Use ViewmodelScope instead of creating coroutine scope

* Use viewModelScope to call the database operations.
* Remove Dispatcher.IO context and coroutine scope for @dao methods.

Room library automatically moves database calls onto a background thread, so you don't explicitly need to move them to a background thread. A suspend @dao method and a `@Query` method returning LiveData in Room automatically uses a background thread for database calls. You don't have to explicitly specify the Dispatcher.IO. Room does it for you in generated implementation.

Author: android-dev-lxl

DevBytes-starter/app/build.gradle

@@ -28,8 +28,8 @@ android {
         targetSdkVersion 30
         versionCode 1
         versionName "1.0"
-        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         vectorDrawables.useSupportLibrary = true
+        multiDexEnabled true
     }
     buildTypes {
         release {
@@ -54,7 +54,7 @@ dependencies {
     implementation 'com.google.android.material:material:1.0.0'
 
     // Android KTX
-    implementation 'androidx.core:core-ktx:1.0.2'
+    implementation 'androidx.core:core-ktx:1.3.1'
 
     // constraint layout
     implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
@@ -87,6 +87,7 @@ dependencies {
     // ViewModel and LiveData
     def lifecycle_version = "2.2.0"
     implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"
+    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
 
     // logging
     implementation 'com.jakewharton.timber:timber:4.7.1'

DevBytes-starter/app/src/main/java/com/example/android/devbyteviewer/viewmodels/DevByteViewModel.kt

@@ -22,6 +22,7 @@ import androidx.lifecycle.LiveData
 import androidx.lifecycle.MutableLiveData
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.ViewModelProvider
+import androidx.lifecycle.viewModelScope
 import com.example.android.devbyteviewer.domain.DevByteVideo
 import com.example.android.devbyteviewer.network.DevByteNetwork
 import com.example.android.devbyteviewer.network.asDomainModel
@@ -40,21 +41,6 @@ import java.io.IOException
  */
 class DevByteViewModel(application: Application) : AndroidViewModel(application) {
 
-    /**
-     * This is the job for all coroutines started by this ViewModel.
-     *
-     * Cancelling this job will cancel all coroutines started by this ViewModel.
-     */
-    private val viewModelJob = SupervisorJob()
-
-    /**
-     * This is the main scope for all coroutines launched by MainViewModel.
-     *
-     * Since we pass viewModelJob, you can cancel all coroutines launched by uiScope by calling
-     * viewModelJob.cancel()
-     */
-    private val viewModelScope = CoroutineScope(viewModelJob + Dispatchers.Main)
-
     /**
      * A playlist of videos that can be shown on the screen. This is private to avoid exposing a
      * way to set this value to observers.
@@ -129,15 +115,6 @@ class DevByteViewModel(application: Application) : AndroidViewModel(application)
         _isNetworkErrorShown.value = true
     }
 
-
-    /**
-     * Cancel all coroutines when the ViewModel is cleared
-     */
-    override fun onCleared() {
-        super.onCleared()
-        viewModelJob.cancel()
-    }
-
     /**
      * Factory for constructing DevByteViewModel with parameter
      */

RecyclerViewClickHandler-Starter/app/build.gradle

@@ -63,6 +63,10 @@ dependencies {
     implementation 'androidx.legacy:legacy-support-v4:1.0.0'
     kapt "androidx.room:room-compiler:$room_version"
     implementation "androidx.lifecycle:lifecycle-extensions:2.2.0"
+    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0"
+
+    // Kotlin Extensions and Coroutines support for Room
+    implementation "androidx.room:room-ktx:$room_version"
 
     // Coroutines
     implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$coroutine_version"

RecyclerViewClickHandler-Starter/app/src/main/java/com/example/android/trackmysleepquality/database/SleepDatabaseDao.kt

@@ -31,7 +31,7 @@ import androidx.room.Update
 interface SleepDatabaseDao {
 
     @Insert
-    fun insert(night: SleepNight)
+    suspend fun insert(night: SleepNight)
 
