四篇文章帶你快速入門Jetpck（終）之Navigation，Paging

Jetpack

Jetpack 是一個由多個庫組成的套件豌熄，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 版本和設備中一致運行的代碼物咳，讓開發(fā)者精力集中編寫重要的代碼锣险。

Android Architecture Component (AAC)蹄皱。

image.png

官方推薦架構

img

請注意，每個組件僅依賴于其下一級的組件芯肤。例如巷折，Activity 和 Fragment 僅依賴于視圖模型。存儲區(qū)是唯一依賴于其他多個類的類崖咨；在本例中锻拘，存儲區(qū)依賴于持久性數(shù)據(jù)模型和遠程后端數(shù)據(jù)源。

Navigation

Navigation的誕生

單個Activity嵌套多個Fragment的UI架構模式,已經被大多數(shù)Android工程師所接受和采用击蹲。但是署拟，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之間的切換歌豺。

• 頁面的切換通常還包括對應用程序App bar管理推穷，F(xiàn)ragment間的切換動畫，以及Fragment間的參數(shù)傳遞类咧，

• Fragment和App bar管理和使用的過程顯得很混亂馒铃。

Navigation 的優(yōu)點

• 可視化的頁面導航圖。
• 通過destination和action完成頁面間的導航痕惋。
• 方便添加頁面切換動畫区宇。
• 頁面間類型安全的參數(shù)傳遞。
• 通過NavigationUI類值戳，對菜單议谷，底部導航，抽屜菜單導航進行統(tǒng)一的管理述寡。

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柿隙，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鲫凶。
   • app:defaultNavHost="true"禀崖，系統(tǒng)自動處理返回鍵。
   • app:navGraph="@navigation/nav_graph"螟炫，設置該Fragment對應的導航圖波附。

3. 創(chuàng)建destination

4. 完成Fragment之間的切換

5. 使用NavController完成導航

   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文件夾昼钻，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"
}

2. 添加依賴內層build.gradle

apply plugin: 'androidx.navigation.safeargs'

3. 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)

4. 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掸屡。

2. 在Acitivity中重寫onCreateOptionsMenu實例化菜單。

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

4. 重寫onOptionsItemSelected碗淌，onSupportNavigateUp方法盏求，實現(xiàn)點擊抖锥，頁面切換。

5. 實現(xiàn)addOnDestinationChangedListener接口碎罚，監(jiān)聽頁面變化磅废。

   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" />
   

2. 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" />
   

3. 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

分頁加載是在應用程序開發(fā)過程中十分常見的需求，我們經常需要以列表的形式加載大量的數(shù)據(jù)荆烈，這些數(shù)據(jù)通常來自網絡或本地數(shù)據(jù)庫拯勉。若一次加載大量的數(shù)據(jù)，這些數(shù)據(jù)消耗大量的時間和數(shù)據(jù)流量憔购。

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

然而用戶實際需要的可能只是部分數(shù)據(jù)宫峦，因此有了分頁加載，分業(yè)加載按需加載倦始，在不影響用戶體驗的同時斗遏，還能節(jié)省數(shù)據(jù)流量，提升應用性能鞋邑。

數(shù)據(jù)來源

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

工作原理

img

1. 在RecyclerView的滑動過程中，會觸發(fā)PagedListAdapter類中的onBindViewHolder()方法账蓉。數(shù)據(jù)與RecycleView中Item布局的UI控件正是在該方法中進行綁定的枚碗。
2. 當RecyclerView滑動到底部時，在onBindViewHolder()方法中所調用的getItem()方法會通知PagedList,當前需要載入更多數(shù)據(jù)铸本。
3. 接著肮雨，PagedList會根據(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從網絡/本地數(shù)據(jù)庫取得數(shù)據(jù)后箱玷，交給PagedList,PagedList將持有這些數(shù)據(jù)怨规。
5. PagedList將數(shù)據(jù)交給PagedListAdapter中的DiffUtil進行比對和處理。
6. 數(shù)據(jù)在經過處理后锡足，交由RecyclerView進行展示波丰。

