Send statusBarTouch events via dedicated messages (flutter#179643)

Instead of using fake touch events. 

Before this patch `FlutterViewController` sends two fake touch events
(down and up), at `(0, 0)` to the framework to signal that the status
bar is tapped on iOS. The scaffold widget and the cupertino page
scaffold widget set up gesture detectors to listen for these fake taps,
and scroll the "primary" scrollable container to the top in response.

This messaging mechanism is sometimes ambiguous, as the framework may
interpret that as a pair of regular pointer tap events (for instance in
flutter#177992 the modal barrier claims the tap gesture and as a result the
modal barrier is dismissed by the fake touch events). This PR changes
that to communicate the status bar tap event via a new system channel,
and dispatch the events via `WidgetsBindingObserver`s in the framework.

See also
flutter#177992 (comment)

Fixes flutter#177992, fixes flutter#175606

It appears that UIKit also has access to the coordinates of the touch
events to determine which scrollable view(s?) to dispatch the scroll to
top event to.
```objc
  * frame #0: 0x00000001032f6520 UIPlayground.debug.dylib`MyScrollViewController.scrollViewShouldScrollToTop(scrollView=0x0000000106014800) at UIScrollView.swift:13:3
    frame #2: 0x00000001867c9300 UIKitCore`-[UIScrollView _scrollToTopIfPossible:] + 316
    frame #3: 0x00000001867c9604 UIKitCore`-[UIScrollView _scrollToTopFromTouchAtScreenLocation:resultHandler:] + 40
    frame flutter#4: 0x0000000186299bbc UIKitCore`__71-[UIWindow _scrollToTopViewsUnderScreenPointIfNecessary:resultHandler:]_block_invoke.358 + 168
    frame flutter#5: 0x000000018629981c UIKitCore`-[UIWindow _scrollToTopViewsUnderScreenPointIfNecessary:resultHandler:] + 1212
    frame flutter#6: 0x000000018581ed8c UIKitCore`-[UIStatusBarManager _handleScrollToTopAtXPosition:] + 192
    frame flutter#7: 0x000000018581eb60 UIKitCore`-[UIStatusBarManager handleTapAction:] + 60
```
Unfortunately that information is not available to user application. The
iOS accessibility bridge currently does create dummy UIScrollViews for
each scrollable in the accessibility tree so may be we can take
advantage of that in the future.

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Tree Hygiene]:
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[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
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[Features we expect every widget to implement]:
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Author: LongCatIsLooong

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -147,6 +147,9 @@ @interface FlutterEngine () <FlutterIndirectScribbleDelegate,
 @property(nonatomic, strong) FlutterMethodChannel* navigationChannel;
 @property(nonatomic, strong) FlutterMethodChannel* restorationChannel;
 @property(nonatomic, strong) FlutterMethodChannel* platformChannel;
+// This channel only sends status bar related events to the framework thus has
+// no handlers.
+@property(nonatomic, strong) FlutterMethodChannel* statusBarChannel;
 @property(nonatomic, strong) FlutterMethodChannel* platformViewsChannel;
 @property(nonatomic, strong) FlutterMethodChannel* textInputChannel;
 @property(nonatomic, strong) FlutterMethodChannel* undoManagerChannel;
@@ -577,6 +580,7 @@ - (void)resetChannels {
   self.navigationChannel = nil;
   self.restorationChannel = nil;
   self.platformChannel = nil;
+  self.statusBarChannel = nil;
   self.platformViewsChannel = nil;
   self.textInputChannel = nil;
   self.undoManagerChannel = nil;
@@ -641,6 +645,12 @@ - (void)setUpChannels {
                                  binaryMessenger:self.binaryMessenger
                                            codec:[FlutterJSONMethodCodec sharedInstance]];
 
+  self.statusBarChannel =
+      [[FlutterMethodChannel alloc] initWithName:@"flutter/status_bar"
+                                 binaryMessenger:self.binaryMessenger
+                                           codec:[FlutterJSONMethodCodec sharedInstance]];
+  [self.statusBarChannel resizeChannelBuffer:0];  // No buffering.
+
   self.platformViewsChannel =
       [[FlutterMethodChannel alloc] initWithName:@"flutter/platform_views"
                                  binaryMessenger:self.binaryMessenger
@@ -1483,6 +1493,13 @@ - (void)onLocaleUpdated:(NSNotification*)notification {
   [self.localizationChannel invokeMethod:@"setLocale" arguments:localeData];
 }
 
