Delay embedded UIViews touch events until the framework says so.

This allows the framework to decide whether and when a touch event
sequence arrives to the embedded view. Which gives the framework the
ability to manage hit testing and gesture disambiguation for embedded
UIViews.

We achieve this by wrapping each embedded UIView with another UIView that has
a custom UIGestureRecognizer that delays touch events from being
delivered, and another UIGestureRecognizer that makes sure to let
Flutter know of events that are hit tested to the embedded view.

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

@@ -10,6 +10,8 @@
 #include "flutter/fml/platform/darwin/scoped_nsobject.h"
 #include "flutter/shell/platform/darwin/ios/framework/Headers/FlutterChannels.h"
 
+#import <UIKit/UIGestureRecognizerSubclass.h>
+
 namespace shell {
 
 FlutterPlatformViewsController::FlutterPlatformViewsController(
@@ -30,6 +32,8 @@
     OnCreate(call, result);
   } else if ([[call method] isEqualToString:@"dispose"]) {
     OnDispose(call, result);
+  } else if ([[call method] isEqualToString:@"acceptGesture"]) {
+    OnAcceptGesture(call, result);
   } else {
     result(FlutterMethodNotImplemented);
   }
@@ -57,9 +61,10 @@
   }
 
   // TODO(amirh): decode and pass the creation args.
-  views_[viewId] = fml::scoped_nsobject<UIView>([[factory createWithFrame:CGRectZero
-                                                           viewIdentifier:viewId
-                                                                arguments:nil] retain]);
+  TouchInterceptingView* view = [[[TouchInterceptingView alloc]
+      initWithSubView:[factory createWithFrame:CGRectZero viewIdentifier:viewId arguments:nil]
+          flutterView:flutter_view_] autorelease];
+  views_[viewId] = fml::scoped_nsobject<TouchInterceptingView>([view retain]);
 
   FlutterView* flutter_view = flutter_view_.get();
   [flutter_view addSubview:views_[viewId].get()];
@@ -83,6 +88,24 @@
   result(nil);
 }
 
+void FlutterPlatformViewsController::OnAcceptGesture(FlutterMethodCall* call,
+                                                     FlutterResult& result) {
+  NSDictionary<NSString*, id>* args = [call arguments];
+  int64_t viewId = [args[@"id"] longLongValue];
+
+  if (views_[viewId] == nil) {
+    result([FlutterError errorWithCode:@"unknown_view"
+                               message:@"trying to set gesture state for an unknown view"
+                               details:[NSString stringWithFormat:@"view id: '%lld'", viewId]]);
+    return;
+  }
+
+  TouchInterceptingView* view = views_[viewId].get();
+  [view releaseGesture];
+
+  result(nil);
+}
+
 void FlutterPlatformViewsController::RegisterViewFactory(
     NSObject<FlutterPlatformViewFactory>* factory,
     NSString* factoryId) {
@@ -106,3 +129,133 @@
 }
 
