kotlin--综合运用Hilt、Paging3、Flow、Room、Retrofit、Coil等实现MVVM架构

前面我们使用Java来运用JetPack中的一系列组件，又使用kotlin运用这些组件实现了一系列功能：

• kotlin--Flow文件下载
• kotlin--Flow结合Room运用
• kotlin--Flow结合retrofit运用
• kotlin--StateFlow运用
• kotlin--SharedFlow运用

接着，Jetpack的Paging3中，我们使用的语言是kotlin，相信通过这些项目的对比，你就能发现koltin取代Java的理由了，kotlin拥有更好的扩展性，更高的性能，更简洁的代码，更好的Jetpack组件支持，如果你还对kotlin不熟悉，那么可以查阅我的kotlin专题博客，在此也要感谢动脑学院Jason老师的辛勤付出，动脑学院在B站上也有投稿koltin基础的视频，通过视频可以快速学习和上手kotlin

今天来综合使用各种组件，搭建最新MVVM项目框架，利用Paging3实现列表功能，Paging3和Paging2一样，支持数据库缓存

 

一、依赖

主项目gradle中导入hilt插件

    dependencies {
        classpath "com.android.tools.build:gradle:7.0.2"
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.20"
        classpath 'com.google.dagger:hilt-android-gradle-plugin:2.28.1-alpha'
    }  

module依赖hilt、kapt插件

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'kotlin-kapt'
    id 'dagger.hilt.android.plugin'
}  

DataBinding、ViewBinding支持：

    buildFeatures {
        dataBinding = true
        viewBinding = true
    }  

在kotlin1.5.20下使用Hilt编译会出现问题： Expected @HiltAndroidApp to have a value. Did you forget to apply the Gradle Plugin? 解决方法：

    kapt {
        javacOptions {
            option("-Adagger.hilt.android.internal.disableAndroidSuperclassValidation=true")
        }
    }  

依赖各大组件：

    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.5.1'
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation "com.squareup.retrofit2:converter-gson:2.9.0"
    implementation 'com.squareup.okhttp3:logging-interceptor:3.4.1'
    implementation "io.coil-kt:coil:1.1.0"

    def room_version = "2.3.0"
    implementation "androidx.room:room-runtime:$room_version"
    implementation "androidx.room:room-ktx:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    
    implementation 'androidx.swiperefreshlayout:swiperefreshlayout:1.1.0-rc01'
    implementation "androidx.startup:startup-runtime:1.0.0"

    def hilt_version = "2.28-alpha"
    implementation "com.google.dagger:hilt-android:$hilt_version"
    kapt "com.google.dagger:hilt-android-compiler:$hilt_version"
    def hilt_view_version = "1.0.0-alpha01"
    implementation "androidx.hilt:hilt-lifecycle-viewmodel:$hilt_view_version"
    kapt "androidx.hilt:hilt-compiler:$hilt_view_version"
    
    implementation "androidx.activity:activity-ktx:1.1.0"
    implementation "androidx.fragment:fragment-ktx:1.2.5"
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.2.0"
    implementation 'androidx.paging:paging-runtime-ktx:3.0.0-beta03'  

二、Hilt注入

Hilt注解释义:

• @HiltAndroidApp：触发Hilt的代码生成
• @AndroidEntryPoint：创建一个依赖容器，该容器遵循Android类的生命周期
• @Module：告诉Hilt如何提供不同类型的实例
• @InstallIn：用来告诉Hilt这个模块会被安装到哪个组件上
• @Provides：告诉Hilt如何获取具体实例
• @Singleton：单例
• @ViewModelInject：通过构造函数，给ViewModel注入实例

1.Application注入HiltAndroidApp

@HiltAndroidApp
class APP : Application()  

别忘了在Manifest中配置

2.Activity中开始查找注入对象

使用AndroidEntryPoint注解来表示，Hilt开始查找注入对象

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    private val binding by lazy {
        ActivityMainBinding.inflate(layoutInflater)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
    }
}  

