Add AppLifecycleListener to save data model on app hide (#197)

* Add AppLifecycleListener to save data model on app hide
Fixes #187

* Add condition to check for test environment in AppLifecycleListener

lib/view_models/view_model.dart

@@ -52,6 +52,20 @@ abstract class SerializableViewModel<D extends DataModel> extends ViewModel {
 
     // save the data model on a regular basis.
     Timer.periodic(const Duration(minutes: 3), (_) => save(model.toJson()));
+
+    /// Check if we are running in a test environment.
+    /// If so, do not listen to app lifecycle events.
+    /// [AppLifecycleListener] is not supported in a test environment.
+    if (!Platform.environment.containsKey('FLUTTER_TEST')) {
+      AppLifecycleListener(
+        onStateChange: (value) {
+          info('AppLifecycleState changed to $value');
+        },
+        onHide: () async {
+          await save(model.toJson());
+        },
+      );
+    }
   }
 
   /// Current path and filename of the data.