Let the FlutterAppDelegate's messenger be customizable if the...

Let the FlutterAppDelegate's messenger be customizable if the rootViewController isn't a FlutterViewController (#3916)

* Customizable messenger

* rename

shell/platform/darwin/ios/framework/Headers/FlutterAppDelegate.h

@@ -29,6 +29,12 @@ FLUTTER_EXPORT
 
 @property(strong, nonatomic) UIWindow* window;
 
+// Can be overriden by subclasses to provide a custom FlutterBinaryMessenger,
+// typically a FlutterViewController, for plugin interop.
+//
+// Defaults to window's rootViewController.
+- (NSObject<FlutterBinaryMessenger>*) binaryMessenger;
+
 @end
 
 #endif  // FLUTTER_FLUTTERDARTPROJECT_H_

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

@@ -212,6 +212,11 @@
   }
 }
 
+// TODO(xster): move when doing https://github.com/flutter/flutter/issues/3671.
+- (NSObject<FlutterBinaryMessenger>*)binaryMessenger {
+  return self.rootFlutterViewController;
+}
+
 - (NSObject<FlutterPluginRegistrar>*)registrarForPlugin:(NSString*)pluginKey {
   NSAssert(self.pluginPublications[pluginKey] == nil, @"Duplicate plugin key: %@", pluginKey);
   self.pluginPublications[pluginKey] = [NSNull null];
@@ -248,7 +253,7 @@
 }
 
 - (NSObject<FlutterBinaryMessenger>*)messenger {
-  return (FlutterViewController*)_appDelegate.window.rootViewController;
+  return [_appDelegate binaryMessenger];
 }
 
 - (void)publish:(NSObject*)value {