Implement working paging

app/build.gradle

@@ -53,6 +53,7 @@ dependencies {
     implementation "androidx.recyclerview:recyclerview:1.0.0"
     implementation "androidx.lifecycle:lifecycle-runtime:2.0.0"
     implementation "androidx.lifecycle:lifecycle-extensions:2.0.0"
+    implementation "androidx.paging:paging-runtime-ktx:2.1.0"
     implementation "androidx.room:room-runtime:2.1.0-alpha04"
     kapt "androidx.room:room-compiler:2.1.0-alpha04"
 

app/src/androidTest/java/com/karumi/jetpack/superheroes/ui/view/AcceptanceTest.kt

@@ -29,7 +29,7 @@ abstract class AcceptanceTest<T : Activity>(clazz: Class<T>) : ScreenshotTest {
     val testRule: IntentsTestRule<T> = IntentsTestRule(clazz, true, false)
 
     private val executorServiceOnUiThread = mock<ExecutorService> {
-        on(it.submit(any())).thenAnswer { invocation ->
+        on(it.execute(any())).thenAnswer { invocation ->
             testRule.runOnUiThread { (invocation.getArgument(0) as Runnable).run() }
             FutureTask { null }
         }

app/src/androidTest/java/com/karumi/jetpack/superheroes/ui/view/MainActivityTest.kt

@@ -1,5 +1,8 @@
 package com.karumi.jetpack.superheroes.ui.view
 
+import android.os.Looper
+import androidx.paging.PagedList
+import androidx.paging.PositionalDataSource
 import com.karumi.jetpack.superheroes.data.repository.SuperHeroRepository
 import com.karumi.jetpack.superheroes.data.singleValueLiveData
 import com.karumi.jetpack.superheroes.domain.model.SuperHero
@@ -9,6 +12,7 @@ import org.kodein.di.Kodein
 import org.kodein.di.erased.bind
 import org.kodein.di.erased.instance
 import org.mockito.Mock
+import java.util.concurrent.Executors.newSingleThreadExecutor
 
 class MainActivityTest : AcceptanceTest<MainActivity>(MainActivity::class.java) {
 
@@ -64,6 +68,11 @@ class MainActivityTest : AcceptanceTest<MainActivity>(MainActivity::class.java)
         compareScreenshot(activity)
     }
 
+    private fun compareScreenshot(activity: MainActivity) {
+        Thread.sleep(100)
+        super.compareScreenshot(activity)
+    }
+
     private fun givenThereAreSomeAvengers(numberOfAvengers: Int): List<SuperHero> =
         givenThereAreSomeSuperHeroes(numberOfAvengers, areAvengers = true)
 
@@ -84,15 +93,42 @@ class MainActivityTest : AcceptanceTest<MainActivity>(MainActivity::class.java)
             )
         }
 
-        whenever(repository.getAllSuperHeroes()).thenReturn(singleValueLiveData(superHeroes))
+        whenever(repository.getAllSuperHeroes())
+            .thenReturn(singleValueLiveData(superHeroes.toPagedList()))
+
         return superHeroes
     }
 
     private fun givenThereAreNoSuperHeroes() {
-        whenever(repository.getAllSuperHeroes()).thenReturn(singleValueLiveData(emptyList()))
+        whenever(repository.getAllSuperHeroes())
+            .thenReturn(singleValueLiveData(emptyList<SuperHero>().toPagedList()))
     }
 
     override val testDependencies = Kodein.Module("Test dependencies", allowSilentOverride = true) {
         bind<SuperHeroRepository>() with instance(repository)
     }
+
+    private fun List<SuperHero>.toPagedList(): PagedList<SuperHero> =
+        PagedList.Builder(object : PositionalDataSource<SuperHero>() {
+            override fun loadRange(
+                params: LoadRangeParams,
+                callback: LoadRangeCallback<SuperHero>
+            ) {
+                callback.onResult(this@toPagedList)
+            }
+
+            override fun loadInitial(
+                params: LoadInitialParams,
+                callback: LoadInitialCallback<SuperHero>
+            ) {
+                callback.onResult(
+                    this@toPagedList,
+                    0,
+                    this@toPagedList.size
+                )
+            }
+        }, 100)
+            .setNotifyExecutor(newSingleThreadExecutor { Looper.getMainLooper().thread })
+            .setFetchExecutor(newSingleThreadExecutor { Looper.getMainLooper().thread })
+            .build()
 }

