Enable delayed event delivery for macOS (#21231)

This enables delayed event delivery for macOS, so that shortcuts can handle keys that are headed for a text field and intercept them. This fixes the problem where pressing TAB (or other shortcuts) in a text field also inserts a tab character into the text field.

Author: gspencergoog

ci/licenses_golden/licenses_flutter

@@ -1073,6 +1073,8 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterGLCom
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterGLCompositorUnittests.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterIOSurfaceHolder.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterIOSurfaceHolder.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterIntermediateKeyResponder.h
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterIntermediateKeyResponder.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterMouseCursorPlugin.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterMouseCursorPlugin.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterResizeSynchronizer.h

shell/platform/darwin/macos/BUILD.gn

@@ -60,6 +60,8 @@ source_set("flutter_framework_source") {
     "framework/Source/FlutterGLCompositor.mm",
     "framework/Source/FlutterIOSurfaceHolder.h",
     "framework/Source/FlutterIOSurfaceHolder.mm",
+    "framework/Source/FlutterIntermediateKeyResponder.h",
+    "framework/Source/FlutterIntermediateKeyResponder.mm",
     "framework/Source/FlutterMouseCursorPlugin.h",
     "framework/Source/FlutterMouseCursorPlugin.mm",
     "framework/Source/FlutterResizeSynchronizer.h",

shell/platform/darwin/macos/framework/Source/FlutterIntermediateKeyResponder.h

@@ -0,0 +1,28 @@
+// 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 <Cocoa/Cocoa.h>
+
+/*
+ * An interface for a key responder that can declare itself as the final
+ * responder of the event, terminating the event propagation.
+ *
+ * It differs from an NSResponder in that it returns a boolean from the
+ * handleKeyUp and handleKeyDown calls, where true means it has handled the
+ * given event.
+ */
+@interface FlutterIntermediateKeyResponder : NSObject
+/*
+ * Informs the receiver that the user has released a key.
+ *
+ * Default implementation returns NO.
+ */
+- (BOOL)handleKeyUp:(nonnull NSEvent*)event;
+/*
+ * Informs the receiver that the user has pressed a key.
+ *
+ * Default implementation returns NO.
+ */
+- (BOOL)handleKeyDown:(nonnull NSEvent*)event;
+@end

shell/platform/darwin/macos/framework/Source/FlutterIntermediateKeyResponder.mm

@@ -0,0 +1,19 @@
+// 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 "flutter/shell/platform/darwin/macos/framework/Source/FlutterIntermediateKeyResponder.h"
+
+@implementation FlutterIntermediateKeyResponder {
+}
+
+#pragma mark - Default key handling methods
+
+- (BOOL)handleKeyUp:(NSEvent*)event {
+  return NO;
+}
+
+- (BOOL)handleKeyDown:(NSEvent*)event {
+  return NO;
+}
+@end

shell/platform/darwin/macos/framework/Source/FlutterTextInputPlugin.h

@@ -6,6 +6,7 @@
 
 #import "flutter/shell/platform/darwin/common/framework/Headers/FlutterBinaryMessenger.h"
 #import "flutter/shell/platform/darwin/macos/framework/Headers/FlutterViewController.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterIntermediateKeyResponder.h"
 
 /**
  * A plugin to handle text input.
@@ -16,7 +17,7 @@
  * This is not an FlutterPlugin since it needs access to FlutterViewController internals, so needs
  * to be managed differently.
  */
-@interface FlutterTextInputPlugin : NSResponder
+@interface FlutterTextInputPlugin : FlutterIntermediateKeyResponder
 
 /**
  * Initializes a text input plugin that coordinates key event handling with |viewController|.

shell/platform/darwin/macos/framework/Source/FlutterTextInputPlugin.mm

@@ -129,17 +129,20 @@ - (void)updateEditState {
 }
 
 #pragma mark -
-#pragma mark NSResponder
+#pragma mark FlutterIntermediateKeyResponder
 
