[macOS] Forward key events to NSTextInputContext when there's composing text

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

@@ -72,6 +72,12 @@ - (void)handleEvent:(nonnull NSEvent*)event {
       event.type != NSEventTypeFlagsChanged) {
     return;
   }
+
+  if (_viewDelegate.isComposing) {
+    [self dispatchToSecondaryResponders:event];
+    return;
+  }
+
   // Having no primary responders require extra logic, but Flutter hard-codes
   // all primary responders, so this is a situation that Flutter will never
   // encounter.

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

@@ -75,6 +75,7 @@ - (void)recordChannelCallsTo:(nonnull NSMutableArray<FlutterAsyncKeyCallback>*)s
 
 @property(nonatomic) FlutterKeyboardManager* manager;
 @property(nonatomic) NSResponder* nextResponder;
+@property(nonatomic, assign) BOOL isComposing;
 
 #pragma mark - Private
 
@@ -105,6 +106,7 @@ - (nonnull instancetype)init {
   [self respondChannelCallsWith:FALSE];
   [self respondEmbedderCallsWith:FALSE];
   [self respondTextInputWith:FALSE];
+  _isComposing = NO;
 
   id messengerMock = OCMStrictProtocolMock(@protocol(FlutterBinaryMessenger));
   OCMStub([messengerMock sendOnChannel:@"flutter/keyevent"
@@ -117,6 +119,7 @@ - (nonnull instancetype)init {
   OCMStub([viewDelegateMock onTextInputKeyEvent:[OCMArg any]])
       .andCall(self, @selector(handleTextInputKeyEvent:));
   OCMStub([viewDelegateMock getBinaryMessenger]).andReturn(messengerMock);
+  OCMStub([viewDelegateMock isComposing]).andCall(self, @selector(isComposing));
   OCMStub([viewDelegateMock sendKeyEvent:FlutterKeyEvent {} callback:nil userData:nil])
       .ignoringNonObjectArgs()
       .andCall(self, @selector(handleEmbedderEvent:callback:userData:));
@@ -188,6 +191,7 @@ - (bool)nextResponderShouldThrowOnKeyUp;
 - (bool)singlePrimaryResponder;
 - (bool)doublePrimaryResponder;
 - (bool)textInputPlugin;
+- (bool)forwardKeyEventsToSystemWhenComposing;
 - (bool)emptyNextResponder;
 @end
 
@@ -208,6 +212,10 @@ - (bool)emptyNextResponder;
   ASSERT_TRUE([[FlutterKeyboardManagerUnittestsObjC alloc] textInputPlugin]);
 }
 
+TEST(FlutterKeyboardManagerUnittests, handlingComposingText) {
+  ASSERT_TRUE([[FlutterKeyboardManagerUnittestsObjC alloc] forwardKeyEventsToSystemWhenComposing]);
+}
+
 TEST(FlutterKeyboardManagerUnittests, EmptyNextResponder) {
   ASSERT_TRUE([[FlutterKeyboardManagerUnittestsObjC alloc] emptyNextResponder]);
 }
@@ -356,6 +364,31 @@ - (bool)textInputPlugin {
   return true;
 }
 
+- (bool)forwardKeyEventsToSystemWhenComposing {
+  KeyboardTester* tester = OCMPartialMock([[KeyboardTester alloc] init]);
+
+  NSMutableArray<FlutterAsyncKeyCallback>* channelCallbacks =
+      [NSMutableArray<FlutterAsyncKeyCallback> array];
+  NSMutableArray<FlutterAsyncKeyCallback>* embedderCallbacks =
+      [NSMutableArray<FlutterAsyncKeyCallback> array];
+  [tester recordEmbedderCallsTo:embedderCallbacks];
+  [tester recordChannelCallsTo:channelCallbacks];
+  // The event shouldn't propagate further even if TextInputPlugin does not
+  // claim the event.
+  [tester respondTextInputWith:NO];
+
+  tester.isComposing = YES;
+  // Send a down event with composing == YES.
+  [tester.manager handleEvent:keyUpEvent(0x50)];
+
+  // Nobody gets the event except for the text input plugin.
+  EXPECT_EQ([channelCallbacks count], 0u);
+  EXPECT_EQ([embedderCallbacks count], 0u);
+  OCMVerify(times(1), [tester handleTextInputKeyEvent:checkKeyDownEvent(0x50)]);
+
+  return true;
+}
+
 - (bool)emptyNextResponder {
   KeyboardTester* tester = [[KeyboardTester alloc] init];
   tester.nextResponder = nil;

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

@@ -50,4 +50,14 @@
  */
 - (BOOL)onTextInputKeyEvent:(nonnull NSEvent*)event;
 
+/**
+ * Whether this FlutterKeyboardViewDelegate is actively taking provisional user text input.
+ *
+ * This is typically true when a Flutter text field is focused, and the user is entering composing
+ * text into the text field.
+ */
+// TODO (LongCatIsLooong): remove this method and implement a long-term fix for
+// https://github.com/flutter/flutter/issues/85328.
+- (BOOL)isComposing;
+
 @end

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

@@ -45,6 +45,15 @@
  */
 - (BOOL)isFirstResponder;
 
+/**
+ * Whether this plugin has composing text.
+ *
+ * This is only true when the text input plugin is actively taking user input with composing text.
+ */
+// TODO (LongCatIsLooong): remove this method and implement a long-term fix for
+// https://github.com/flutter/flutter/issues/85328.
+- (BOOL)isComposing;
+
 /**
  * Handles key down events received from the view controller, responding YES if
  * the event was handled.

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

@@ -457,6 +457,10 @@ - (NSString*)textAffinityString {
                                                             : kTextAffinityDownstream;
 }
 
+- (BOOL)isComposing {
+  return _activeModel && !_activeModel->composing_range().collapsed();
+}
+
 - (BOOL)handleKeyEvent:(NSEvent*)event {
   if (event.type == NSEventTypeKeyUp ||
       (event.type == NSEventTypeFlagsChanged && event.modifierFlags < _previouslyPressedFlags)) {

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

@@ -661,6 +661,10 @@ - (void)viewDidReshape:(NSView*)view {
 
 #pragma mark - FlutterKeyboardViewDelegate
 
+- (BOOL)isComposing {
+  return [_textInputPlugin isComposing];
+}
+
 - (void)sendKeyEvent:(const FlutterKeyEvent&)event
             callback:(nullable FlutterKeyEventCallback)callback
             userData:(nullable void*)userData {