app/src/main/java/com/karumi/jetpack/superheroes/SuperHeroesApplication.kt

@@ -6,6 +6,7 @@ import com.karumi.jetpack.superheroes.common.module
 import com.karumi.jetpack.superheroes.data.repository.LocalSuperHeroDataSource
 import com.karumi.jetpack.superheroes.data.repository.RemoteSuperHeroDataSource
 import com.karumi.jetpack.superheroes.data.repository.SuperHeroRepository
+import com.karumi.jetpack.superheroes.data.repository.SuperHeroesBoundaryCallback
 import com.karumi.jetpack.superheroes.data.repository.room.SuperHeroDao
 import org.kodein.di.DKodein
 import org.kodein.di.Kodein
@@ -41,9 +42,12 @@ class SuperHeroesApplication : Application(), KodeinAware {
             database.superHeroesDao()
         }
         bind<SuperHeroRepository>() with provider {
-            SuperHeroRepository(instance(), instance())
+            SuperHeroRepository(instance(), instance(), instance())
         }
-        bind<LocalSuperHeroDataSource>() with singleton {
+        bind<SuperHeroesBoundaryCallback>() with provider {
+            SuperHeroesBoundaryCallback(instance(), instance())
+        }
+        bind<LocalSuperHeroDataSource>() with provider {
             LocalSuperHeroDataSource(instance(), instance())
         }
         bind<RemoteSuperHeroDataSource>() with provider {

app/src/main/java/com/karumi/jetpack/superheroes/data/repository/LocalSuperHeroDataSource.kt

@@ -2,6 +2,8 @@ package com.karumi.jetpack.superheroes.data.repository
 
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.Transformations
+import androidx.paging.LivePagedListBuilder
+import androidx.paging.PagedList
 import com.karumi.jetpack.superheroes.data.repository.room.SuperHeroDao
 import com.karumi.jetpack.superheroes.data.repository.room.SuperHeroEntity
 import com.karumi.jetpack.superheroes.domain.model.SuperHero
@@ -11,14 +13,20 @@ class LocalSuperHeroDataSource(
     private val dao: SuperHeroDao,
     private val executor: ExecutorService
 ) {
-    fun getAllSuperHeroes(): LiveData<List<SuperHero>> =
-        Transformations.map(dao.getAll()) { it.toSuperHeroes() }
+    fun getAllSuperHeroes(
+        pageSize: Int,
+        boundaryCallback: PagedList.BoundaryCallback<SuperHero>
+    ): LiveData<PagedList<SuperHero>> {
+        val superHeroesFactory = dao.getAll().map { it.toSuperHero() }
+        return LivePagedListBuilder(superHeroesFactory, pageSize)
+            .setBoundaryCallback(boundaryCallback)
+            .build()
+    }
 
     fun get(id: String): LiveData<SuperHero?> =
         Transformations.map(dao.getById(id)) { it?.toSuperHero() }
 
     fun saveAll(all: List<SuperHero>) = executor.execute {
-        dao.deleteAll()
         dao.insertAll(all.map { it.toEntity() })
     }
 
@@ -27,7 +35,6 @@ class LocalSuperHeroDataSource(
         return superHero
     }
 
-    private fun List<SuperHeroEntity>.toSuperHeroes(): List<SuperHero> = map { it.toSuperHero() }
     private fun SuperHeroEntity.toSuperHero(): SuperHero = superHero
     private fun SuperHero.toEntity(): SuperHeroEntity = SuperHeroEntity(this)
 }

app/src/main/java/com/karumi/jetpack/superheroes/data/repository/RemoteSuperHeroDataSource.kt

@@ -15,11 +15,15 @@ class RemoteSuperHeroDataSource(
     private val superHeroes: MutableMap<String, SuperHero> =
         fakeData().associateBy { it.id }.toMutableMap()
 
-    fun getAllSuperHeroes(): LiveData<List<SuperHero>> {
+    fun getSuperHeroesPage(pageIndex: Int, pageSize: Int): LiveData<List<SuperHero>> {
         val allSuperHeroes = MutableLiveData<List<SuperHero>>()
         executor.execute {
             waitABit()
-            allSuperHeroes.postValue(superHeroes.values.toList().sortedBy { it.id })
+            val superHeroesPage = superHeroes.values.toList()
+                .sortedBy { it.id }
+                .drop(pageIndex * pageSize)
+                .take(pageSize)
+            allSuperHeroes.postValue(superHeroesPage)
         }
         return allSuperHeroes
     }

app/src/main/java/com/karumi/jetpack/superheroes/data/repository/SuperHeroRepository.kt