 /**
+ * Handles key down events received from the view controller, responding TRUE if
+ * the event was handled.
+ *
  * Note, the Apple docs suggest that clients should override essentially all the
  * mouse and keyboard event-handling methods of NSResponder. However, experimentation
  * indicates that only key events are processed by the native layer; Flutter processes
  * mouse events. Additionally, processing both keyUp and keyDown results in duplicate
- * processing of the same keys. So for now, limit processing to just keyDown.
+ * processing of the same keys. So for now, limit processing to just handleKeyDown.
  */
-- (void)keyDown:(NSEvent*)event {
-  [_textInputContext handleEvent:event];
+- (BOOL)handleKeyDown:(NSEvent*)event {
+  return [_textInputContext handleEvent:event];
 }
 
 #pragma mark -

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

@@ -82,9 +82,14 @@ void Reset() {
 @interface FlutterViewController () <FlutterViewReshapeListener>
 
 /**
- * A list of additional responders to keyboard events. Keybord events are forwarded to all of them.
+ * A list of additional responders to keyboard events.
+ *
+ * Keyboard events received by FlutterViewController are first dispatched to
+ * each additional responder in order. If any of them handle the event (by
+ * returning true), the event is not dispatched to later additional responders
+ * or to the nextResponder.
  */
-@property(nonatomic) NSMutableOrderedSet<NSResponder*>* additionalKeyResponders;
+@property(nonatomic) NSMutableOrderedSet<FlutterIntermediateKeyResponder*>* additionalKeyResponders;
 
 /**
  * The tracking area used to generate hover events, if enabled.
@@ -135,7 +140,15 @@ - (void)dispatchMouseEvent:(nonnull NSEvent*)event;
 - (void)dispatchMouseEvent:(nonnull NSEvent*)event phase:(FlutterPointerPhase)phase;
 
 /**
- * Converts |event| to a key event channel message, and sends it to the engine.
+ * Sends |event| to all responders in additionalKeyResponders and then to the
+ * nextResponder if none of the additional responders handled the event.
+ */
+- (void)propagateKeyEvent:(NSEvent*)event ofType:(NSString*)type;
+
+/**
+ * Converts |event| to a key event channel message, and sends it to the engine to
+ * hand to the framework. Once the framework responds, if the event was not handled,
+ * propagates the event to any additional key responders.
  */
 - (void)dispatchKeyEvent:(NSEvent*)event ofType:(NSString*)type;
 
@@ -206,9 +219,11 @@ @implementation FlutterViewController {
  * Performs initialization that's common between the different init paths.
  */
 static void CommonInit(FlutterViewController* controller) {
-  controller->_engine = [[FlutterEngine alloc] initWithName:@"io.flutter"
-                                                    project:controller->_project
-                                     allowHeadlessExecution:NO];
+  if (!controller->_engine) {
+    controller->_engine = [[FlutterEngine alloc] initWithName:@"io.flutter"
+                                                      project:controller->_project
+                                       allowHeadlessExecution:NO];
+  }
   controller->_additionalKeyResponders = [[NSMutableOrderedSet alloc] init];
   controller->_mouseTrackingMode = FlutterMouseTrackingModeInKeyWindow;
 }
@@ -238,6 +253,27 @@ - (instancetype)initWithProject:(nullable FlutterDartProject*)project {
   return self;
 }
 
+- (instancetype)initWithEngine:(nonnull FlutterEngine*)engine
+                       nibName:(nullable NSString*)nibName
+                        bundle:(nullable NSBundle*)nibBundle {
+  NSAssert(engine != nil, @"Engine is required");
+  self = [super initWithNibName:nibName bundle:nibBundle];
+  if (self) {
+    if (engine.viewController) {
+      NSLog(@"The supplied FlutterEngine %@ is already used with FlutterViewController "
+             "instance %@. One instance of the FlutterEngine can only be attached to one "
+             "FlutterViewController at a time. Set FlutterEngine.viewController "
+             "to nil before attaching it to another FlutterViewController.",
+            [engine description], [engine.viewController description]);
+    }
+    _engine = engine;
+    CommonInit(self);
+    [engine setViewController:self];
+  }
+
+  return self;
+}
+
 - (void)loadView {
   NSOpenGLContext* resourceContext = _engine.resourceContext;
   if (!resourceContext) {
@@ -288,11 +324,12 @@ - (FlutterView*)flutterView {
   return static_cast<FlutterView*>(self.view);
 }
 
