return success with endOfPaginationReached set to true when nextkey is null

app/build.gradle

@@ -49,6 +49,10 @@ android {
     viewBinding {
         enabled = true
     }
+
+    viewBinding {
+        enabled = true
+    }
 }
 
 dependencies {

app/src/main/java/com/example/android/codelabs/paging/Injection.kt

@@ -20,6 +20,7 @@ import android.content.Context
 import androidx.lifecycle.ViewModelProvider
 import com.example.android.codelabs.paging.api.GithubService
 import com.example.android.codelabs.paging.data.GithubRepository
+import com.example.android.codelabs.paging.db.RepoDatabase
 import com.example.android.codelabs.paging.ui.ViewModelFactory
 
 /**
@@ -33,15 +34,15 @@ object Injection {
      * Creates an instance of [GithubRepository] based on the [GithubService] and a
      * [GithubLocalCache]
      */
-    private fun provideGithubRepository(): GithubRepository {
-        return GithubRepository(GithubService.create())
+    private fun provideGithubRepository(context: Context): GithubRepository {
+        return GithubRepository(GithubService.create(), RepoDatabase.getInstance(context))
     }
 
     /**
      * Provides the [ViewModelProvider.Factory] that is then used to get a reference to
      * [ViewModel] objects.
      */
-    fun provideViewModelFactory(): ViewModelProvider.Factory {
-        return ViewModelFactory(provideGithubRepository())
+    fun provideViewModelFactory(context: Context): ViewModelProvider.Factory {
+        return ViewModelFactory(provideGithubRepository(context))
     }
 }

app/src/main/java/com/example/android/codelabs/paging/data/GithubRemoteMediator.kt