3.Hilt注入网络模块

我们准备使用Retrofit封装一个网络模块，需要对该模块使用Module注解和InstallIn注解绑定到对应Android类的生命周期，显然整个APP运行过程中，我们都要使用网络模块，所以选择绑定Application

@InstallIn(ApplicationComponent::class)
@Module
object RetrofitModule {
    
}  

提供一个方法给Hilt获取Okhttp对象，此方法为单例，所以使用Provides和Singleton

{
    private val TAG: String = RetrofitModule.javaClass.simpleName

    @Singleton
    @Provides
    fun getOkHttpClient(): OkHttpClient {
        val interceptor = HttpLoggingInterceptor {
            Log.d(TAG, it)
        }.apply { level = HttpLoggingInterceptor.Level.BODY }
        
        return OkHttpClient.Builder().addInterceptor(interceptor).build()
    }
}  

再提供一个获取Retrofit的方法：

{
    @Singleton
    @Provides
    fun getRetrofit(okHttpClient: OkHttpClient): Retrofit {
        return Retrofit.Builder()
            .baseUrl(BASE_URL)
            .client(okHttpClient)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
    }
}  

完整的网络模块代码：

const val BASE_URL = "http://192.168.17.114:8080/pagingserver_war/"

@InstallIn(ApplicationComponent::class)
@Module
object RetrofitModule {

    private val TAG: String = RetrofitModule.javaClass.simpleName

    @Singleton
    @Provides
    fun getOkHttpClient(): OkHttpClient {
        val interceptor = HttpLoggingInterceptor {
            Log.d(TAG, it)
        }.apply { level = HttpLoggingInterceptor.Level.BODY }
        
        return OkHttpClient.Builder().addInterceptor(interceptor).build()
    }

    @Singleton
    @Provides
    fun getRetrofit(okHttpClient: OkHttpClient): Retrofit {
        return Retrofit.Builder()
            .baseUrl(BASE_URL)
            .client(okHttpClient)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
    }
}  

三、接口与实体类

1.根据接口和接口返回的json数据，分别创建API和实体类

api地址：ikds.do?since=0&pagesize=5 服务器数据：

[
    {
        "id":1,
        "title":"扎克·施奈德版正义联盟",
        "cover":"https://img9.doubanio.com/view/photo/s_ratio_poster/public/p2634360594.webp",
        "rate":"8.9"
    },
    {
        "id":2,
        "title":"侍神令",
        "cover":"https://img2.doubanio.com/view/photo/s_ratio_poster/public/p2629260713.webp",
        "rate":"5.8"
    },
    {
        "id":3,
        "title":"双层肉排",
        "cover":"https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2633977758.webp",
        "rate":"6.7"
    },
    {
        "id":4,
        "title":"大地",
        "cover":"https://img9.doubanio.com/view/photo/s_ratio_poster/public/p2628845704.webp",
        "rate":"6.6"
    },
    {
        "id":5,
        "title":"租来的朋友",
        "cover":"https://img2.doubanio.com/view/photo/s_ratio_poster/public/p2616903233.webp",
        "rate":"6.1"
    }
]  

实体类：

data class MovieItemModel(
    val id: Int,
    val title: String,
    val cover: String,
    val rate: String
)  

API接口：

interface MovieService {
    @GET("ikds.do")
    suspend fun getMovieList(
        @Query("since") since: Int,
        @Query("pagesize") pagesize: Int
    ): List<MovieItemModel>
}  

2.在网络模块RetrofitModule中新增获取MovieService的方法

{
    @Singleton
    @Provides
    fun provideMovieService(retrofit: Retrofit): MovieService {
        return retrofit.create(MovieService::class.java)
    }
}  

四、Hilt注入数据库模块

1.Room相关基类

使用Room数据库，首先创建Entity，这边加了一个页码的字段：

@Entity
data class MovieEntity(
    @PrimaryKey
    val id: Int,
    val title: String,
    val cover: String,
    val rate: String,
    val page: Int//页码
)  

创建Dao，Room支持返回PagingSource对象，可以直接和我们的Paging结合使用了：

 

@Dao
interface MovieDao {

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(movieList: List<MovieEntity>)

    @Query("SELECT * FROM MovieEntity")
    fun getMovieList(): PagingSource<Int, MovieEntity>

