Fix lint warnings [Solution]

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

@@ -16,10 +16,9 @@
 
 package com.example.android.codelabs.paging.data
 
-import android.arch.lifecycle.MutableLiveData
+import android.arch.paging.LivePagedListBuilder
 import android.util.Log
 import com.example.android.codelabs.paging.api.GithubService
-import com.example.android.codelabs.paging.api.searchRepos
 import com.example.android.codelabs.paging.db.GithubLocalCache
 import com.example.android.codelabs.paging.model.RepoSearchResult
 
@@ -31,49 +30,31 @@ class GithubRepository(
         private val cache: GithubLocalCache
 ) {
 
-    // keep the last requested page. When the request is successful, increment the page number.
-    private var lastRequestedPage = 1
-
-    // LiveData of network errors.
-    private val networkErrors = MutableLiveData<String>()
-
-    // avoid triggering multiple requests in the same time
-    private var isRequestInProgress = false
-
     /**
      * Search repositories whose names match the query.
      */
     fun search(query: String): RepoSearchResult {
         Log.d("GithubRepository", "New query: $query")
-        lastRequestedPage = 1
-        requestAndSaveData(query)
-
-        // Get data from the local cache
-        val data = cache.reposByName(query)
 
-        return RepoSearchResult(data, networkErrors)
-    }
+        // Get data source factory from the local cache
+        val dataSourceFactory = cache.reposByName(query)
 
-    fun requestMore(query: String) {
-        requestAndSaveData(query)
-    }
+        // every new query creates a new BoundaryCallback
+        // The BoundaryCallback will observe when the user reaches to the edges of
+        // the list and update the database with extra data
+        val boundaryCallback = RepoBoundaryCallback(query, service, cache)
+        val networkErrors = boundaryCallback.networkErrors
 
-    private fun requestAndSaveData(query: String) {
-        if (isRequestInProgress) return
+        // Get the paged list
+        val data = LivePagedListBuilder(dataSourceFactory, DATABASE_PAGE_SIZE)
+                .setBoundaryCallback(boundaryCallback)
+                .build()
 
-        isRequestInProgress = true
-        searchRepos(service, query, lastRequestedPage, NETWORK_PAGE_SIZE, { repos ->
-            cache.insert(repos, {
-                lastRequestedPage++
-                isRequestInProgress = false
-            })
-        }, { error ->
-            networkErrors.postValue(error)
-            isRequestInProgress = false
-        })
+        // Get the network errors exposed by the boundary callback
+        return RepoSearchResult(data, networkErrors)
     }
 
     companion object {
-        private const val NETWORK_PAGE_SIZE = 50
+        private const val DATABASE_PAGE_SIZE = 20
     }
 }

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

@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2018 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 android.arch.lifecycle.LiveData
+import android.arch.lifecycle.MutableLiveData
+import android.arch.paging.PagedList
+import android.util.Log
+import com.example.android.codelabs.paging.api.GithubService
+import com.example.android.codelabs.paging.api.searchRepos
+import com.example.android.codelabs.paging.db.GithubLocalCache
+import com.example.android.codelabs.paging.model.Repo
+
+/**
+ * This boundary callback gets notified when user reaches to the edges of the list for example when
+ * the database cannot provide any more data.
+ **/
+class RepoBoundaryCallback(
+        private val query: String,
+        private val service: GithubService,
+        private val cache: GithubLocalCache
+) : PagedList.BoundaryCallback<Repo>() {
+
+    companion object {
+        private const val NETWORK_PAGE_SIZE = 50
+    }
+
+    // keep the last requested page. When the request is successful, increment the page number.
+    private var lastRequestedPage = 1
+
+    private val _networkErrors = MutableLiveData<String>()
+    // LiveData of network errors.
+    val networkErrors: LiveData<String>
+        get() = _networkErrors
+
+    // avoid triggering multiple requests in the same time
+    private var isRequestInProgress = false
+
+    /**
+     * Database returned 0 items. We should query the backend for more items.
+     */
+    override fun onZeroItemsLoaded() {
+        Log.d("RepoBoundaryCallback", "onZeroItemsLoaded")
+        requestAndSaveData(query)
+    }
+
+    /**
+     * When all items in the database were loaded, we need to query the backend for more items.
+     */
+    override fun onItemAtEndLoaded(itemAtEnd: Repo) {
+        Log.d("RepoBoundaryCallback", "onItemAtEndLoaded")
+        requestAndSaveData(query)
+    }
+
+    private fun requestAndSaveData(query: String) {
+        if (isRequestInProgress) return
+
+        isRequestInProgress = true
+        searchRepos(service, query, lastRequestedPage, NETWORK_PAGE_SIZE, { repos ->
+            cache.insert(repos) {
+                lastRequestedPage++
+                isRequestInProgress = false
+            }
+        }, { error ->
+            _networkErrors.postValue(error)
+            isRequestInProgress = false
+        })
+    }
+}