Paging的3個核心類

1. PagedListAdapter:

• 適配器，RecyclerView的適配器舶得，通過分析數(shù)據(jù)是否發(fā)生了變化掰烟，負責處理UI展示的邏輯(增加/刪除/替換等)。

2. PageList:

• 負責通知DataSource何時獲取數(shù)據(jù)沐批，以及如何獲取數(shù)據(jù)纫骑。例如，何時加載第一頁/下一頁九孩、第一頁加載的數(shù)量先馆、提前多少條數(shù)據(jù)開始執(zhí)行預加載等。需要注意的是躺彬，從DataSource獲取的數(shù)據(jù)將存儲在PagedList中煤墙。

3. DataSource

• 數(shù)據(jù)源缤底，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)行番捂。數(shù)據(jù)可以來自網絡个唧，也可以來自本地數(shù)據(jù)庫，如Room.根據(jù)分頁機制的不同设预，Paging為我們提供了3種DataSource徙歼。

添加依賴權限

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)，且目標數(shù)據(jù)源數(shù)量固定的情況鳖枕。例如魄梯，若請求時攜帶的參數(shù)為start=2&count=5,則表示向服務端請求從第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"的方式進行請求的情況宾符。例如彬碱，若請求時攜帶的參數(shù)為page=2&pageSize=5,則表示數(shù)據(jù)源以5條數(shù)據(jù)為一頁或辖，當前返回第二頁的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

• 適用于當目標數(shù)據(jù)的下一頁需要依賴于上一頁數(shù)據(jù)中最后一個對象中的某個字段作為key的情況。此類分頁形式常見于評論功能的實現(xiàn)遣妥。例如，若上一頁數(shù)據(jù)中最后一個對象的key為9527,那么在請求下一頁時兴猩，需要攜帶參數(shù)since=9527&pageSize=5,則服務器會返回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)建網絡請求框架

   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負責創(chuàng)建UserDataSource埂陆，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。默認為true退客。當被設置為true時链嘀，要求在DataSource中提供數(shù)據(jù)源的總量，否則會報錯茫藏。
                   // 這是因為RecyclerView需要知道數(shù)據(jù)總量包个，為這些數(shù)據(jù)預留位置
                   .setEnablePlaceholders(true)
                   //設置每頁加載的數(shù)量
                   .setPageSize(UserDataSource.PER_PAGE)
                   //設置距離底部還有多少條數(shù)據(jù)時加載下一頁數(shù)據(jù)
                   .setPrefetchDistance(0)
                   //重點碧囊，這里需要設置預獲取的值>0树灶，否則getKey()和loadBefore()以及l(fā)oadAfter()不會被調用
                   //首次初始化加載的數(shù)量，需是分頁加載數(shù)量的倍數(shù)天通。若沒有設置，則默認是PER_PAGE的三倍
                   .setInitialLoadSizeHint(UserDataSource.PER_PAGE * 4)
                   //好像沒有效果
                   //設置PagedList所能承受的最大數(shù)量熄驼，一般來說是PER_PAGE的許多許多倍像寒，超過可能會出現(xiàn)異常烘豹。
                   .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 {
           /**
            * 用于計算列表中兩個非空項之間的差異的回調。
            *
            * 之前數(shù)據(jù)更新了诺祸，需要通過notifyDataSetChanged()通知整個RecyclerView携悯，效率不高
            * 使用DiffUtil只會更新需要更新的Item，不需要刷新整個RecyclerView筷笨，并且可以在Item刪除的時候加上動畫效果
            */
           private val DIFF_CALLBACK: DiffUtil.ItemCallback<User> = object : DiffUtil.ItemCallback<User>() {
               /**
                * 當DiffUtil想要檢測兩個對象是否代表同一個Item時憔鬼，調用該方法進行判斷
                */
               override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
                   return oldItem._id == newItem._id
               }
   
               /**
                * 當DiffUtil想要檢測兩個Item是否有一樣的數(shù)據(jù)時，調用該方法進行判斷
                *
                * 內容如果更新了胃夏，展示給用戶看的東西可能也需要更新轴或，所以需要這個判斷
                */
               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)建網絡請求框架