    @Query("DELETE FROM MovieEntity")
    suspend fun clear()
}  

定义Database抽象类：

@Database(entities = [MovieEntity::class], version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {
    abstract fun movieDao(): MovieDao
}  

2.Hilt注入数据库模块

数据库模块同样需要伴随应用的生命周期，所以还是和Application绑定 提供方法给Hilt获取AppDatabase、MovieDao

@InstallIn(ApplicationComponent::class)
@Module
object RoomModule {

    @Singleton
    @Provides
    fun getAppDatabase(application: Application): AppDatabase {
        return Room.databaseBuilder(application, AppDatabase::class.java, "my.db")
            .build()
    }

    @Singleton
    @Provides
    fun provideMovieDao(appDatabase: AppDatabase): MovieDao {
        return appDatabase.movieDao()
    }

}  

五、Pager配置

我们有了网络模块，数据库模块，接下来就要实现配置Pager，PagingSource我们已经实现了从数据库获取，现在需要的实现的是：网络数据使用RemoteMediator获取

1.网络数据获取：RemoteMediator

结合最初的架构图，RemoteMediator是用于获取网络数据，并将数据存入数据库，我们就可以从数据库获取PagingSource，传递给后续的Pager

@OptIn(ExperimentalPagingApi::class)
class MovieRemoteMediator(
    private val api: MovieService,
    private val appDatabase: AppDatabase
) : RemoteMediator<Int, MovieEntity>() {
    
    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, MovieEntity>
    ): MediatorResult {
        TODO("Not yet implemented")
    }
    
}  

load函数先放一边，先来实现架构中其他模块

2.对ViewModel暴露获取数据接口：Repository

定义一个Repository接口获取Flow<PagingData<T>>数据，T应该为MovieItemModel，因为对外（ViewModel）而言，使用的都是MovieItemModel网络对象，对内使用的才是MovieEntity数据库对象

interface Repository<T : Any> {
    fun fetchList(): Flow<PagingData<T>>
}  

实现类，使用MovieItemModel作为泛型类型，并返回Pager的Flow：

class MovieRepositoryImpl(
    private val api: MovieService,
    private val appDatabase: AppDatabase
) : Repository<MovieItemModel> {

    override fun fetchList(): Flow<PagingData<MovieItemModel>> {
        val pageSize = 10

        return Pager(
            config = PagingConfig(
                initialLoadSize = pageSize * 2,
                pageSize = pageSize,
                prefetchDistance = 1
            ),
            remoteMediator = MovieRemoteMediator(api, appDatabase)
        ) {
            appDatabase.movieDao().getMovieList()
        }.flow.flowOn(Dispatchers.IO).map { 
            
        }
    }

}  

编译器上可以看到map中的it对象为Paging<MovieEntity>类型的，因为我们MovieDao返回的是一个PagingSource<Int, MovieEntity>对象，所以需要把MovieEntity转换为MovieItemModel

3.Data Mapper

Data Mapper广泛应用于MyBatis，Data Mapper将数据源的Model（MovieEntity）转换为页面显示Model（MovieItemModel），两者分开的原因就是为了Model层和View层进一步解耦

定义统一转换接口：

interface Mapper<I, O> {
    fun map(input: I): O
}  

针对MovieEntity和MovieItemModel实现接口

class MovieEntity2ItemModelMapper : Mapper<MovieEntity, MovieItemModel> {
    override fun map(input: MovieEntity): MovieItemModel {
        return input.run {
            MovieItemModel(
                id = id,
                title = title,
                cover = cover,
                rate = rate
            )
        }
    }
}  

4.利用Mapper对Repository转换

有了Mapper后，就可以将2.中我们的MovieEntity转换为MovieItemModel了

class MovieRepositoryImpl(
    private val api: MovieService,
    private val appDatabase: AppDatabase,
    private val mapper: MovieEntity2ItemModelMapper
) : Repository<MovieItemModel> {