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

@@ -16,7 +16,6 @@
 
 package com.example.android.codelabs.paging.db
 
-import android.arch.lifecycle.LiveData
 import android.arch.paging.DataSource
 import android.util.Log
 import com.example.android.codelabs.paging.model.Repo
@@ -34,7 +33,7 @@ class GithubLocalCache(
     /**
      * Insert a list of repos in the database, on a background thread.
      */
-    fun insert(repos: List<Repo>, insertFinished: ()-> Unit) {
+    fun insert(repos: List<Repo>, insertFinished: () -> Unit) {
         ioExecutor.execute {
             Log.d("GithubLocalCache", "inserting ${repos.size} repos")
             repoDao.insert(repos)
@@ -48,7 +47,7 @@ class GithubLocalCache(
      * any characters between the words.
      * @param name repository name
      */
-    fun reposByName(name: String): LiveData<List<Repo>> {
+    fun reposByName(name: String): DataSource.Factory<Int, Repo> {
         // appending '%' so we can allow other characters to be before and after the query string
         val query = "%${name.replace(' ', '%')}%"
         return repoDao.reposByName(query)

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

@@ -16,7 +16,7 @@
 
 package com.example.android.codelabs.paging.db
 
-import android.arch.lifecycle.LiveData
+import android.arch.paging.DataSource
 import android.arch.persistence.room.Dao
 import android.arch.persistence.room.Insert
 import android.arch.persistence.room.OnConflictStrategy
@@ -37,6 +37,6 @@ interface RepoDao {
     // and order those results descending, by the number of stars and then by name
     @Query("SELECT * FROM repos WHERE (name LIKE :queryString) OR (description LIKE " +
             ":queryString) ORDER BY stars DESC, name ASC")
-    fun reposByName(queryString: String): LiveData<List<Repo>>
+    fun reposByName(queryString: String): DataSource.Factory<Int, Repo>
 
 }

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

@@ -17,12 +17,13 @@
 package com.example.android.codelabs.paging.model
 
 import android.arch.lifecycle.LiveData
+import android.arch.paging.PagedList
 
 /**
  * RepoSearchResult from a search, which contains LiveData<List<Repo>> holding query data,
  * and a LiveData<String> of network error state.
  */
 data class RepoSearchResult(
-        val data: LiveData<List<Repo>>,
+        val data: LiveData<PagedList<Repo>>,
         val networkErrors: LiveData<String>
 )

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

@@ -16,7 +16,7 @@
 
 package com.example.android.codelabs.paging.ui
 
-import android.support.v7.recyclerview.extensions.ListAdapter
+import android.arch.paging.PagedListAdapter
 import android.support.v7.util.DiffUtil
 import android.support.v7.widget.RecyclerView
 import android.view.ViewGroup
@@ -25,7 +25,7 @@ import com.example.android.codelabs.paging.model.Repo
 /**
  * Adapter for the list of repositories.
  */
-class ReposAdapter : ListAdapter<Repo, RecyclerView.ViewHolder>(REPO_COMPARATOR) {
+class ReposAdapter : PagedListAdapter<Repo, RecyclerView.ViewHolder>(REPO_COMPARATOR) {
 
     override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
         return RepoViewHolder.create(parent)

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

@@ -18,6 +18,7 @@ package com.example.android.codelabs.paging.ui
 
 import android.arch.lifecycle.Observer
 import android.arch.lifecycle.ViewModelProviders
+import android.arch.paging.PagedList
 import android.os.Bundle
 import android.support.v7.app.AppCompatActivity
 import android.support.v7.widget.DividerItemDecoration
@@ -31,6 +32,7 @@ import android.widget.Toast
 import com.example.android.codelabs.paging.R
 import com.example.android.codelabs.paging.Injection
 import com.example.android.codelabs.paging.model.Repo
+import kotlinx.android.synthetic.clearFindViewByIdCache
 import kotlinx.android.synthetic.main.activity_search_repositories.*
 
 
@@ -50,22 +52,26 @@ class SearchRepositoriesActivity : AppCompatActivity() {
         // add dividers between RecyclerView's row items
         val decoration = DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
         list.addItemDecoration(decoration)
-        setupScrollListener()
 
         initAdapter()
         val query = savedInstanceState?.getString(LAST_SEARCH_QUERY) ?: DEFAULT_QUERY
         viewModel.searchRepo(query)
         initSearch(query)
     }
 