@@ -2,23 +2,21 @@ package com.karumi.jetpack.superheroes.data.repository
 
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.MediatorLiveData
+import androidx.paging.PagedList
 import com.karumi.jetpack.superheroes.domain.model.SuperHero
 
 class SuperHeroRepository(
     private val local: LocalSuperHeroDataSource,
-    private val remote: RemoteSuperHeroDataSource
+    private val remote: RemoteSuperHeroDataSource,
+    private val boundaryCallback: SuperHeroesBoundaryCallback
 ) {
-    fun getAllSuperHeroes(): LiveData<List<SuperHero>> = MediatorLiveData<List<SuperHero>>().apply {
-        val localSource = local.getAllSuperHeroes()
-        val remoteSource = remote.getAllSuperHeroes()
-
-        addSource(remoteSource) { superHeroes ->
-            removeSource(remoteSource)
-            addSource(localSource) { postValue(it) }
-            local.saveAll(superHeroes)
-        }
+    companion object {
+        const val PAGE_SIZE = 4
     }
 
+    fun getAllSuperHeroes(): LiveData<PagedList<SuperHero>> =
+        local.getAllSuperHeroes(PAGE_SIZE, boundaryCallback)
+
     fun get(id: String): LiveData<SuperHero?> = MediatorLiveData<SuperHero?>().apply {
         addSource(local.get(id)) {
             if (it == null) {

app/src/main/java/com/karumi/jetpack/superheroes/data/repository/SuperHeroesBoundaryCallback.kt

@@ -0,0 +1,35 @@
+package com.karumi.jetpack.superheroes.data.repository
+
+import androidx.lifecycle.Observer
+import androidx.paging.PagedList
+import com.karumi.jetpack.superheroes.domain.model.SuperHero
+
+class SuperHeroesBoundaryCallback(
+    private val local: LocalSuperHeroDataSource,
+    private val remote: RemoteSuperHeroDataSource
+) : PagedList.BoundaryCallback<SuperHero>() {
+
+    private var pageIndexAboutToLoad = 0
+
+    override fun onZeroItemsLoaded() {
+        pageIndexAboutToLoad = 0
+        loadNextPage()
+    }
+
+    override fun onItemAtEndLoaded(itemAtEnd: SuperHero) {
+        loadNextPage()
+    }
+
+    private fun loadNextPage() {
+        val remoteSuperHeroesPage =
+            remote.getSuperHeroesPage(pageIndexAboutToLoad, SuperHeroRepository.PAGE_SIZE)
+
+        remoteSuperHeroesPage.observeForever(object : Observer<List<SuperHero>> {
+            override fun onChanged(superHeroes: List<SuperHero>) {
+                local.saveAll(superHeroes)
+                remoteSuperHeroesPage.removeObserver(this)
+                pageIndexAboutToLoad++
+            }
+        })
+    }
+}

app/src/main/java/com/karumi/jetpack/superheroes/data/repository/room/SuperHeroDao.kt

@@ -1,6 +1,7 @@
 package com.karumi.jetpack.superheroes.data.repository.room
 
 import androidx.lifecycle.LiveData
+import androidx.paging.DataSource
 import androidx.room.Dao
 import androidx.room.Insert
 import androidx.room.OnConflictStrategy
@@ -10,7 +11,7 @@ import androidx.room.Update
 @Dao
 interface SuperHeroDao {
     @Query("SELECT * FROM superheroes ORDER BY superhero_id ASC")
-    fun getAll(): LiveData<List<SuperHeroEntity>>
+    fun getAll(): DataSource.Factory<Int, SuperHeroEntity>
 
     @Query("SELECT * FROM superheroes WHERE superhero_id = :id")
     fun getById(id: String): LiveData<SuperHeroEntity?>

app/src/main/java/com/karumi/jetpack/superheroes/domain/usecase/GetSuperHeroes.kt

@@ -1,9 +1,11 @@
 package com.karumi.jetpack.superheroes.domain.usecase
 
 import androidx.lifecycle.LiveData
+import androidx.paging.PagedList
 import com.karumi.jetpack.superheroes.data.repository.SuperHeroRepository
 import com.karumi.jetpack.superheroes.domain.model.SuperHero
 
 class GetSuperHeroes(private val superHeroesRepository: SuperHeroRepository) {
-    operator fun invoke(): LiveData<List<SuperHero>> = superHeroesRepository.getAllSuperHeroes()
+    operator fun invoke(): LiveData<PagedList<SuperHero>> =
+        superHeroesRepository.getAllSuperHeroes()
 }

app/src/main/java/com/karumi/jetpack/superheroes/ui/view/MainActivity.kt

@@ -3,6 +3,7 @@ package com.karumi.jetpack.superheroes.ui.view
 import android.os.Bundle
 import androidx.appcompat.widget.Toolbar
 import androidx.lifecycle.Observer
+import androidx.paging.PagedList
 import androidx.recyclerview.widget.LinearLayoutManager
 import com.karumi.jetpack.superheroes.R
 import com.karumi.jetpack.superheroes.common.bindViewModel
@@ -32,7 +33,7 @@ class MainActivity : BaseActivity<MainActivityBinding>() {
         initializeRecyclerView()
         viewModel.prepare()
         viewModel.idOfSuperHeroToOpen.observe(this, Observer<String> { openDetail(it) })
-        viewModel.superHeroes.observe(this, Observer<List<SuperHero>> { showSuperHeroes(it) })
+        viewModel.superHeroes.observe(this, Observer<PagedList<SuperHero>> { showSuperHeroes(it) })
     }
 
     override fun configureBinding(binding: MainActivityBinding) {
@@ -49,10 +50,8 @@ class MainActivity : BaseActivity<MainActivityBinding>() {
         recycler_view.adapter = adapter
     }
 