    @OptIn(ExperimentalPagingApi::class)
    override fun fetchList(): Flow<PagingData<MovieItemModel>> {
        val pageSize = 10

        return Pager(
            config = PagingConfig(
                initialLoadSize = pageSize * 2,
                pageSize = pageSize,
                prefetchDistance = 1
            ),
            remoteMediator = MovieRemoteMediator(api, appDatabase)
        ) {
            appDatabase.movieDao().getMovieList()
        }.flow.flowOn(Dispatchers.IO).map { pagingData ->
            pagingData.map { mapper.map(it) }
        }
    }

}  

5.Hilt注入Repository

Repository的生命周期并不是伴随应用的，而是伴随Activity，所以安装到ActivityComponent 同样方法也不是单例的，而是根据Activity，使用ActivityScoped注解

@InstallIn(ActivityComponent::class)
@Module
object RepositoryModule {

    @ActivityScoped
    @Provides
    fun provideMovieRepository(
        api: MovieService,
        appDatabase: AppDatabase
    ): MovieRepositoryImpl {
        return MovieRepositoryImpl(api, appDatabase, MovieEntity2ItemModelMapper())
    }

}  

六、ViewModel

Model层的架构搭建完毕后，我们需要ViewModel层与Model层作数据交互

Hilt注入ViewModel构造函数

ViewModel中需要Repository对象作为属性，而Hilt支持使用ViewModelInject注解给ViewModel构造函数注入

class MovieViewModel @ViewModelInject constructor(
    private val repository: MovieRepositoryImpl
) : ViewModel() {
    val data = repository.fetchList().cachedIn(viewModelScope).asLiveData()
}  

七、Adapter与Coil

ViewModel完成后，接下来需要RecyclerView的Adapter，这块和之前的Paggin3一样

1.布局文件

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingVertical="10dip">


        <ImageView
            android:id="@+id/imageView"
            android:layout_width="100dip"
            android:layout_height="100dip"
            app:image="@{movie.cover}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toStartOf="@+id/guideline2"
            app:layout_constraintHorizontal_bias="0.432"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.054"
            tools:srcCompat="@tools:sample/avatars" />

        <TextView
            android:id="@+id/textViewTitle"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{movie.title}"
            android:textSize="16sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.0"
            app:layout_constraintStart_toStartOf="@+id/guideline"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.255"
            tools:text="泰坦尼克号" />

        <TextView
            android:id="@+id/textViewRate"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="24dp"
            android:text="@{movie.rate}"
            android:textSize="16sp"
            app:layout_constraintStart_toStartOf="@+id/guideline"
            app:layout_constraintTop_toBottomOf="@+id/textViewTitle"
            tools:text="评分:8.9分" />

        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            app:layout_constraintGuide_percent="0.4" />

        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            app:layout_constraintGuide_percent="0.5" />

    </androidx.constraintlayout.widget.ConstraintLayout>

    <data>

        <variable
            name="movie"
            type="com.aruba.mvvmapplication.model.MovieItemModel" />
    </data>
</layout>  

2.BindingAdapter

使用BindingAdapter自定义一个image属性 这边选用Coil作为图片加载框架，Coil相较于其他框架拥有更好的性能、更小的体积、易用性、结合了协程、androidx等最新技术、还拥有缓存、动态采样、加载暂停/终止等功能

@BindingAdapter("image")
fun setImage(imageView: ImageView, imageUrl: String) {
    imageView.load(imageUrl) {
        placeholder(R.drawable.ic_launcher_foreground)//占位图
        crossfade(true)//淡入淡出
    }
}  

3.Adapter实现

使用ViewDataBinding作为属性，定义一个基类ViewHolder

class BindingViewHolder(val binding: ViewDataBinding) : RecyclerView.ViewHolder(binding.root)  

Adapter继承PagingDataAdapter，并传入一个DiffUtil.ItemCallback

class MoviePagingAdapter : PagingDataAdapter<MovieItemModel, BindingViewHolder>(
    object : DiffUtil.ItemCallback<MovieItemModel>() {
        override fun areItemsTheSame(oldItem: MovieItemModel, newItem: MovieItemModel): Boolean {
            return oldItem.id == newItem.id
        }

        override fun areContentsTheSame(oldItem: MovieItemModel, newItem: MovieItemModel): Boolean {
            return oldItem == newItem
        }
    }
) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BindingViewHolder {
        val binding = ItemBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return BindingViewHolder(binding)
    }

    override fun onBindViewHolder(holder: BindingViewHolder, position: Int) {
        if (getItem(position) != null)
            (holder.binding as ItemBinding).movie = getItem(position)
    }

