Handle transitions to AppLifecycleState.detached in lifecycle state generation (#142523)

Generates the correct lifecycle state transitions in ServicesBinding when detaching.

packages/flutter/lib/src/services/binding.dart

@@ -289,12 +289,6 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
     if (previousState == state) {
       return const <AppLifecycleState>[];
     }
-    if (previousState == AppLifecycleState.paused && state == AppLifecycleState.detached) {
-      // Handle the wrap-around from paused to detached
-      return const <AppLifecycleState>[
-        AppLifecycleState.detached,
-      ];
-    }
     final List<AppLifecycleState> stateChanges = <AppLifecycleState>[];
     if (previousState == null) {
       // If there was no previous state, just jump directly to the new state.
@@ -304,7 +298,12 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
       final int stateIndex = AppLifecycleState.values.indexOf(state);
       assert(previousStateIndex != -1, 'State $previousState missing in stateOrder array');
       assert(stateIndex != -1, 'State $state missing in stateOrder array');
-      if (previousStateIndex > stateIndex) {
+      if (state == AppLifecycleState.detached) {
+        for (int i = previousStateIndex + 1; i < AppLifecycleState.values.length; ++i) {
+          stateChanges.add(AppLifecycleState.values[i]);
+        }
+        stateChanges.add(AppLifecycleState.detached);
+      } else if (previousStateIndex > stateIndex) {
         for (int i = stateIndex; i < previousStateIndex; ++i) {
           stateChanges.insert(0, AppLifecycleState.values[i]);
         }

packages/flutter/test/widgets/app_lifecycle_listener_test.dart

@@ -129,10 +129,11 @@ void main() {
     await setAppLifeCycleState(AppLifecycleState.resumed);
     expect(transitions, equals(<String>['restart', 'show', 'resume']));
 
-    // Generates intermediate states.
+    // Generates intermediate states from lower to higher lifecycle states.
     transitions.clear();
     await setAppLifeCycleState(AppLifecycleState.paused);
     expect(transitions, equals(<String>['inactive', 'hide', 'pause']));
+
     // Wraps around from pause to detach.
     await setAppLifeCycleState(AppLifecycleState.detached);
     expect(transitions, equals(<String>['inactive', 'hide', 'pause', 'detach']));
@@ -140,14 +141,16 @@ void main() {
     expect(transitions, equals(<String>['inactive', 'hide', 'pause', 'detach', 'resume']));
     await setAppLifeCycleState(AppLifecycleState.paused);
     expect(transitions, equals(<String>['inactive', 'hide', 'pause', 'detach', 'resume', 'inactive', 'hide', 'pause']));
+
+    // Generates intermediate states from higher to lower lifecycle states.
     transitions.clear();
     await setAppLifeCycleState(AppLifecycleState.resumed);
     expect(transitions, equals(<String>['restart', 'show', 'resume']));
 
-    // Asserts on bad transitions
-    await expectLater(() => setAppLifeCycleState(AppLifecycleState.detached), throwsAssertionError);
-    await setAppLifeCycleState(AppLifecycleState.paused);
+    // Go to detached
+    transitions.clear();
     await setAppLifeCycleState(AppLifecycleState.detached);
+    expect(transitions, equals(<String>['inactive', 'hide', 'pause', 'detach']));
   });
 
   testWidgets('Receives exit requests', (WidgetTester tester) async {

packages/flutter/test/widgets/binding_test.dart

@@ -224,7 +224,13 @@ void main() {
     ]);
 
     observer.accumulatedStates.clear();
-    await expectLater(() async => setAppLifeCycleState(AppLifecycleState.detached), throwsAssertionError);
+    await setAppLifeCycleState(AppLifecycleState.detached);
+    expect(observer.accumulatedStates, <AppLifecycleState>[
+      AppLifecycleState.inactive,
+      AppLifecycleState.hidden,
+      AppLifecycleState.paused,
+      AppLifecycleState.detached,
+    ]);
     WidgetsBinding.instance.removeObserver(observer);
   });