRST_PAGE + 1) }

                }
            }

            override fun onFailure(call: Call<UserResponse>, t: Throwable) {
                TODO("Not yet implemented")
            }
        })
    }

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

    }

    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<Int, User>) {
        Log.e("loadAfter()", "called");
        RetrofitClient.getApi()?.getUsers(params.key, PER_PAGE, SITE)?.enqueue(object : Callback<UserResponse> {
            override fun onResponse(call: Call<UserResponse>, response: Response<UserResponse>) {
                if (response.body() != null) {
                    Log.e("loadAfter()", " response:" + response.body())
                    val nextKey = if (response.body()!!.has_more!!) params.key + 1 else null
                    response.body()!!.items?.let { callback.onResult(it, nextKey) }
                }
            }

            override fun onFailure(call: Call<UserResponse>, t: Throwable) {
                TODO("Not yet implemented")
            }
        })
    }

  5. 創(chuàng)建UserDataSourceFactory負(fù)責(zé)創(chuàng)建UserDataSource，并使用LiveData包裝UserDataSource講其暴露給UserViewModel

    class UserDataSourceFactory : DataSource.Factory<Int, User>() {
    private val liveDataSource = MutableLiveData<UserDataSource>()
    override fun create(): DataSource<Int, User> {
        val dataSource = UserDataSource()
        liveDataSource.postValue(dataSource)
        return dataSource
    }
}

  6. 創(chuàng)建ViewModel

    class UserViewModel : ViewModel() {
    var userPagedList: LiveData<PagedList<User>>? = null

    init {
        val config = PagedList.Config.Builder() // 是否顯示占位。默認(rèn)為true。當(dāng)被設(shè)置為true時(shí)，要求在DataSource中提供數(shù)據(jù)源的總量，否則會(huì)報(bào)錯(cuò)。
                // 這是因?yàn)镽ecyclerView需要知道數(shù)據(jù)總量，為這些數(shù)據(jù)預(yù)留位置
                .setEnablePlaceholders(true)
                //設(shè)置每頁加載的數(shù)量
                .setPageSize(UserDataSource.PER_PAGE)
                //設(shè)置距離底部還有多少條數(shù)據(jù)時(shí)加載下一頁數(shù)據(jù)
                .setPrefetchDistance(3)
                //重點(diǎn)，這里需要設(shè)置預(yù)獲取的值>0，否則getKey()和loadBefore()以及l(fā)oadAfter()不會(huì)被調(diào)用
                //首次初始化加載的數(shù)量，需是分頁加載數(shù)量的倍數(shù)。若沒有設(shè)置，則默認(rèn)是PER_PAGE的三倍
                .setInitialLoadSizeHint(UserDataSource.PER_PAGE * 3)
                //好像沒有效果
                //設(shè)置PagedList所能承受的最大數(shù)量，一般來說是PER_PAGE的許多許多倍，超過可能會(huì)出現(xiàn)異常。
                .setMaxSize(65536 * UserDataSource.PER_PAGE).build()
        // 通過LivePagedListBuilder創(chuàng)建PagedList
        userPagedList = LivePagedListBuilder(UserDataSourceFactory(), config).build()
    }
}

  7. 創(chuàng)建PagedListAdapter

    class UserAdapter(context: Context) : PagedListAdapter<User, UserAdapter.UserViewHolder>(DIFF_CALLBACK) {
    private var context: Context? = null

    init {
        this.context = context
    }

    companion object {
        /**
         * 用于計(jì)算列表中兩個(gè)非空項(xiàng)之間的差異的回調(diào)。
         *
         * 之前數(shù)據(jù)更新了，需要通過notifyDataSetChanged()通知整個(gè)RecyclerView，效率不高
         * 使用DiffUtil只會(huì)更新需要更新的Item，不需要刷新整個(gè)RecyclerView，并且可以在Item刪除的時(shí)候加上動(dòng)畫效果
         */
        private val DIFF_CALLBACK: DiffUtil.ItemCallback<User> = object : DiffUtil.ItemCallback<User>() {
            /**
             * 當(dāng)DiffUtil想要檢測兩個(gè)對象是否代表同一個(gè)Item時(shí)，調(diào)用該方法進(jìn)行判斷
             */
            override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
                return oldItem.account_id == newItem.account_id
            }

            /**
             * 當(dāng)DiffUtil想要檢測兩個(gè)Item是否有一樣的數(shù)據(jù)時(shí)，調(diào)用該方法進(jìn)行判斷
             *
             * 內(nèi)容如果更新了，展示給用戶看的東西可能也需要更新，所以需要這個(gè)判斷
             */
            override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {
                return oldItem == newItem
            }
        }
    }