 }  // namespace shell
+
+// This recognizers delays touch events from being dispatched to the responder chain until it failed
+// recognizing a gesture.
+//
+// We only fail this recognizer when asked to do so by the Flutter framework (which does so by
+// invoking an acceptGesture method on the platform_views channel). And this is how we allow the
+// Flutter framework to delay or prevent the embedded view from getting a touch sequence.
+@interface DelayingGestureRecognizer : UIGestureRecognizer <UIGestureRecognizerDelegate>
+@end
+
+// While the DelayingGestureRecognizer is preventing touches from hitting the responder chain
+// the touch events are not arriving to the FlutterView (and thus not arriving to the Flutter
+// framework). We use this gesture recognizer to dispatch the events directly to the FlutterView
+// while during this phase.
+//
+// If the Flutter framework decides to dispatch events to the embedded view, we fail the
+// DelayingGestureRecognizer which sends the events up the responder chain. But since the events
+// are handled by the embedded view they are not delivered to the Flutter framework in this phase
+// as well. So during this phase as well the ForwardingGestureRecognizer dispatched the events
+// directly to the FlutterView.
+@interface ForwardingGestureRecognizer : UIGestureRecognizer <UIGestureRecognizerDelegate>
+- (instancetype)initWithTarget:(id)target flutterView:(UIView*)flutterView;
+@end
+
+@implementation TouchInterceptingView {
+  ForwardingGestureRecognizer* forwardingRecognizer;
+  DelayingGestureRecognizer* delayingRecognizer;
+}
+- (instancetype)initWithSubView:(UIView*)embeddedView flutterView:(UIView*)flutterView {
+  self = [super initWithFrame:embeddedView.frame];
+  if (self) {
+    self.multipleTouchEnabled = YES;
+    embeddedView.autoresizingMask =
+        (UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight);
+
+    [self addSubview:embeddedView];
+
+    forwardingRecognizer =
+        [[[ForwardingGestureRecognizer alloc] initWithTarget:self
+                                                 flutterView:flutterView] autorelease];
+
+    delayingRecognizer = [[[DelayingGestureRecognizer alloc] initWithTarget:self
+                                                                     action:nil] autorelease];
+
+    [self addGestureRecognizer:delayingRecognizer];
+    [self addGestureRecognizer:forwardingRecognizer];
+  }
+  return self;
+}
+
+- (void)releaseGesture {
+  delayingRecognizer.state = UIGestureRecognizerStateFailed;
+}
+@end
+
+@implementation DelayingGestureRecognizer
+- (instancetype)initWithTarget:(id)target action:(SEL)action {
+  self = [super initWithTarget:target action:action];
+  if (self) {
+    self.delaysTouchesBegan = YES;
+    self.delegate = self;
+  }
+  return self;
+}
+
+- (BOOL)gestureRecognizer:(UIGestureRecognizer*)gestureRecognizer
+    shouldBeRequiredToFailByGestureRecognizer:(UIGestureRecognizer*)otherGestureRecognizer {
+  return otherGestureRecognizer != self;
+}
+
+- (BOOL)gestureRecognizer:(UIGestureRecognizer*)gestureRecognizer
+    shouldRequireFailureOfGestureRecognizer:(UIGestureRecognizer*)otherGestureRecognizer {
+  return otherGestureRecognizer == self;
+}
+
+- (void)touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event {
+  // The gesture has ended, and the delaying gesture recognizer was not failed, we recognize
+  // the gesture to prevent the touches from being dispatched to the embedded view.
+  //
+  // This doesn't work well with gestures that are recognized by the Flutter framework after
+  // all pointers are up.
+  //
+  // TODO(amirh): explore if we can instead set this to recognized when the next touch sequence
+  //  begins, or we can use a framework signal for restarting the recognizers (e.g when the
+  //  gesture arena is resolved).
+  self.state = UIGestureRecognizerStateRecognized;
+}
+
+- (void)touchesCancelled:(NSSet*)touches withEvent:(UIEvent*)event {
+  self.state = UIGestureRecognizerStateCancelled;
+}
+@end
+
+@implementation ForwardingGestureRecognizer {
+  UIView* _flutterView;
+}
+
+- (instancetype)initWithTarget:(id)target flutterView:(UIView*)flutterView {
+  self = [super initWithTarget:target action:nil];
+  if (self) {
+    self.delegate = self;
+    _flutterView = flutterView;
+  }
+  return self;
+}
+
+- (void)touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {
+  [_flutterView touchesBegan:touches withEvent:event];
+}
+
+- (void)touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event {
+  [_flutterView touchesMoved:touches withEvent:event];
+}
+
+- (void)touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event {
+  [_flutterView touchesEnded:touches withEvent:event];
+  self.state = UIGestureRecognizerStateRecognized;
+}
+
+- (void)touchesCancelled:(NSSet*)touches withEvent:(UIEvent*)event {
+  [_flutterView touchesCancelled:touches withEvent:event];
+  self.state = UIGestureRecognizerStateCancelled;
+}
+
+- (BOOL)gestureRecognizer:(UIGestureRecognizer*)gestureRecognizer
+    shouldRecognizeSimultaneouslyWithGestureRecognizer:
+        (UIGestureRecognizer*)otherGestureRecognizer {
+  return YES;
+}
+@end

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

@@ -13,6 +13,18 @@
 #include "flutter/shell/platform/darwin/ios/framework/Headers/FlutterChannels.h"
 #include "flutter/shell/platform/darwin/ios/framework/Headers/FlutterPlatformViews.h"
 
+// A UIView that is used as the parent for embedded UIViews.
+//
+// This view has 2 roles:
+// 1. Delay or prevent touch events from arriving the embedded view.
+// 2. Dispatching all events that are hittested to the embedded view to the FlutterView.
+@interface TouchInterceptingView : UIView
+- (instancetype)initWithSubView:(UIView*)embeddedView flutterView:(UIView*)flutterView;
+
+// Stop delaying any active touch sequence (and let it arrive the embedded view).
+- (void)releaseGesture;
+@end
+
 namespace shell {
 
 class FlutterPlatformViewsController : public flow::ExternalViewEmbedder {
@@ -28,11 +40,12 @@ class FlutterPlatformViewsController : public flow::ExternalViewEmbedder {
   fml::scoped_nsobject<FlutterMethodChannel> channel_;
   fml::scoped_nsobject<FlutterView> flutter_view_;
   std::map<std::string, fml::scoped_nsobject<NSObject<FlutterPlatformViewFactory>>> factories_;
-  std::map<int64_t, fml::scoped_nsobject<UIView>> views_;
+  std::map<int64_t, fml::scoped_nsobject<TouchInterceptingView>> views_;
 
   void OnMethodCall(FlutterMethodCall* call, FlutterResult& result);
   void OnCreate(FlutterMethodCall* call, FlutterResult& result);
   void OnDispose(FlutterMethodCall* call, FlutterResult& result);
+  void OnAcceptGesture(FlutterMethodCall* call, FlutterResult& result);
 
   FML_DISALLOW_COPY_AND_ASSIGN(FlutterPlatformViewsController);
 };