Send AppLifecycleState notifications on FlutterViewController lifecycle events. (#3801)

shell/common/engine.cc

@@ -323,7 +323,7 @@ bool Engine::HandleLifecyclePlatformMessage(blink::PlatformMessage* message) {
     activity_running_ = false;
     StopAnimator();
   } else if (state == "AppLifecycleState.resumed" ||
-             state == "AppLifecycle.inactive") {
+             state == "AppLifecycleState.inactive") {
     activity_running_ = true;
     StartAnimatorIfPossible();
   }

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -273,11 +273,19 @@ class PlatformMessageResponseDarwin : public blink::PlatformMessageResponse {
   [view release];
 }
 
+#pragma mark - UIViewController lifecycle notifications
+
 - (void)viewWillAppear:(BOOL)animated {
   [self connectToEngineAndLoad];
   [super viewWillAppear:animated];
 }
 
+- (void)viewDidDisappear:(BOOL)animated {
+  [_lifecycleChannel.get() sendMessage:@"AppLifecycleState.paused"];
+
+  [super viewDidDisappear];
+}
+
 #pragma mark - Application lifecycle notifications
 
 - (void)applicationBecameActive:(NSNotification*)notification {
@@ -533,6 +541,7 @@ static inline PointerChangeMapperPhase PointerChangePhaseFromUITouchPhase(UITouc
   [self surfaceUpdated:YES];
   [self onLocaleUpdated:nil];
   [self onVoiceOverChanged:nil];
+  [_lifecycleChannel.get() sendMessage:@"AppLifecycleState.resumed"];
 
   [super viewDidAppear:animated];
 }