Keep engine alive if VC is not deallocated (#7544)

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

@@ -144,13 +144,15 @@
   FML_DCHECK(self.iosPlatformView);
   _viewController = [viewController getWeakPtr];
   self.iosPlatformView->SetOwnerViewController(_viewController);
-  if (!viewController && !_allowHeadlessExecution) {
+  [self maybeSetupPlatformViewChannels];
+}
+
+- (void)notifyViewControllerDeallocated {
+  if (!_allowHeadlessExecution) {
     [self resetChannels];
 
     _shell.reset();
     _threadHost.Reset();
-  } else {
-    [self maybeSetupPlatformViewChannels];
   }
 }
 

shell/platform/darwin/ios/framework/Source/FlutterEngine_Internal.h

@@ -43,6 +43,7 @@
 - (FlutterTextInputPlugin*)textInputPlugin;
 - (void)launchEngine:(NSString*)entrypoint libraryURI:(NSString*)libraryOrNil;
 - (BOOL)createShell:(NSString*)entrypoint libraryURI:(NSString*)libraryOrNil;
+- (void)notifyViewControllerDeallocated;
 
 @end
 

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

@@ -430,6 +430,7 @@
 }
 
 - (void)dealloc {
+  [_engine.get() notifyViewControllerDeallocated];
   [[NSNotificationCenter defaultCenter] removeObserver:self];
   [super dealloc];
 }