    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {
        val view = LayoutInflater.from(context).inflate(R.layout.item_user, parent, false)
        return UserViewHolder(view)
    }

    override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
        val user = getItem(position)
        if (user != null) {
            Picasso.get().load(user.profile_image).placeholder(R.drawable.ic_launcher_background).error(R.drawable.ic_launcher_background).into(holder.ivAvatar)
            holder.tvName.text = user.display_name
        } else {
            holder.ivAvatar.setImageResource(R.drawable.ic_launcher_background)
            holder.tvName.text = ""
        }
    }

    class UserViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        var ivAvatar: ImageView = itemView.findViewById(R.id.iv_Avatar)
        var tvName: TextView = itemView.findViewById(R.id.tv_Name)

    }

}

  8. 配置Activity

    class PageKeyedDataSourceActivity : AppCompatActivity() {
    val TAG = this.javaClass.simpleName
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_page_keyed_data_source)
        rv_recycler.layoutManager = LinearLayoutManager(this)
        rv_recycler.setHasFixedSize(true)
        val userAdapter = UserAdapter(this)
        val movieViewModel = ViewModelProviders.of(this)[UserViewModel::class.java]
        movieViewModel.userPagedList!!.observe(this, {
            Log.e(TAG, "onChange()->$it")
            userAdapter.submitList(it)
        })
        rv_recycler.adapter = userAdapter
        title = "PageKeyedDataSourceActivity"
    }
}

  9. 效果

[圖片上傳失敗...(image-3f5622-1610060544137)]

ItemKeyedDataSource使用

  1. 創(chuàng)建實(shí)體類

    data class User(var id: Int, @SerializedName("login") var name: String?, @SerializedName("avatar_url") var avatar: String?)

  2. 創(chuàng)建API接口

    interface Api {
    @GET("users")
    fun getUsers(@Query("since") since: Int, @Query("per_page") perPage: Int): Call<List<User>>
}

  3. 創(chuàng)建網(wǎng)絡(luò)請求框架

    object RetrofitClient {
    private const val BASE_URL = "https://api.github.com/"
    private var retrofit: Retrofit? = null

    init {
        retrofit = Retrofit.Builder().baseUrl(BASE_URL).addConverterFactory(GsonConverterFactory.create()).build()
    }

    fun getApi(): Api? {
        return retrofit?.create(Api::class.java)
    }
}

  4. 創(chuàng)建UserDataSource繼承ItemKeyedDataSource

    class UserDataSource : ItemKeyedDataSource<Int, User>() {

    companion object {
        const val PER_PAGE = 12
    }

    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<User>) {
        Log.e("loadInitial()", "called")
        val since = 0
        RetrofitClient.getApi()?.getUsers(since, PER_PAGE)?.enqueue(object : Callback<List<User>> {
            override fun onResponse(call: Call<List<User>>, response: Response<List<User>>) {
                if (response.body() != null) {
                    Log.e("loadInitial()", " response:" + response.body())
                    callback.onResult(response.body()!!)
                }
            }

            override fun onFailure(call: Call<List<User>>, t: Throwable) {
                TODO("Not yet implemented")
            }

        })
    }

    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<User>) {
        Log.e("loadAfter()", "called");

        RetrofitClient.getApi()?.getUsers(params.key, PER_PAGE)?.enqueue(object : Callback<List<User>> {
            override fun onResponse(call: Call<List<User>>, response: Response<List<User>>) {
                if (response.body() != null) {
                    Log.e("loadAfter()", " response:" + response.body())
                    callback.onResult(response.body()!!)
                }
            }

            override fun onFailure(call: Call<List<User>>, t: Throwable) {
                TODO("Not yet implemented")
            }

        })
    }

    override fun loadBefore(params: LoadParams<Int>, callback: LoadCallback<User>) {
        Log.e("loadBefore()", "called")
    }

    override fun getKey(item: User): Int {
        Log.e("getkey()", "called")
        return item.id
    }
}

  5. 創(chuàng)建UserDataSourceFactory負(fù)責(zé)創(chuàng)建UserDataSource，并使用LiveData包裝UserDataSource講其暴露給UserViewModel

    class UserDataSourceFactory : DataSource.Factory<Int, User>() {
    private val liveDataSource = MutableLiveData<UserDataSource>()
    override fun create(): DataSource<Int, User> {
        val dataSource = UserDataSource()
        liveDataSource.postValue(dataSource)
        return dataSource
    }
}