+- (void)onStatusBarTap {
+  // Called by FlutterViewController to notify the framework that a tap landed
+  // on the status bar, and the most relevant vertical scroll view visible in the
+  // app, if applicable, should scroll to top.
+  [self.statusBarChannel invokeMethod:@"handleScrollToTop" arguments:nil];
+}
+
 - (void)waitForFirstFrameSync:(NSTimeInterval)timeout
                      callback:(NS_NOESCAPE void (^_Nonnull)(BOOL didTimeout))callback {
   fml::TimeDelta waitTime = fml::TimeDelta::FromMilliseconds(timeout * 1000);

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine_Internal.h

@@ -124,6 +124,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)sendDeepLinkToFramework:(NSURL*)url completionHandler:(void (^)(BOOL success))completion;
 
+- (void)onStatusBarTap;
 @end
 
 @interface FlutterImplicitEngineBridgeImpl : NSObject <FlutterImplicitEngineBridge>

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -554,29 +554,13 @@ - (void)loadView {
   return pointer_data;
 }
 
-static void SendFakeTouchEvent(UIScreen* screen,
-                               FlutterEngine* engine,
-                               CGPoint location,
-                               flutter::PointerData::Change change) {
-  const CGFloat scale = screen.scale;
-  flutter::PointerData pointer_data = [[engine viewController] generatePointerDataForFake];
-  pointer_data.physical_x = location.x * scale;
-  pointer_data.physical_y = location.y * scale;
-  auto packet = std::make_unique<flutter::PointerDataPacket>(/*count=*/1);
-  pointer_data.change = change;
-  packet->SetPointerData(0, pointer_data);
-  [engine dispatchPointerDataPacket:std::move(packet)];
-}
-
 - (BOOL)scrollViewShouldScrollToTop:(UIScrollView*)scrollView {
   if (!self.engine) {
     return NO;
   }
-  CGPoint statusBarPoint = CGPointZero;
-  UIScreen* screen = self.flutterScreenIfViewLoaded;
-  if (screen) {
-    SendFakeTouchEvent(screen, self.engine, statusBarPoint, flutter::PointerData::Change::kDown);
-    SendFakeTouchEvent(screen, self.engine, statusBarPoint, flutter::PointerData::Change::kUp);
+  if (self.isViewLoaded) {
+    // Status bar taps before the UI is visible should be ignored.
+    [self.engine onStatusBarTap];
   }
   return NO;
 }

engine/src/flutter/testing/ios_scenario_app/README.md

@@ -18,11 +18,26 @@ See also:
 
 ## Adding a New Scenario
 
-Create a new subclass of [Scenario](lib/src/scenario.dart) and add it to the map
-in [scenarios.dart](lib/src/scenarios.dart). For an example, see
+Like a regular Flutter iOS app, the Scenario app consists of the [iOS embedding
+code](ios/Scenarios/Scenarios/AppDelegate.m) and the dart logic that are
+`Scenario`s.
+
+To introduce a new subclass of [Scenario](lib/src/scenario.dart), add it to the map
+in [scenarios.dart](lib/src/scenarios.dart). For an example,
 [animated_color_square.dart](lib/src/animated_color_square.dart), which draws a
 continuously animating colored square that bounces off the sides of the
 viewport.
 
-Then set the scenario from the iOS app by calling `set_scenario` on platform
-channel `driver`.
+The Scenarios app loads a `Scenario` when it receives a `set_scenario` method call
+on the `driver` platform channel from the objective-c code. However if you're
+adding a UI test this is typically not needed as you typically should add a new
+launch argument. See
+[ScenariosUITests](ios/Scenarios/ScenariosUITests/README.md) for more details.
+
+## Running a specific test
+
+The `run_ios_tests.sh` script runs all tests in the `Scenarios` project. If you're
+debugging a specific test, rebuild the `ios_debug_sim_unopt_arm64` engine variant
+(assuming testing on a simulator on Apple Silicon chips), and open
+`src/out/ios_debug_sim_unopt_arm64/ios_scenario_app/Scenarios.xcworkspace` in xcode.
+Use the xcode UI to run the test.

