[UPDATE/OPTIMISE] Introduce viewmodel-ktx and simplify PlantDetailViewMode (#348)

* Update lifecycle-lib-2.1.0-alpha02

* Introduce viewmodel-ktx and simplify PlantDetailViewModel

Author: XinyueZ

app/build.gradle

@@ -56,6 +56,7 @@ dependencies {
     implementation "androidx.constraintlayout:constraintlayout:$rootProject.constraintLayoutVersion"
     implementation "androidx.core:core-ktx:$rootProject.ktxVersion"
     implementation "androidx.lifecycle:lifecycle-extensions:$rootProject.lifecycleVersion"
+    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$rootProject.lifecycleVersion"
     implementation "androidx.recyclerview:recyclerview:$rootProject.recyclerViewVersion"
     implementation "androidx.room:room-runtime:$rootProject.roomVersion"
     implementation "com.github.bumptech.glide:glide:$rootProject.glideVersion"

app/src/main/java/com/google/samples/apps/sunflower/viewmodels/PlantDetailViewModel.kt

@@ -19,13 +19,13 @@ package com.google.samples.apps.sunflower.viewmodels
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.Transformations
 import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
 import com.google.samples.apps.sunflower.PlantDetailFragment
 import com.google.samples.apps.sunflower.data.GardenPlantingRepository
 import com.google.samples.apps.sunflower.data.Plant
 import com.google.samples.apps.sunflower.data.PlantRepository
-import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.Dispatchers.Main
-import kotlinx.coroutines.Job
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 
 /**
@@ -40,27 +40,13 @@ class PlantDetailViewModel(
     val isPlanted: LiveData<Boolean>
     val plant: LiveData<Plant>
 
-    /**
-     * 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 = Job()
-
-    /**
-     * This is the scope for all coroutines launched by [PlantDetailViewModel].
-     *
-     * Since we pass [viewModelJob], you can cancel all coroutines launched by [viewModelScope] by calling
-     * viewModelJob.cancel().  This is called in [onCleared].
-     */
-    private val viewModelScope = CoroutineScope(Main + viewModelJob)
-
     /**
      * Cancel all coroutines when the ViewModel is cleared.
      */
+    @ExperimentalCoroutinesApi
     override fun onCleared() {
         super.onCleared()
-        viewModelJob.cancel()
+        viewModelScope.cancel()
     }
 
     init {