ShowsViewModel: replace AsyncTask to avoid task rejection crash.

The assumption is that the onChanged callback might not always
run on the main thread, hence the task getting rejected.
AsyncTask is deprecated anyhow, so replace with coroutine.

app/src/main/java/com/battlelancer/seriesguide/ui/shows/ShowsViewModel.kt

@@ -1,26 +1,31 @@
 package com.battlelancer.seriesguide.ui.shows
 
 import android.app.Application
-import android.os.AsyncTask
 import android.text.format.DateUtils
 import androidx.lifecycle.AndroidViewModel
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.MediatorLiveData
 import androidx.lifecycle.MutableLiveData
 import androidx.lifecycle.Transformations
 import androidx.sqlite.db.SimpleSQLiteQuery
+import com.battlelancer.seriesguide.SgApp
 import com.battlelancer.seriesguide.model.SgShow
 import com.battlelancer.seriesguide.provider.SeriesGuideContract
 import com.battlelancer.seriesguide.provider.SeriesGuideDatabase
 import com.battlelancer.seriesguide.provider.SgRoomDatabase
 import com.battlelancer.seriesguide.settings.AdvancedSettings
 import com.battlelancer.seriesguide.util.TimeTools
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.sync.Semaphore
+import kotlinx.coroutines.sync.withPermit
 
 class ShowsViewModel(application: Application) : AndroidViewModel(application) {
 
     private val queryString = MutableLiveData<String>()
     private val sgShowsLiveData: LiveData<List<SgShow>>
     val showItemsLiveData = MediatorLiveData<List<ShowsAdapter.ShowItem>>()
+    private val showItemsLiveDataSemaphore = Semaphore(1)
 
     init {
         sgShowsLiveData = Transformations.switchMap(queryString) { queryString ->
@@ -30,11 +35,15 @@ class ShowsViewModel(application: Application) : AndroidViewModel(application) {
 
         showItemsLiveData.addSource(sgShowsLiveData) { sgShows ->
             // calculate actually displayed values on a background thread
-            AsyncTask.THREAD_POOL_EXECUTOR.execute {
-                val mapped = sgShows?.map {
-                    ShowsAdapter.ShowItem.map(it, getApplication())
+            SgApp.coroutineScope.launch(Dispatchers.IO) {
+                // Use Semaphore with 1 permit to ensure results are delivered in order and never
+                // processed in parallel.
+                showItemsLiveDataSemaphore.withPermit {
+                    val mapped = sgShows?.map {
+                        ShowsAdapter.ShowItem.map(it, getApplication())
+                    }
+                    showItemsLiveData.postValue(mapped)
                 }
-                showItemsLiveData.postValue(mapped)
             }
         }
     }