engine/src/flutter/testing/ios_scenario_app/ios/Scenarios/Scenarios.xcodeproj/xcshareddata/xcschemes/Scenarios.xcscheme

@@ -77,6 +77,10 @@
             argument = "--screen-before-flutter"
             isEnabled = "NO">
          </CommandLineArgument>
+         <CommandLineArgument
+            argument = "--tap-status-bar"
+            isEnabled = "NO">
+         </CommandLineArgument>
          <CommandLineArgument
             argument = "--bogus-font-text"
             isEnabled = "NO">

engine/src/flutter/testing/ios_scenario_app/ios/Scenarios/Scenarios/AppDelegate.m

@@ -16,6 +16,10 @@ @interface NoStatusBarViewController : UIViewController
 
 @end
 
+@interface FlutterEngine ()
+@property(nonatomic, strong) FlutterMethodChannel* statusBarChannel;
+@end
+
 @implementation NoStatusBarViewController
 - (BOOL)prefersStatusBarHidden {
   return YES;
@@ -210,11 +214,28 @@ - (void)setupFlutterViewControllerTest:(NSString*)scenarioIdentifier {
           FlutterPlatformViewGestureRecognizersBlockingPolicyWaitUntilTouchesEnded];
 
   UIViewController* rootViewController = flutterViewController;
-  // Make Flutter View's origin x/y not 0.
   if ([scenarioIdentifier isEqualToString:@"non_full_screen_flutter_view_platform_view"]) {
+    // Make Flutter View's origin x/y not 0.
     rootViewController = [[NoStatusBarViewController alloc] init];
     [rootViewController.view addSubview:flutterViewController.view];
     flutterViewController.view.frame = CGRectMake(150, 150, 500, 500);
+  } else if ([scenarioIdentifier isEqualToString:@"tap_status_bar"]) {
+    [engine.binaryMessenger
+        setMessageHandlerOnChannel:@"flutter/status_bar"
+              binaryMessageHandler:^(NSData* _Nullable message, FlutterBinaryReply _Nonnull reply) {
+                NSDictionary* dict = [NSJSONSerialization JSONObjectWithData:message
+                                                                     options:0
+                                                                       error:nil];
+                FlutterBasicMessageChannel* channel = [[FlutterBasicMessageChannel alloc]
+                       initWithName:@"display_data"
+                    binaryMessenger:engine.binaryMessenger
+                              codec:[FlutterJSONMessageCodec sharedInstance]];
+                [channel sendMessage:@{@"data" : dict}];
+                UITextField* text =
+                    [[UITextField alloc] initWithFrame:CGRectMake(0, 400, 300, 100)];
+                text.text = dict[@"method"];
+                [flutterViewController.view addSubview:text];
+              }];
   }
 
   self.window.rootViewController = rootViewController;

engine/src/flutter/testing/ios_scenario_app/ios/Scenarios/ScenariosUITests/README.md

@@ -1,3 +1,19 @@
+# Adding a New Scenario for a XCUITest
+
+An XCUITest is different from a regular XCTest in that the test subject app runs
+in a different process than the test suite, making it trickier for the test code
+to communicate with the app. For instance, you won't have access to
+the view controller or engine instances from within the test code.
+
+For this reason, the test code typically uses **launch arguments** to configure
+the app (for example, use [launchArgsMap](../Scenarios/AppDelegate.m) to inform
+the app which `Scenario` to load), and use UIKit UI components to collect test
+results (for example, every messsage received on the `display_data` channel adds
+a new `UITextField` to the app, which will be visible to the test code. See [touches_scenario.dart](../../../lib/src/touches_scenario.dart) for an example).
+
+Refer to the [Adding a New Scenario](./../../../README.md) section for how to
+register a new dart `Scenario`.
+
 #  Golden UI Tests
 
 This folder contains golden image tests. It renders UI (for instance, a platform
@@ -17,3 +33,8 @@ indicating the file name it expected to find. The test will continue and fail,
 but will contain an attachment with the expected screen shot. If the screen
 shot looks good, add it with the correct name to the project and run the test
 again - it should pass this time.
+
+## Running a specific Scenario
+
+Add and enable the new launch argument to the `Arguments Passed On Launch`
+section of the `Debug - Run` scheme, and build and run the app.