     /**
      * When updating a row with a value already set in a column,
@@ -40,23 +40,23 @@ interface SleepDatabaseDao {
      * @param night new value to write
      */
     @Update
-    fun update(night: SleepNight)
+    suspend fun update(night: SleepNight)
 
     /**
      * Selects and returns the row that matches the supplied start time, which is our key.
      *
      * @param key startTimeMilli to match
      */
     @Query("SELECT * from daily_sleep_quality_table WHERE nightId = :key")
-    fun get(key: Long): SleepNight
+    suspend fun get(key: Long): SleepNight
 
     /**
      * Deletes all values from the table.
      *
      * This does not delete the table, only its contents.
      */
     @Query("DELETE FROM daily_sleep_quality_table")
-    fun clear()
+    suspend fun clear()
 
     /**
      * Selects and returns all rows in the table,
@@ -70,7 +70,7 @@ interface SleepDatabaseDao {
      * Selects and returns the latest night.
      */
     @Query("SELECT * FROM daily_sleep_quality_table ORDER BY nightId DESC LIMIT 1")
-    fun getTonight(): SleepNight?
+    suspend fun getTonight(): SleepNight?
 
     /**
      * Selects and returns the night with given nightId.

RecyclerViewClickHandler-Starter/app/src/main/java/com/example/android/trackmysleepquality/sleepdetail/SleepDetailFragment.kt

@@ -67,7 +67,7 @@ class SleepDetailFragment : Fragment() {
         binding.setLifecycleOwner(this)
 
         // Add an Observer to the state variable for Navigating when a Quality icon is tapped.
-        sleepDetailViewModel.navigateToSleepTracker.observe(this, Observer {
+        sleepDetailViewModel.navigateToSleepTracker.observe(viewLifecycleOwner, Observer {
             if (it == true) { // Observed state is true.
                 this.findNavController().navigate(
                         SleepDetailFragmentDirections.actionSleepDetailFragmentToSleepTrackerFragment())

RecyclerViewClickHandler-Starter/app/src/main/java/com/example/android/trackmysleepquality/sleepdetail/SleepDetailViewModel.kt

@@ -42,13 +42,6 @@ class SleepDetailViewModel(
      */
     val database = dataSource
 
-    /** Coroutine setup variables */
-
-    /**
-     * viewModelJob allows us to cancel all coroutines started by this ViewModel.
-     */
-    private val viewModelJob = Job()
-
     private val night: LiveData<SleepNight>
 
     fun getNight() = night
@@ -72,16 +65,6 @@ class SleepDetailViewModel(
     val navigateToSleepTracker: LiveData<Boolean?>
         get() = _navigateToSleepTracker
 
-    /**
-     * Cancels all coroutines when the ViewModel is cleared, to cleanup any pending work.
-     *
-     * onCleared() gets called when the ViewModel is destroyed.
-     */
-    override fun onCleared() {
-        super.onCleared()
-        viewModelJob.cancel()
-    }
-
 
     /**
      * Call this immediately after navigating to [SleepTrackerFragment]

RecyclerViewClickHandler-Starter/app/src/main/java/com/example/android/trackmysleepquality/sleepquality/SleepQualityFragment.kt

@@ -70,7 +70,7 @@ class SleepQualityFragment : Fragment() {
         binding.sleepQualityViewModel = sleepQualityViewModel
 
         // Add an Observer to the state variable for Navigating when a Quality icon is tapped.
-        sleepQualityViewModel.navigateToSleepTracker.observe(this, Observer {
+        sleepQualityViewModel.navigateToSleepTracker.observe(viewLifecycleOwner, Observer {
             if (it == true) { // Observed state is true.
                 this.findNavController().navigate(
                         SleepQualityFragmentDirections.actionSleepQualityFragmentToSleepTrackerFragment())

RecyclerViewClickHandler-Starter/app/src/main/java/com/example/android/trackmysleepquality/sleepquality/SleepQualityViewModel.kt