}  

4.为RecyclerView添加扩展函数

为了后续Paging的使用，为RecyclerView添加设置Adapter和liveData的扩展函数：

fun <VH : RecyclerView.ViewHolder, T : Any> RecyclerView.setPagingAdapter(
    owner: LifecycleOwner,
    adapter: PagingDataAdapter<T, VH>,
    liveData: LiveData<PagingData<T>>
) {
    liveData.observe(owner) {
        adapter.submitData(owner.lifecycle, it)
    }
}  

Activity的代码如下：

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    private val binding by lazy {
        ActivityMainBinding.inflate(layoutInflater)
    }
    private val viewModel: MovieViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)

        binding.recyclerview.setPagingAdapter(
            owner = this,
            adapter = MoviePagingAdapter(),
            liveData = viewModel.data
        )
    }
}  

八、实现RemoteMediator

之前未实现load函数的代码：

@OptIn(ExperimentalPagingApi::class)
class MovieRemoteMediator(
    private val api: MovieService,
    private val appDatabase: AppDatabase
) : RemoteMediator<Int, MovieEntity>() {
    
    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, MovieEntity>
    ): MediatorResult {
        TODO("Not yet implemented")
    }
    
}  

1.MediatorResult

load函数需要一个MediatorResult类型的返回值，MediatorResult有三种返回参数：

• MediatorResult.Error(e)：出现错误
• MediatorResult.Success(endOfPaginationReached = true)：请求成功且有数据(还有下一页)
• MediatorResult.Success(endOfPaginationReached = false)：请求成功但没有数据(到底了)

返回MediatorResult.Success，pager就会从数据库中拿数据，load函数初步实现：

{
        try {
            //1.判断loadType

            //2.请求网络分页数据

            //3.存入数据库

            val endOfPaginationReached = true
            return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
        } catch (e: Exception) {
            return MediatorResult.Error(e)
        }
}  

2.LoadType

LoadType为枚举类，有三个对象：

• Refresh：首次加载数据和调用PagingDataAdapter.refresh()时触发
• Append：加载更多数据时触发
• Prepend：在列表头部添加数据时触发，Refresh触发时也会触发

第一步就需要判断LoadType的状态，如果是Refresh，那么数据库中没有数据，就要从网络获取数据，Refresh状态下load函数执行完毕后会自动再次调用load函数，此时的LoadType为Append，此时数据库中有数据了，直接返回Success通知Pager可以从数据库取数据了

{
        try {
            //1.判断loadType
            val pageKey = when (loadType) {
                //首次加载
                LoadType.REFRESH -> null
                //REFRESH之后还会调用load（REFRESH时数据库中没有数据），来加载开头的数据，直接返回成功就可以了
                LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = false)
                //加载更多
                LoadType.APPEND -> {

                }
            }
            
            //2.请求网络分页数据
            val page = pageKey ?: 0
            
            //3.存入数据库

            val endOfPaginationReached = true
            return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
        } catch (e: Exception) {
            return MediatorResult.Error(e)
        }
}  

3.PagingState

对于下一页的数据，则要使用PagingState获取了，PagingState分为两部分组成：

• pages：上一页的数据，主要用来获取最后一个item，作为下一页的开始位置
• config：配置Pager时的PagingConfig，可以获取到pageSize等一系列初始化配置的值

如果上一页最后一个item为空，那么表示列表加载到底了，否则获取到需要加载的当前page

{
                //加载更多
                LoadType.APPEND -> {
                    val lastItem = state.lastItemOrNull() ?: return MediatorResult.Success(
                        endOfPaginationReached = true
                    )
                    lastItem.page//返回当前页
                }
}  