engine/src/flutter/testing/ios_scenario_app/ios/Scenarios/ScenariosUITests/StatusBarTest.m

@@ -16,6 +16,10 @@ - (void)setUp {
 }
 
 - (void)testTapStatusBar {
+  XCUIElement* textField = self.application.textFields[@"handleScrollToTop"];
+  BOOL exists = [textField waitForExistenceWithTimeout:1];
+  XCTAssertFalse(exists, @"");
+
   XCUIApplication* systemApp =
       [[XCUIApplication alloc] initWithBundleIdentifier:@"com.apple.springboard"];
   XCUIElement* statusBar = [systemApp.statusBars firstMatch];
@@ -25,21 +29,7 @@ - (void)testTapStatusBar {
     XCUICoordinate* coordinates = [statusBar coordinateWithNormalizedOffset:CGVectorMake(0, 0)];
     [coordinates tap];
   }
-
-  XCUIElement* addTextField =
-      self.application
-          .textFields[@"0,PointerChange.add,device=0,buttons=0,signalKind=PointerSignalKind.none"];
-  BOOL exists = [addTextField waitForExistenceWithTimeout:1];
-  XCTAssertTrue(exists, @"");
-  XCUIElement* downTextField =
-      self.application
-          .textFields[@"1,PointerChange.down,device=0,buttons=0,signalKind=PointerSignalKind.none"];
-  exists = [downTextField waitForExistenceWithTimeout:1];
-  XCTAssertTrue(exists, @"");
-  XCUIElement* upTextField =
-      self.application
-          .textFields[@"2,PointerChange.up,device=0,buttons=0,signalKind=PointerSignalKind.none"];
-  exists = [upTextField waitForExistenceWithTimeout:1];
+  exists = [textField waitForExistenceWithTimeout:1];
   XCTAssertTrue(exists, @"");
 }
 

engine/src/flutter/testing/ios_scenario_app/lib/src/platform_channel_echo.dart

@@ -0,0 +1,30 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:typed_data' show ByteData;
+import 'dart:ui';
+
+import 'scenario.dart';
+
+/// A scenario which intercepts all messages on the given channel, and sends back
+/// the same message to the engine on a channel with the same name.
+class EchoPlatformChannelScenario extends Scenario {
+  /// Constructor for `EchoPlatformChannelScenario`.
+  EchoPlatformChannelScenario(super.view, {required this.channel}) {
+    channelBuffers.setListener(channel, _onHandlePlatformMessage);
+  }
+
+  /// The name of the channel where all messages should be intercepted.
+  final String channel;
+
+  void _onHandlePlatformMessage(ByteData? data, PlatformMessageResponseCallback _) {
+    view.platformDispatcher.sendPlatformMessage(channel, data, null);
+  }
+
+  @override
+  void unmount() {
+    channelBuffers.clearListener(channel);
+    super.unmount();
+  }
+}

engine/src/flutter/testing/ios_scenario_app/lib/src/scenarios.dart

@@ -11,6 +11,7 @@ import 'darwin_system_font.dart';
 import 'get_bitmap_scenario.dart';
 import 'initial_route_reply.dart';
 import 'locale_initialization.dart';
+import 'platform_channel_echo.dart';
 import 'platform_view.dart';
 import 'poppable_screen.dart';
 import 'scenario.dart';
@@ -141,7 +142,8 @@ Map<String, _ScenarioFactory> _scenarios = <String, _ScenarioFactory>{
       TwoPlatformViewClipPath(view, firstId: _viewId++, secondId: _viewId++),
   'two_platform_view_clip_path_multiple_clips': (FlutterView view) =>
       TwoPlatformViewClipPathMultipleClips(view, firstId: _viewId++, secondId: _viewId++),
-  'tap_status_bar': (FlutterView view) => TouchesScenario(view),
+  'tap_status_bar': (FlutterView view) =>
+      EchoPlatformChannelScenario(view, channel: 'flutter/status_bar'),
   'initial_route_reply': (FlutterView view) => InitialRouteReply(view),
   'platform_view_with_continuous_texture': (FlutterView view) =>
       PlatformViewWithContinuousTexture(view, id: _viewId++),