@@ -19,12 +19,9 @@ package com.example.android.trackmysleepquality.sleepquality
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.MutableLiveData
 import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
 import com.example.android.trackmysleepquality.database.SleepDatabaseDao
-import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.Job
 import kotlinx.coroutines.launch
-import kotlinx.coroutines.withContext
 
 /**
  * ViewModel for SleepQualityFragment.
@@ -40,25 +37,6 @@ class SleepQualityViewModel(
      */
     val database = dataSource
 
-    /** Coroutine setup variables */
-
-    /**
-     * viewModelJob allows us to cancel all coroutines started by this ViewModel.
-     */
-    private val viewModelJob = Job()
-
-    /**
-     * A [CoroutineScope] keeps track of all coroutines started by this ViewModel.
-     *
-     * Because we pass it [viewModelJob], any coroutine started in this scope can be cancelled
-     * by calling `viewModelJob.cancel()`
-     *
-     * By default, all coroutines started in uiScope will launch in [Dispatchers.Main] which is
-     * the main thread on Android. This is a sensible default because most coroutines started by
-     * a [ViewModel] update the UI after performing some processing.
-     */
-    private val uiScope = CoroutineScope(Dispatchers.Main + viewModelJob)
-
     /**
      * Variable that tells the fragment whether it should navigate to [SleepTrackerFragment].
      *
@@ -73,17 +51,6 @@ class SleepQualityViewModel(
     val navigateToSleepTracker: LiveData<Boolean?>
         get() = _navigateToSleepTracker
 
-    /**
-     * Cancels all coroutines when the ViewModel is cleared, to cleanup any pending work.
-     *
-     * onCleared() gets called when the ViewModel is destroyed.
-     */
-    override fun onCleared() {
-        super.onCleared()
-        viewModelJob.cancel()
-    }
-
-
     /**
      * Call this immediately after navigating to [SleepTrackerFragment]
      */
@@ -97,14 +64,10 @@ class SleepQualityViewModel(
      * Then navigates back to the SleepTrackerFragment.
      */
     fun onSetSleepQuality(quality: Int) {
-        uiScope.launch {
-            // IO is a thread pool for running operations that access the disk, such as
-            // our Room database.
-            withContext(Dispatchers.IO) {
-                val tonight = database.get(sleepNightKey)
-                tonight.sleepQuality = quality
-                database.update(tonight)
-            }
+        viewModelScope.launch {
+            val tonight = database.get(sleepNightKey)
+            tonight.sleepQuality = quality
+            database.update(tonight)
 
             // Setting this state variable to true will alert the observer and trigger navigation.
             _navigateToSleepTracker.value = true

RecyclerViewClickHandler-Starter/app/src/main/java/com/example/android/trackmysleepquality/sleeptracker/SleepTrackerFragment.kt

@@ -85,10 +85,10 @@ class SleepTrackerFragment : Fragment() {
 
         // Add an Observer on the state variable for showing a Snackbar message
         // when the CLEAR button is pressed.
-        sleepTrackerViewModel.showSnackBarEvent.observe(this, Observer {
+        sleepTrackerViewModel.showSnackBarEvent.observe(viewLifecycleOwner, Observer {
             if (it == true) { // Observed state is true.
                 Snackbar.make(
-                        activity!!.findViewById(android.R.id.content),
+                        requireActivity().findViewById(android.R.id.content),
                         getString(R.string.cleared_message),
                         Snackbar.LENGTH_SHORT // How long to display the message.
                 ).show()
@@ -99,7 +99,7 @@ class SleepTrackerFragment : Fragment() {
         })
 
         // Add an Observer on the state variable for Navigating when STOP button is pressed.
-        sleepTrackerViewModel.navigateToSleepQuality.observe(this, Observer { night ->
+        sleepTrackerViewModel.navigateToSleepQuality.observe(viewLifecycleOwner, Observer { night ->
             night?.let {
                 // We need to get the navController from this, because button is not ready, and it
                 // just has to be a view. For some reason, this only matters if we hit stop again