feat: Add listener for initial state on flame_bloc (#2382)

While analyzing the tests for flame_bloc, I stumbled upon an interesting conundrum. It seems that the initialState provided does not trigger a state change callback through onNewState. While that is in line with bloc, I think for most game use cases it is important to keep track of the "last" state of stuff - and in fact most of our existing examples do use the library in that way, to keep track of the last (i.e., the current) state. However, doing so generically is not simple with the current library because of how the initial state is not provided. While you could initialize your last manually, if there is a dynamic initial state, it would not be trivial to wire it up. This, therefore, adds a new listener of onInitialState, maintaining the behaviour of onNewState. Note: this solves the last unused reference from this issue by using it on the new tests.
flame-engine · May 1, 2023 · 01121c2 · 01121c2
1 parent 532899c
commit 01121c2
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Showing 3 changed files with 50 additions and 3 deletions.
diff --git a/packages/flame_bloc/lib/src/flame_bloc_listenable.dart b/packages/flame_bloc/lib/src/flame_bloc_listenable.dart
@@ -50,6 +50,7 @@ mixin FlameBlocListenable<B extends BlocBase<S>, S> on Component {
     }
     _bloc = bloc;
     _state = bloc.state;
+    onInitialState(_state);
     _subscription = bloc.stream.listen((newState) {
       if (_state != newState) {
         final callNewState = listenWhen(_state, newState);
@@ -73,6 +74,11 @@ mixin FlameBlocListenable<B extends BlocBase<S>, S> on Component {
   /// Default implementation is a no-op.
   void onNewState(S state) {}
 
+  /// Called only once with the initial state provided.
+  ///
+  /// Default implementation is a no-op.
+  void onInitialState(S state) {}
+
   @override
   @mustCallSuper
   void onRemove() {

diff --git a/packages/flame_bloc/test/src/flame_bloc_listenable_test.dart b/packages/flame_bloc/test/src/flame_bloc_listenable_test.dart
@@ -16,6 +16,13 @@ class PlayerListener extends Component
 
     last = state;
   }
+
+  @override
+  void onInitialState(PlayerState state) {
+    super.onInitialState(state);
+
+    last ??= state;
+  }
 }
 
 class SadPlayerListener extends Component
@@ -33,6 +40,13 @@ class SadPlayerListener extends Component
 
     last = state;
   }
+
+  @override
+  void onInitialState(PlayerState state) {
+    super.onInitialState(state);
+
+    last ??= state;
+  }
 }
 
 void main() {
@@ -78,7 +92,7 @@ void main() {
 
         bloc.makeSad();
         await Future.microtask(() {});
-        expect(component.last, isNull);
+        expect(component.last, equals(PlayerState.alive));
       },
     );
 
@@ -96,7 +110,7 @@ void main() {
 
         bloc.kill();
         await Future.microtask(() {});
-        expect(component.last, isNull);
+        expect(component.last, equals(PlayerState.alive));
       },
     );
 

diff --git a/packages/flame_bloc/test/src/flame_bloc_provider_test.dart b/packages/flame_bloc/test/src/flame_bloc_provider_test.dart
@@ -17,6 +17,13 @@ class InventoryListener extends Component
     super.onNewState(state);
     lastState = state;
   }
+
+  @override
+  void onInitialState(InventoryState state) {
+    super.onInitialState(state);
+
+    lastState ??= state;
+  }
 }
 
 void main() {
@@ -66,6 +73,22 @@ void main() {
         expect(component.bloc, bloc);
       });
 
+      testWithFlameGame(
+        'initial state is used to properly track last state',
+        (game) async {
+          final bloc = InventoryCubit();
+          late InventoryListener component;
+          final provider =
+              FlameBlocProvider<InventoryCubit, InventoryState>.value(
+            value: bloc,
+            children: [
+              component = InventoryListener(),
+            ],
+          );
+          await game.ensureAdd(provider);
+          expect(component.lastState, equals(InventoryState.sword));
+        },
+      );
       testWithFlameGame('can listen to new state changes', (game) async {
         final bloc = InventoryCubit();
         late InventoryListener component;
@@ -80,7 +103,11 @@ void main() {
 
         bloc.selectBow();
         await Future<void>.microtask(() {});
-        expect(component.lastState, equals(InventoryState.bow));
+
+        bloc.selectSword();
+        await Future<void>.microtask(() {});
+
+        expect(component.lastState, equals(InventoryState.sword));
       });
     });