  6. 創(chuàng)建ViewModel

    class UserViewModel : ViewModel() {
    var userPagedList: LiveData<PagedList<User>>? = null

    init {
        val config = PagedList.Config.Builder() // 是否顯示占位。默認(rèn)為true。當(dāng)被設(shè)置為true時(shí)，要求在DataSource中提供數(shù)據(jù)源的總量，否則會(huì)報(bào)錯(cuò)。
                // 這是因?yàn)镽ecyclerView需要知道數(shù)據(jù)總量，為這些數(shù)據(jù)預(yù)留位置
                .setEnablePlaceholders(true)
                //設(shè)置每頁加載的數(shù)量
                .setPageSize(UserDataSource.PER_PAGE)
                //設(shè)置距離底部還有多少條數(shù)據(jù)時(shí)加載下一頁數(shù)據(jù)
                .setPrefetchDistance(3)
                //重點(diǎn)，這里需要設(shè)置預(yù)獲取的值>0，否則getKey()和loadBefore()以及l(fā)oadAfter()不會(huì)被調(diào)用
                //首次初始化加載的數(shù)量，需是分頁加載數(shù)量的倍數(shù)。若沒有設(shè)置，則默認(rèn)是PER_PAGE的三倍
                .setInitialLoadSizeHint(UserDataSource.PER_PAGE * 3)
                //好像沒有效果
                //設(shè)置PagedList所能承受的最大數(shù)量，一般來說是PER_PAGE的許多許多倍，超過可能會(huì)出現(xiàn)異常。
                .setMaxSize(65536 * UserDataSource.PER_PAGE).build()

        // 通過LivePagedListBuilder創(chuàng)建PagedList
        userPagedList = LivePagedListBuilder(UserDataSourceFactory(), config).build()
    }
}

  7. 創(chuàng)建PagedListAdapter

    class UserAdapter(context: Context) : PagedListAdapter<User, UserAdapter.UserViewHolder>(DIFF_CALLBACK) {
    private var context: Context? = null

    init {
        this.context = context
    }

    companion object {
        /**
         * 用于計(jì)算列表中兩個(gè)非空項(xiàng)之間的差異的回調(diào)。
         *
         * 之前數(shù)據(jù)更新了，需要通過notifyDataSetChanged()通知整個(gè)RecyclerView，效率不高
         * 使用DiffUtil只會(huì)更新需要更新的Item，不需要刷新整個(gè)RecyclerView，并且可以在Item刪除的時(shí)候加上動(dòng)畫效果
         */
        private val DIFF_CALLBACK: DiffUtil.ItemCallback<User> = object : DiffUtil.ItemCallback<User>() {
            /**
             * 當(dāng)DiffUtil想要檢測兩個(gè)對象是否代表同一個(gè)Item時(shí)，調(diào)用該方法進(jìn)行判斷
             */
            override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
                return oldItem.id == newItem.id
            }

            /**
             * 當(dāng)DiffUtil想要檢測兩個(gè)Item是否有一樣的數(shù)據(jù)時(shí)，調(diào)用該方法進(jìn)行判斷
             *
             * 內(nèi)容如果更新了，展示給用戶看的東西可能也需要更新，所以需要這個(gè)判斷
             */
            override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {
                return oldItem == newItem
            }
        }
    }


    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {
        val view = LayoutInflater.from(context).inflate(R.layout.item_user, parent, false)
        return UserViewHolder(view)
    }

    override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
        val user = getItem(position)
        if (user != null) {
            Picasso.get().load(user.avatar).placeholder(R.drawable.ic_launcher_background).error(R.drawable.ic_launcher_background).into(holder.ivAvatar)
            holder.tvName.text = user.name
        } else {
            holder.ivAvatar.setImageResource(R.drawable.ic_launcher_background)
            holder.tvName.text = ""
        }
    }

    class UserViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        var ivAvatar: ImageView = itemView.findViewById(R.id.iv_Avatar)
        var tvName: TextView = itemView.findViewById(R.id.tv_Name)

    }

}

  8. 配置Activity

    class ItemKeyedDataSourceActivity : AppCompatActivity() {
    val TAG = this.javaClass.simpleName
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_item_keyed_data_source)
        rv_recycler.layoutManager = LinearLayoutManager(this)
        rv_recycler.setHasFixedSize(true)
        val userAdapter = UserAdapter(this)
        val movieViewModel = ViewModelProviders.of(this)[UserViewModel::class.java]
        movieViewModel.userPagedList!!.observe(this, {
            Log.e(TAG, "onChange()->$it")
            userAdapter.submitList(it)
        })
        rv_recycler.adapter = userAdapter
        title = "ItemKeyedDataSourceActivity"
    }
}

  9. 效果
    [圖片上傳失敗...(image-f7efec-1610060544137)]

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