[macos] Add hover support to FLEViewController (#7975)

Adds an option to enable mouse tracking, which will send
add/hover/remove events to the Flutter engine.

Author: stuartmorgan

shell/platform/darwin/macos/framework/Headers/FLEViewController.h

@@ -16,6 +16,17 @@
 #import "FlutterMacros.h"
 #endif
 
+typedef NS_ENUM(NSInteger, FlutterMouseTrackingMode) {
+  // Hover events will never be sent to Flutter.
+  FlutterMouseTrackingModeNone = 0,
+  // Hover events will be sent to Flutter when the view is in the key window.
+  FlutterMouseTrackingModeInKeyWindow,
+  // Hover events will be sent to Flutter when the view is in the active app.
+  FlutterMouseTrackingModeInActiveApp,
+  // Hover events will be sent to Flutter regardless of window and app focus.
+  FlutterMouseTrackingModeAlways,
+};
+
 /**
  * Controls embedder plugins and communication with the underlying Flutter engine, managing a view
  * intended to handle key inputs and drawing protocols (see |view|).
@@ -33,6 +44,12 @@ FLUTTER_EXPORT
  */
 @property(nullable) NSView<FLEOpenGLContextHandling>* view;
 
+/**
+ * The style of mouse tracking to use for the view. Defaults to
+ * FlutterMouseTrackingModeNone.
+ */
+@property(nonatomic) FlutterMouseTrackingMode mouseTrackingMode;
+
 /**
  * Launches the Flutter engine with the provided configuration.
  *

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

@@ -32,7 +32,18 @@ @interface FLEViewController ()
 /**
  * A list of additional responders to keyboard events. Keybord events are forwarded to all of them.
  */
-@property NSMutableOrderedSet<NSResponder*>* additionalKeyResponders;
+@property(nonatomic) NSMutableOrderedSet<NSResponder*>* additionalKeyResponders;
+
+/**
+ * The tracking area used to generate hover events, if enabled.
+ */
+@property(nonatomic) NSTrackingArea* trackingArea;
+
+/**
+ * Updates |trackingArea| for the current tracking settings, creating it with
+ * the correct mode if tracking is enabled, or removing it if not.
+ */
+- (void)configureTrackingArea;
 
 /**
  * Creates and registers plugins used by this view controller.
@@ -202,12 +213,28 @@ - (void)dealloc {
   }
 }
 
+- (void)setView:(NSView*)view {
+  if (_trackingArea) {
+    [self.view removeTrackingArea:_trackingArea];
+  }
+  [super setView:view];
+  [self configureTrackingArea];
+}
+
 - (void)loadView {
   self.view = [[FLEView alloc] init];
 }
 
 #pragma mark - Public methods
 
+- (void)setMouseTrackingMode:(FlutterMouseTrackingMode)mode {
+  if (_mouseTrackingMode == mode) {
+    return;
+  }
+  _mouseTrackingMode = mode;
+  [self configureTrackingArea];
+}
+
 - (BOOL)launchEngineWithAssetsPath:(NSURL*)assets
               commandLineArguments:(NSArray<NSString*>*)arguments {
   return [self launchEngineInternalWithAssetsPath:assets
@@ -241,6 +268,35 @@ - (void)removeKeyResponder:(NSResponder*)responder {
 
 #pragma mark - Private methods
 
+- (void)configureTrackingArea {
+  if (_mouseTrackingMode != FlutterMouseTrackingModeNone && self.view) {
+    NSTrackingAreaOptions options =
+        NSTrackingMouseEnteredAndExited | NSTrackingMouseMoved | NSTrackingInVisibleRect;
+    switch (_mouseTrackingMode) {
+      case FlutterMouseTrackingModeInKeyWindow:
+        options |= NSTrackingActiveInKeyWindow;
+        break;
+      case FlutterMouseTrackingModeInActiveApp:
+        options |= NSTrackingActiveInActiveApp;
+        break;
+      case FlutterMouseTrackingModeAlways:
+        options |= NSTrackingActiveAlways;
+        break;
+      default:
+        NSLog(@"Error: Unrecognized mouse tracking mode: %ld", _mouseTrackingMode);
+        return;
+    }
+    _trackingArea = [[NSTrackingArea alloc] initWithRect:NSZeroRect
+                                                 options:options
+                                                   owner:self
+                                                userInfo:nil];
+    [self.view addTrackingArea:_trackingArea];
+  } else if (_trackingArea) {
+    [self.view removeTrackingArea:_trackingArea];
+    _trackingArea = nil;
+  }
+}
+
 - (void)addInternalPlugins {
   _textInputPlugin = [[FLETextInputPlugin alloc] initWithViewController:self];
   _keyEventChannel =
@@ -471,4 +527,16 @@ - (void)mouseDragged:(NSEvent*)event {
   [self dispatchMouseEvent:event phase:kMove];
 }
 
+- (void)mouseEntered:(NSEvent*)event {
+  [self dispatchMouseEvent:event phase:kAdd];
+}
+
+- (void)mouseExited:(NSEvent*)event {
+  [self dispatchMouseEvent:event phase:kRemove];
+}
+
+- (void)mouseMoved:(NSEvent*)event {
+  [self dispatchMouseEvent:event phase:kHover];
+}
+
 @end