+    override fun onDestroy() {
+        super.onDestroy()
+        clearFindViewByIdCache()
+    }
+
     override fun onSaveInstanceState(outState: Bundle) {
         super.onSaveInstanceState(outState)
         outState.putString(LAST_SEARCH_QUERY, viewModel.lastQueryValue())
     }
 
     private fun initAdapter() {
         list.adapter = adapter
-        viewModel.repos.observe(this, Observer<List<Repo>> {
+        viewModel.repos.observe(this, Observer<PagedList<Repo>> {
             Log.d("Activity", "list: ${it?.size}")
             showEmptyList(it?.size == 0)
             adapter.submitList(it)
@@ -78,22 +84,22 @@ class SearchRepositoriesActivity : AppCompatActivity() {
     private fun initSearch(query: String) {
         search_repo.setText(query)
 
-        search_repo.setOnEditorActionListener({ _, actionId, _ ->
+        search_repo.setOnEditorActionListener { _, actionId, _ ->
             if (actionId == EditorInfo.IME_ACTION_GO) {
                 updateRepoListFromInput()
                 true
             } else {
                 false
             }
-        })
-        search_repo.setOnKeyListener({ _, keyCode, event ->
+        }
+        search_repo.setOnKeyListener { _, keyCode, event ->
             if (event.action == KeyEvent.ACTION_DOWN && keyCode == KeyEvent.KEYCODE_ENTER) {
                 updateRepoListFromInput()
                 true
             } else {
                 false
             }
-        })
+        }
     }
 
     private fun updateRepoListFromInput() {
@@ -116,20 +122,6 @@ class SearchRepositoriesActivity : AppCompatActivity() {
         }
     }
 
-    private fun setupScrollListener() {
-        val layoutManager = list.layoutManager as LinearLayoutManager
-        list.addOnScrollListener(object : RecyclerView.OnScrollListener() {
-            override fun onScrolled(recyclerView: RecyclerView?, dx: Int, dy: Int) {
-                super.onScrolled(recyclerView, dx, dy)
-                val totalItemCount = layoutManager.itemCount
-                val visibleItemCount = layoutManager.childCount
-                val lastVisibleItem = layoutManager.findLastVisibleItemPosition()
-
-                viewModel.listScrolled(visibleItemCount, lastVisibleItem, totalItemCount)
-            }
-        })
-    }
-
     companion object {
         private const val LAST_SEARCH_QUERY: String = "last_search_query"
         private const val DEFAULT_QUERY = "Android"

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

@@ -20,6 +20,7 @@ import android.arch.lifecycle.LiveData
 import android.arch.lifecycle.MutableLiveData
 import android.arch.lifecycle.Transformations
 import android.arch.lifecycle.ViewModel
+import android.arch.paging.PagedList
 import com.example.android.codelabs.paging.data.GithubRepository
 import com.example.android.codelabs.paging.model.Repo
 import com.example.android.codelabs.paging.model.RepoSearchResult
@@ -30,16 +31,12 @@ import com.example.android.codelabs.paging.model.RepoSearchResult
  */
 class SearchRepositoriesViewModel(private val repository: GithubRepository) : ViewModel() {
 
-    companion object {
-        private const val VISIBLE_THRESHOLD = 5
-    }
-
     private val queryLiveData = MutableLiveData<String>()
     private val repoResult: LiveData<RepoSearchResult> = Transformations.map(queryLiveData, {
         repository.search(it)
     })
 
-    val repos: LiveData<List<Repo>> = Transformations.switchMap(repoResult,
+    val repos: LiveData<PagedList<Repo>> = Transformations.switchMap(repoResult,
             { it -> it.data })
     val networkErrors: LiveData<String> = Transformations.switchMap(repoResult,
             { it -> it.networkErrors })
@@ -51,15 +48,6 @@ class SearchRepositoriesViewModel(private val repository: GithubRepository) : Vi
         queryLiveData.postValue(queryString)
     }
 
-    fun listScrolled(visibleItemCount: Int, lastVisibleItemPosition: Int, totalItemCount: Int) {
-        if (visibleItemCount + lastVisibleItemPosition + VISIBLE_THRESHOLD >= totalItemCount) {
-            val immutableQuery = lastQueryValue()
-            if (immutableQuery != null) {
-                repository.requestMore(immutableQuery)
-            }
-        }
-    }
-
     /**
      * Get the last query value.
      */