-- (void)addKeyResponder:(NSResponder*)responder {
+- (void)addKeyResponder:(FlutterIntermediateKeyResponder*)responder {
   [self.additionalKeyResponders addObject:responder];
 }
 
-- (void)removeKeyResponder:(NSResponder*)responder {
+- (void)removeKeyResponder:(FlutterIntermediateKeyResponder*)responder {
+  [self.additionalKeyResponders removeObject:responder];
 }
 
 #pragma mark - Private methods
@@ -460,19 +497,56 @@ - (void)dispatchMouseEvent:(NSEvent*)event phase:(FlutterPointerPhase)phase {
   }
 }
 
+- (void)propagateKeyEvent:(NSEvent*)event ofType:(NSString*)type {
+  if ([type isEqual:@"keydown"]) {
+    for (FlutterIntermediateKeyResponder* responder in self.additionalKeyResponders) {
+      if ([responder handleKeyDown:event]) {
+        return;
+      }
+    }
+    if ([self.nextResponder respondsToSelector:@selector(keyDown:)]) {
+      [self.nextResponder keyDown:event];
+    }
+  } else if ([type isEqual:@"keyup"]) {
+    for (FlutterIntermediateKeyResponder* responder in self.additionalKeyResponders) {
+      if ([responder handleKeyUp:event]) {
+        return;
+      }
+    }
+    if ([self.nextResponder respondsToSelector:@selector(keyUp:)]) {
+      [self.nextResponder keyUp:event];
+    }
+  }
+}
+
 - (void)dispatchKeyEvent:(NSEvent*)event ofType:(NSString*)type {
+  if (event.type != NSEventTypeKeyDown && event.type != NSEventTypeKeyUp &&
+      event.type != NSEventTypeFlagsChanged) {
+    return;
+  }
   NSMutableDictionary* keyMessage = [@{
     @"keymap" : @"macos",
     @"type" : type,
     @"keyCode" : @(event.keyCode),
     @"modifiers" : @(event.modifierFlags),
   } mutableCopy];
-  // Calling these methods on any other type of event will raise an exception.
+  // Calling these methods on any other type of event
+  // (e.g NSEventTypeFlagsChanged) will raise an exception.
   if (event.type == NSEventTypeKeyDown || event.type == NSEventTypeKeyUp) {
     keyMessage[@"characters"] = event.characters;
     keyMessage[@"charactersIgnoringModifiers"] = event.charactersIgnoringModifiers;
   }
-  [_keyEventChannel sendMessage:keyMessage];
+  __weak __typeof__(self) weakSelf = self;
+  FlutterReply replyHandler = ^(id _Nullable reply) {
+    if (!reply) {
+      return;
+    }
+    // Only re-dispatch the event to other responders if the framework didn't handle it.
+    if (![[reply valueForKey:@"handled"] boolValue]) {
+      [weakSelf propagateKeyEvent:event ofType:type];
+    }
+  };
+  [_keyEventChannel sendMessage:keyMessage reply:replyHandler];
 }
 
 - (void)onSettingsChanged:(NSNotification*)notification {
@@ -571,20 +645,10 @@ - (BOOL)acceptsFirstResponder {
 
 - (void)keyDown:(NSEvent*)event {
   [self dispatchKeyEvent:event ofType:@"keydown"];
-  for (NSResponder* responder in self.additionalKeyResponders) {
-    if ([responder respondsToSelector:@selector(keyDown:)]) {
-      [responder keyDown:event];
-    }
-  }
 }
 
 - (void)keyUp:(NSEvent*)event {
   [self dispatchKeyEvent:event ofType:@"keyup"];
-  for (NSResponder* responder in self.additionalKeyResponders) {
-    if ([responder respondsToSelector:@selector(keyUp:)]) {
-      [responder keyUp:event];
-    }
-  }
 }
 
 - (void)flagsChanged:(NSEvent*)event {