@@ -0,0 +1,141 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.android.codelabs.paging.data
+
+import androidx.paging.ExperimentalPagingApi
+import androidx.paging.LoadType
+import androidx.paging.PagingState
+import androidx.paging.RemoteMediator
+import androidx.room.withTransaction
+import com.example.android.codelabs.paging.api.GithubService
+import com.example.android.codelabs.paging.api.IN_QUALIFIER
+import com.example.android.codelabs.paging.db.RemoteKeys
+import com.example.android.codelabs.paging.db.RepoDatabase
+import com.example.android.codelabs.paging.model.Repo
+import retrofit2.HttpException
+import java.io.IOException
+import java.io.InvalidObjectException
+
+// GitHub page API is 1 based: https://developer.github.com/v3/#pagination
+private const val GITHUB_STARTING_PAGE_INDEX = 1
+
+@OptIn(ExperimentalPagingApi::class)
+class GithubRemoteMediator(
+        private val query: String,
+        private val service: GithubService,
+        private val repoDatabase: RepoDatabase
+) : RemoteMediator<Int, Repo>() {
+
+    override suspend fun load(loadType: LoadType, state: PagingState<Int, Repo>): MediatorResult {
+
+        val page = when (loadType) {
+            LoadType.REFRESH -> {
+                val remoteKeys = getRemoteKeyClosestToCurrentPosition(state)
+                remoteKeys?.nextKey?.minus(1) ?: GITHUB_STARTING_PAGE_INDEX
+            }
+            LoadType.PREPEND -> {
+                val remoteKeys = getRemoteKeyForFirstItem(state)
+                if (remoteKeys == null) {
+                    // The LoadType is PREPEND so some data was loaded before,
+                    // so we should have been able to get remote keys
+                    // If the remoteKeys are null, then we're an invalid state and we have a bug
+                    throw InvalidObjectException("Remote key and the prevKey should not be null")
+                }
+                // If the previous key is null, then we can't request more data
+                val prevKey = remoteKeys.prevKey
+                if (prevKey == null) {
+                    return MediatorResult.Success(endOfPaginationReached = true)
+                }
+                remoteKeys.prevKey
+            }
+            LoadType.APPEND -> {
+                val remoteKeys = getRemoteKeyForLastItem(state)
+                if (remoteKeys == null) {
+                    throw InvalidObjectException("Remote key should not be null for $loadType")
+                }
+
+                val nextKey = remoteKeys.nextKey
+                if (nextKey == null) {
+                    return MediatorResult.Success(endOfPaginationReached = true)
+                }
+                remoteKeys.nextKey
+            }
+
+        }
+
+        val apiQuery = query + IN_QUALIFIER
+
+        try {
+            val apiResponse = service.searchRepos(apiQuery, page, state.config.pageSize)
+
+            val repos = apiResponse.items
+            val endOfPaginationReached = repos.isEmpty()
+            repoDatabase.withTransaction {
+                // clear all tables in the database
+                if (loadType == LoadType.REFRESH) {
+                    repoDatabase.remoteKeysDao().clearRemoteKeys()
+                    repoDatabase.reposDao().clearRepos()
+                }
+                val prevKey = if (page == GITHUB_STARTING_PAGE_INDEX) null else page - 1
+                val nextKey = if (endOfPaginationReached) null else page + 1
+                val keys = repos.map {
+                    RemoteKeys(repoId = it.id, prevKey = prevKey, nextKey = nextKey)
+                }
+                repoDatabase.remoteKeysDao().insertAll(keys)
+                repoDatabase.reposDao().insertAll(repos)
+            }
+            return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
+        } catch (exception: IOException) {
+            return MediatorResult.Error(exception)
+        } catch (exception: HttpException) {
+            return MediatorResult.Error(exception)
+        }
+    }
+
+    private suspend fun getRemoteKeyForLastItem(state: PagingState<Int, Repo>): RemoteKeys? {
+        // Get the last page that was retrieved, that contained items.
+        // From that last page, get the last item
+        return state.pages.lastOrNull() { it.data.isNotEmpty() }?.data?.lastOrNull()
+                ?.let { repo ->
+                    // Get the remote keys of the last item retrieved
+                    repoDatabase.remoteKeysDao().remoteKeysRepoId(repo.id)
+                }
+    }
+
+    private suspend fun getRemoteKeyForFirstItem(state: PagingState<Int, Repo>): RemoteKeys? {
+        // Get the first page that was retrieved, that contained items.
+        // From that first page, get the first item
+        return state.pages.firstOrNull { it.data.isNotEmpty() }?.data?.firstOrNull()
+                ?.let { repo ->
+                    // Get the remote keys of the first items retrieved
+                    repoDatabase.remoteKeysDao().remoteKeysRepoId(repo.id)
+                }
+    }
+
+    private suspend fun getRemoteKeyClosestToCurrentPosition(
+            state: PagingState<Int, Repo>
+    ): RemoteKeys? {
+        // The paging library is trying to load data after the anchor position
+        // Get the item closest to the anchor position
+        return state.anchorPosition?.let { position ->
+            state.closestItemToPosition(position)?.id?.let { repoId ->
+                repoDatabase.remoteKeysDao().remoteKeysRepoId(repoId)
+            }
+        }
+    }
+
+}

app/src/main/java/com/example/android/codelabs/paging/data/GithubRepository.kt

@@ -17,95 +17,42 @@
 package com.example.android.codelabs.paging.data
 
 import android.util.Log
+import androidx.paging.Pager
+import androidx.paging.PagingConfig
+import androidx.paging.PagingData
 import com.example.android.codelabs.paging.api.GithubService
-import com.example.android.codelabs.paging.api.IN_QUALIFIER
+import com.example.android.codelabs.paging.db.RepoDatabase
 import com.example.android.codelabs.paging.model.Repo
-import com.example.android.codelabs.paging.model.RepoSearchResult
-import kotlinx.coroutines.ExperimentalCoroutinesApi
-import kotlinx.coroutines.FlowPreview
-import kotlinx.coroutines.channels.ConflatedBroadcastChannel
 import kotlinx.coroutines.flow.Flow
-import kotlinx.coroutines.flow.asFlow
-import retrofit2.HttpException
-import java.io.IOException
-
-// GitHub page API is 1 based: https://developer.github.com/v3/#pagination
-private const val GITHUB_STARTING_PAGE_INDEX = 1
 