4.网络获取数据和存入数据库

接下来就是从网络获取数据了：

    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, MovieEntity>
    ): MediatorResult {
        try {
            //1.判断loadType
            val pageKey = when (loadType) {
                //首次加载
                LoadType.REFRESH -> null
                //REFRESH之后还会调用load（REFRESH时数据库中没有数据），来加载开头的数据，直接返回成功就可以了
                LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = false)
                //加载更多
                LoadType.APPEND -> {
                    val lastItem = state.lastItemOrNull() ?: return MediatorResult.Success(
                        endOfPaginationReached = true
                    )
                    lastItem.page//返回当前页
                }
            }

            //2.请求网络分页数据
            val page = pageKey ?: 0
            val result = api.getMovieList(
                page * state.config.pageSize,
                state.config.pageSize
            )

            //3.存入数据库

            val endOfPaginationReached = true
            return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
        } catch (e: Exception) {
            return MediatorResult.Error(e)
        }
    }  

将服务器对象转换为本地数据库对象后，存入数据库，完整RemoteMediator代码：

@OptIn(ExperimentalPagingApi::class)
class MovieRemoteMediator(
    private val api: MovieService,
    private val appDatabase: AppDatabase
) : RemoteMediator<Int, MovieEntity>() {

    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, MovieEntity>
    ): MediatorResult {
        try {
            //1.判断loadType
            val pageKey = when (loadType) {
                //首次加载
                LoadType.REFRESH -> null
                //REFRESH之后还会调用load（REFRESH时数据库中没有数据），来加载开头的数据，直接返回成功就可以了
                LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = false)
                //加载更多
                LoadType.APPEND -> {
                    val lastItem = state.lastItemOrNull() ?: return MediatorResult.Success(
                        endOfPaginationReached = true
                    )
                    lastItem.page//返回当前页
                }
            }

            //2.请求网络分页数据
            val page = pageKey ?: 0
            val result = api.getMovieList(
                page * state.config.pageSize,
                state.config.pageSize
            )

            //服务器对象转换为本地数据库对象
            val entity = result.map {
                MovieEntity(
                    id = it.id,
                    title = it.title,
                    cover = it.cover,
                    rate = it.rate,
                    page = page + 1
                )
            }
            //3.存入数据库
            val movieDao = appDatabase.movieDao()
            appDatabase.withTransaction {
                if (loadType == LoadType.REFRESH) {
                    movieDao.clear()
                }

                movieDao.insert(entity)
            }

            val endOfPaginationReached = result.isEmpty()
            return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
        } catch (e: Exception) {
            return MediatorResult.Error(e)
        }
    }

}  

运行后的效果：

 

联动.gif

九、刷新

1.上拉刷新、重试按钮、错误信息

上拉刷新、重试按钮、错误信息布局文件如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="10dp"
    android:layout_marginBottom="20dp"
    android:gravity="center"
    android:orientation="vertical"
    android:paddingBottom="20dp">

    <Button
        android:id="@+id/retryButton"
        style="@style/Widget.AppCompat.Button.Colored"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/footer_retry"
        android:textColor="@android:color/background_dark" />

    <ProgressBar
        android:id="@+id/progress"
        style="?android:attr/progressBarStyle"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/errorMsg"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@android:color/background_dark"
        tools:text="连接超时"/>

</LinearLayout>  

之前我们使用Paging的LoadStateAdapter，直接设置到PagingDataAdapter上就可以了，刷新对应的ViewHolder如下：

class NetWorkStateItemViewHolder(
    private val binding: NetworkStateItemBinding,
    val retryCallback: () -> Unit
) : RecyclerView.ViewHolder(binding.root) {

    fun bindData(data: LoadState){
        binding.apply {
            // 正在加载，显示进度条
            progress.isVisible = data is LoadState.Loading
            // 加载失败，显示并点击重试按钮
            retryButton.isVisible = data is LoadState.Error
            retryButton.setOnClickListener { retryCallback() }
            // 加载失败显示错误原因
            errorMsg.isVisible = !(data as? LoadState.Error)?.error?.message.isNullOrBlank()
            errorMsg.text = (data as? LoadState.Error)?.error?.message
        }
    }

