Set initial UI state in onResumeFragments.

onResumeFragments is when the fragments state has been restored.
The app doesn't save any UI state (onSaveInstanceState implementations
are empty, and all UI state is derived from the state of the services
for playback and sample downloads) it's better this way in case some
operations are performed at this stage.

app/src/main/java/com/studio4plus/homerplayer/ui/MainActivity.java

@@ -96,6 +96,12 @@ protected void onStart() {
         handleIntent(getIntent());
     }
 
+    @Override
+    protected void onResumeFragments() {
+        super.onResumeFragments();
+        controller.onActivityResumeFragments();
+    }
+
     @SuppressLint("MissingSuperCall")
     @Override
     protected void onSaveInstanceState(Bundle outState) {

app/src/main/java/com/studio4plus/homerplayer/ui/UiControllerMain.java

@@ -47,6 +47,7 @@ public class UiControllerMain implements ServiceConnection {
     private @Nullable PlaybackService playbackService;
 
     private @NonNull State currentState = new InitState();
+    private boolean isRunning = false;
 
     @Inject
     UiControllerMain(@NonNull AppCompatActivity activity,
@@ -76,11 +77,16 @@ void onActivityCreated() {
     void onActivityStart() {
         Crashlytics.log(Log.DEBUG, TAG,"activity start");
         scanAudioBookFiles();
+    }
+
+    void onActivityResumeFragments() {
+        isRunning = true;
         maybeSetInitialState();
     }
 
     void onActivityPause() {
         currentState.onActivityPause();
+        isRunning = false;
     }
 
     void onActivityStop() {
@@ -193,7 +199,7 @@ private void scanAudioBookFiles() {
 
     private void  maybeSetInitialState() {
         if (currentState instanceof InitState && playbackService != null &&
-                audioBookManager.isInitialized()) {
+                audioBookManager.isInitialized() && isRunning) {
 
             if (playbackService.getState() != PlaybackService.State.IDLE) {
                 Preconditions.checkState(hasAnyBooks());
@@ -215,6 +221,10 @@ private AudioBook currentAudioBook() {
     }
 
     private void changeState(StateFactory newStateFactory) {
+        Preconditions.checkState(isRunning,
+                "attempt to switch state from " + currentState.debugName() + " to "
+                        + newStateFactory.name() + " while activity is paused");
+
         Crashlytics.log(Log.DEBUG, TAG, "UI: leave state: " + currentState.debugName());
         currentState.onLeaveState();
         Crashlytics.log(Log.DEBUG, TAG,"UI: create state: " + newStateFactory.name());