fix(cubit): async state emission (#57)

Author: felangel

packages/cubit/CHANGELOG.md

@@ -1,3 +1,7 @@
+# 0.1.1
+
+- fix: offload state emission to next event loop iteration
+
 # 0.1.0
 
 - docs: add `onTransition` documentation to README

packages/cubit/lib/src/cubit_stream.dart

@@ -37,9 +37,12 @@ abstract class CubitStream<State> extends Stream<State> {
   /// {@endtemplate}
   @protected
   void emit(State state) {
-    if (state == _state || _controller.isClosed) return;
-    _state = state;
-    _controller.add(_state);
+    // Wait for next event-loop iteration before adding new state
+    Future<void>.delayed(Duration.zero, () {
+      if (state == _state || _controller.isClosed) return;
+      _state = state;
+      _controller.add(_state);
+    });
   }
 
   /// Adds a subscription to the `Stream<State>`.
@@ -72,6 +75,9 @@ abstract class CubitStream<State> extends Stream<State> {
   /// which resolves when it is done or an error occurred.
   @mustCallSuper
   Future<void> close() async {
+    // Wait for next event-loop iteration
+    // to allow propagation of newly emitted states.
+    await Future<void>.delayed(Duration.zero);
     await _controller.close();
     await _controller.stream.drain<State>();
   }

packages/cubit/pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/felangel/cubit
 issue_tracker: https://github.com/felangel/cubit/issues
 homepage: https://github.com/felangel/cubit
 
-version: 0.1.0
+version: 0.1.1
 
 environment:
   sdk: ">=2.7.0 <3.0.0"

packages/cubit/test/cubit_test.dart

@@ -52,8 +52,8 @@ void main() {
           () async {
         final transitions = <Transition<int>>[];
         final cubit = CounterCubit(onTransitionCallback: transitions.add);
-        await Future<void>.delayed(Duration.zero);
-        cubit..increment()..increment();
+        await Future<void>.delayed(Duration.zero, cubit.increment);
+        await Future<void>.delayed(Duration.zero, cubit.increment);
         await cubit.close();
         expect(
           transitions,
@@ -125,6 +125,14 @@ void main() {
         expect(states, [equals(0)]);
       });
 
+      test('receives fake async states immediately upon subscribing', () async {
+        final states = <int>[];
+        final cubit = FakeAsyncCounterCubit()..skip(1).listen(states.add);
+        await cubit.increment();
+        await cubit.close();
+        expect(states, [equals(1)]);
+      });
+
       test('can call listen multiple times', () async {
         final states = <int>[];
         final cubit = CounterCubit()..listen(states.add)..listen(states.add);

packages/cubit/test/cubits/cubits.dart

@@ -1,2 +1,3 @@
 export 'counter_cubit.dart';
+export 'fake_async_cubit.dart';
 export 'seeded_cubit.dart';

packages/cubit/test/cubits/fake_async_cubit.dart

@@ -0,0 +1,7 @@
+import 'package:cubit/cubit.dart';
+
+class FakeAsyncCounterCubit extends Cubit<int> {
+  FakeAsyncCounterCubit() : super(0);
+
+  Future<void> increment() async => emit(state + 1);
+}