}

inline var View.isVisible: Boolean
    get() = visibility == View.VISIBLE
    set(value) {
        visibility = if (value) View.VISIBLE else View.GONE
    }  

Adapter代码：

class FooterAdapter(
    val adapter: MoviePagingAdapter
) : LoadStateAdapter<NetWorkStateItemViewHolder>() {

    override fun onBindViewHolder(holder: NetWorkStateItemViewHolder, loadState: LoadState) {
        //水平居中
        val params = holder.itemView.layoutParams
        if (params is StaggeredGridLayoutManager.LayoutParams) {
            params.isFullSpan = true
        }
        holder.bindData(loadState)
    }

    override fun onCreateViewHolder(
        parent: ViewGroup,
        loadState: LoadState
    ): NetWorkStateItemViewHolder {
        val binding =
            NetworkStateItemBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return NetWorkStateItemViewHolder(binding) { adapter.retry() }
    }
}  

Activity中配置下PagingDataAdapter，并为RecyclerView设置ConcatAdapter，一定要设置成withLoadStateFooter函数返回的Adapter，否则不会有效果！！

        val adapter = MoviePagingAdapter()

        binding.recyclerview.adapter = adapter
            .run { withLoadStateFooter(FooterAdapter(this)) }  

2.下拉刷新

下拉刷新和之前也是相同的，布局中嵌套一个SwipeRefreshLayout：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".activity.MainActivity">

    <androidx.swiperefreshlayout.widget.SwipeRefreshLayout
        android:id="@+id/refreshLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/recyclerview"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layoutManager="androidx.recyclerview.widget.StaggeredGridLayoutManager"
            app:spanCount="2" />

    </androidx.swiperefreshlayout.widget.SwipeRefreshLayout>

</androidx.constraintlayout.widget.ConstraintLayout>  

Activity中对PagingDataAdapter的loadState进行监听：

        lifecycleScope.launchWhenCreated {
            //监听adapter状态
            adapter.loadStateFlow.collect {
                //根据刷新状态来通知swiprefreshLayout是否刷新完毕
                binding.refreshLayout.isRefreshing = it.refresh is LoadState.Loading
            }
        }  

十、App Starup实现无网络数据组件初始化

RemoteMediator中可以在无网络时从数据库获取数据，所以load函数中我们还需要对网络状态进行判断，无网络时，直接返回Success

1.获取网络状态的扩展函数

定义一个扩展函数用来获取网络状态：

@Suppress("DEPRECATION")
@SuppressLint("MissingPermission")
fun Context.isConnectedNetwork(): Boolean = run {
    val cm = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
    val activeNetwork: NetworkInfo? = cm.activeNetworkInfo
    activeNetwork?.isConnectedOrConnecting == true
}  

Manifest中不要忘了加权限

2.新建帮助类，初始化Context

object AppHelper {
    lateinit var mContext: Context

    fun init(context: Context) {
        this.mContext = context
    }
}  

3.RemoteMediator中判断网络状态并返回

            //无网络从本地数据库获取数据
            if (!AppHelper.mContext.isConnectedNetwork()) {
                return MediatorResult.Success(endOfPaginationReached = false)
            }  

此时AppHelper的init函数还没有调用

4.App Starup

 

image.png

App Starup是JetPack的新成员，提供了在App启动时初始化组件简单、高效的方法，还可以指定初始化顺序，我们新建一个类继承于Initializer：

class AppInitializer : Initializer<Unit> {

    override fun create(context: Context) {
        AppHelper.init(context)
    }

    //按顺序执行初始化
    override fun dependencies(): MutableList<Class<out Initializer<*>>> = mutableListOf()
}  

最后还需要在Manifest中注册：

        <provider
            android:name="androidx.startup.InitializationProvider"
            android:authorities="${applicationId}.androidx-startup"
            android:exported="false"
            tools:node="merge">
            <meta-data
                android:name="com.aruba.mvvmapplication.init.AppInitializer"
                android:value="androidx.startup" />
        </provider>  

最终效果：

 

项目地址：https://gitee.com/aruba/mvvmapplication.git