   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負責創(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() // 是否顯示占位仰禀。默認為true照雁。當被設置為true時，要求在DataSource中提供數(shù)據(jù)源的總量答恶，否則會報錯饺蚊。
                   // 這是因為RecyclerView需要知道數(shù)據(jù)總量亥宿，為這些數(shù)據(jù)預留位置
                   .setEnablePlaceholders(true)
                   //設置每頁加載的數(shù)量
                   .setPageSize(UserDataSource.PER_PAGE)
                   //設置距離底部還有多少條數(shù)據(jù)時加載下一頁數(shù)據(jù)
                   .setPrefetchDistance(3)
                   //重點，這里需要設置預獲取的值>0映企，否則getKey()和loadBefore()以及l(fā)oadAfter()不會被調用
                   //首次初始化加載的數(shù)量，需是分頁加載數(shù)量的倍數(shù)双絮。若沒有設置囤攀，則默認是PER_PAGE的三倍
                   .setInitialLoadSizeHint(UserDataSource.PER_PAGE * 3)
                   //好像沒有效果
                   //設置PagedList所能承受的最大數(shù)量，一般來說是PER_PAGE的許多許多倍蝌衔，超過可能會出現(xiàn)異常噩斟。
                   .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 {
           /**
            * 用于計算列表中兩個非空項之間的差異的回調沛简。
            *
            * 之前數(shù)據(jù)更新了，需要通過notifyDataSetChanged()通知整個RecyclerView撒顿，效率不高
            * 使用DiffUtil只會更新需要更新的Item凤壁，不需要刷新整個RecyclerView，并且可以在Item刪除的時候加上動畫效果
            */
           private val DIFF_CALLBACK: DiffUtil.ItemCallback<User> = object : DiffUtil.ItemCallback<User>() {
               /**
                * 當DiffUtil想要檢測兩個對象是否代表同一個Item時唧席，調用該方法進行判斷
                */
               override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
                   return oldItem.account_id == newItem.account_id
               }
   
               /**
                * 當DiffUtil想要檢測兩個Item是否有一樣的數(shù)據(jù)時，調用該方法進行判斷
                *
                * 內容如果更新了徒仓，展示給用戶看的東西可能也需要更新掉弛，所以需要這個判斷
                */
               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)建網絡請求框架

   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負責創(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() // 是否顯示占位谬晕。默認為true。當被設置為true時不撑，要求在DataSource中提供數(shù)據(jù)源的總量焕檬，否則會報錯实愚。
                   // 這是因為RecyclerView需要知道數(shù)據(jù)總量腊敲，為這些數(shù)據(jù)預留位置
                   .setEnablePlaceholders(true)
                   //設置每頁加載的數(shù)量
                   .setPageSize(UserDataSource.PER_PAGE)
                   //設置距離底部還有多少條數(shù)據(jù)時加載下一頁數(shù)據(jù)
                   .setPrefetchDistance(3)
                   //重點，這里需要設置預獲取的值>0介时，否則getKey()和loadBefore()以及l(fā)oadAfter()不會被調用
                   //首次初始化加載的數(shù)量榕吼，需是分頁加載數(shù)量的倍數(shù)。若沒有設置乱凿，則默認是PER_PAGE的三倍
                   .setInitialLoadSizeHint(UserDataSource.PER_PAGE * 3)
                   //好像沒有效果
                   //設置PagedList所能承受的最大數(shù)量徒蟆，一般來說是PER_PAGE的許多許多倍，超過可能會出現(xiàn)異常闹蒜。
                   .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 {
           /**
            * 用于計算列表中兩個非空項之間的差異的回調姥闪。
            *
            * 之前數(shù)據(jù)更新了催式，需要通過notifyDataSetChanged()通知整個RecyclerView荣月，效率不高
            * 使用DiffUtil只會更新需要更新的Item，不需要刷新整個RecyclerView顿天，并且可以在Item刪除的時候加上動畫效果
            */
           private val DIFF_CALLBACK: DiffUtil.ItemCallback<User> = object : DiffUtil.ItemCallback<User>() {
               /**
                * 當DiffUtil想要檢測兩個對象是否代表同一個Item時咽白，調用該方法進行判斷
                */
               override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
                   return oldItem.id == newItem.id
               }
   
               /**
                * 當DiffUtil想要檢測兩個Item是否有一樣的數(shù)據(jù)時，調用該方法進行判斷
                *
                * 內容如果更新了授段，展示給用戶看的東西可能也需要更新侵贵，所以需要這個判斷
                */
               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)]

項目代碼
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