 /**
  * Repository class that works with local and remote data sources.
  */
-@ExperimentalCoroutinesApi
-class GithubRepository(private val service: GithubService) {
-
-    // keep the list of all results received
-    private val inMemoryCache = mutableListOf<Repo>()
-
-    // keep channel of results. The channel allows us to broadcast updates so
-    // the subscriber will have the latest data
-    private val searchResults = ConflatedBroadcastChannel<RepoSearchResult>()
-
-    // keep the last requested page. When the request is successful, increment the page number.
-    private var lastRequestedPage = GITHUB_STARTING_PAGE_INDEX
-
-    // avoid triggering multiple requests in the same time
-    private var isRequestInProgress = false
+class GithubRepository(
+        private val service: GithubService,
+        private val database: RepoDatabase
+) {
 
     /**
      * Search repositories whose names match the query, exposed as a stream of data that will emit
      * every time we get more data from the network.
      */
-    suspend fun getSearchResultStream(query: String): Flow<RepoSearchResult> {
+    fun getSearchResultStream(query: String): Flow<PagingData<Repo>> {
         Log.d("GithubRepository", "New query: $query")
-        lastRequestedPage = 1
-        inMemoryCache.clear()
-        requestAndSaveData(query)
-
-        return searchResults.asFlow()
-    }
-
-    suspend fun requestMore(query: String) {
-        if (isRequestInProgress) return
-        val successful = requestAndSaveData(query)
-        if (successful) {
-            lastRequestedPage++
-        }
-    }
-
-    suspend fun retry(query: String) {
-        if (isRequestInProgress) return
-        requestAndSaveData(query)
-    }
-
-    private suspend fun requestAndSaveData(query: String): Boolean {
-        isRequestInProgress = true
-        var successful = false
-
-        val apiQuery = query + IN_QUALIFIER
-        try {
-            val response = service.searchRepos(apiQuery, lastRequestedPage, NETWORK_PAGE_SIZE)
-            Log.d("GithubRepository", "response $response")
-            val repos = response.items ?: emptyList()
-            inMemoryCache.addAll(repos)
-            val reposByName = reposByName(query)
-            searchResults.offer(RepoSearchResult.Success(reposByName))
-            successful = true
-        } catch (exception: IOException) {
-            searchResults.offer(RepoSearchResult.Error(exception))
-        } catch (exception: HttpException) {
-            searchResults.offer(RepoSearchResult.Error(exception))
-        }
-        isRequestInProgress = false
-        return successful
-    }
 
-    private fun reposByName(query: String): List<Repo> {
-        // from the in memory cache select only the repos whose name or description matches
-        // the query. Then order the results.
-        return inMemoryCache.filter {
-            it.name.contains(query, true) ||
-                    (it.description != null && it.description.contains(query, true))
-        }.sortedWith(compareByDescending<Repo> { it.stars }.thenBy { it.name })
+        // appending '%' so we can allow other characters to be before and after the query string
+        val dbQuery = "%${query.replace(' ', '%')}%"
+        val pagingSourceFactory = { database.reposDao().reposByName(dbQuery) }
+
+        return Pager(
+                config = PagingConfig(pageSize = NETWORK_PAGE_SIZE),
+                remoteMediator = GithubRemoteMediator(
+                        query,
+                        service,
+                        database
+                ),
+                pagingSourceFactory = pagingSourceFactory
+        ).flow
     }
 
     companion object {

app/src/main/java/com/example/android/codelabs/paging/db/RemoteKeys.kt

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.android.codelabs.paging.db
+
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+
+@Entity(tableName = "remote_keys")
+data class RemoteKeys(
+        @PrimaryKey val repoId: Long,
+        val prevKey: Int?,
+        val nextKey: Int?
+)

app/src/main/java/com/example/android/codelabs/paging/db/RemoteKeysDao.kt

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.android.codelabs.paging.db
+
+import androidx.room.Dao
+import androidx.room.Insert
+import androidx.room.OnConflictStrategy
+import androidx.room.Query
+
+@Dao
+interface RemoteKeysDao {
+
+    @Insert(onConflict = OnConflictStrategy.REPLACE)
+    suspend fun insertAll(remoteKey: List<RemoteKeys>)
+
+    @Query("SELECT * FROM remote_keys WHERE repoId = :repoId")
+    suspend fun remoteKeysRepoId(repoId: Long): RemoteKeys?
+
+    @Query("DELETE FROM remote_keys")
+    suspend fun clearRemoteKeys()
+}
+