-    private fun showSuperHeroes(superHeroes: List<SuperHero>) {
-        adapter.clear()
-        adapter.addAll(superHeroes)
-        adapter.notifyDataSetChanged()
+    private fun showSuperHeroes(superHeroes: PagedList<SuperHero>) {
+        adapter.submitList(superHeroes)
     }
 
     private fun openDetail(id: String) {

app/src/main/java/com/karumi/jetpack/superheroes/ui/view/SingleLiveEvent.kt

@@ -27,7 +27,6 @@ import androidx.lifecycle.MutableLiveData
 import androidx.lifecycle.Observer
 import java.util.concurrent.atomic.AtomicBoolean
 
-
 /**
  * A lifecycle-aware observable that sends only new updates after subscription, used for events like
  * navigation and Snackbar messages.

app/src/main/java/com/karumi/jetpack/superheroes/ui/view/adapter/SuperHeroesAdapter.kt

@@ -3,21 +3,16 @@ package com.karumi.jetpack.superheroes.ui.view.adapter
 import android.view.LayoutInflater
 import android.view.ViewGroup
 import androidx.databinding.DataBindingUtil
-import androidx.recyclerview.widget.RecyclerView
+import androidx.paging.PagedListAdapter
+import androidx.recyclerview.widget.DiffUtil
 import com.karumi.jetpack.superheroes.R
 import com.karumi.jetpack.superheroes.databinding.SuperHeroRowBinding
 import com.karumi.jetpack.superheroes.domain.model.SuperHero
 import com.karumi.jetpack.superheroes.ui.viewmodel.SuperHeroesViewModel
 
 internal class SuperHeroesAdapter(
     private val viewModel: SuperHeroesViewModel
-) : RecyclerView.Adapter<SuperHeroViewHolder>() {
-    private val superHeroes: MutableList<SuperHero> = ArrayList()
-
-    fun addAll(collection: Collection<SuperHero>) {
-        superHeroes.addAll(collection)
-    }
-
+) : PagedListAdapter<SuperHero, SuperHeroViewHolder>(diffCallback) {
     override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): SuperHeroViewHolder {
         val binding: SuperHeroRowBinding = DataBindingUtil.inflate(
             LayoutInflater.from(parent.context),
@@ -30,14 +25,17 @@ internal class SuperHeroesAdapter(
     }
 
     override fun onBindViewHolder(holder: SuperHeroViewHolder, position: Int) {
-        holder.render(superHeroes[position], viewModel)
+        val superHero = getItem(position) ?: return
+        holder.render(superHero, viewModel)
     }
 
-    override fun getItemCount(): Int {
-        return superHeroes.size
-    }
+    companion object {
+        private val diffCallback = object : DiffUtil.ItemCallback<SuperHero>() {
+            override fun areItemsTheSame(oldItem: SuperHero, newItem: SuperHero): Boolean =
+                oldItem.id == newItem.id
 
-    fun clear() {
-        superHeroes.clear()
+            override fun areContentsTheSame(oldItem: SuperHero, newItem: SuperHero): Boolean =
+                oldItem == newItem
+        }
     }
 }

app/src/main/java/com/karumi/jetpack/superheroes/ui/viewmodel/SuperHeroesViewModel.kt

@@ -4,6 +4,7 @@ import android.app.Application
 import androidx.lifecycle.AndroidViewModel
 import androidx.lifecycle.MediatorLiveData
 import androidx.lifecycle.MutableLiveData
+import androidx.paging.PagedList
 import com.karumi.jetpack.superheroes.domain.model.SuperHero
 import com.karumi.jetpack.superheroes.domain.usecase.GetSuperHeroes
 import com.karumi.jetpack.superheroes.ui.view.SingleLiveEvent
@@ -15,7 +16,7 @@ class SuperHeroesViewModel(
 
     val isLoading = MutableLiveData<Boolean>()
     val isShowingEmptyCase = MutableLiveData<Boolean>()
-    val superHeroes = MediatorLiveData<List<SuperHero>>()
+    val superHeroes = MediatorLiveData<PagedList<SuperHero>>()
     val idOfSuperHeroToOpen = SingleLiveEvent<String>()
 
     fun prepare() {