“Cocoa”NSMenus 实际上完全基于 Carbon 构建，因此虽然 Cocoa API 没有公开太多功能，但您可以深入研究 Carbon-land 并获得更多功能。无论如何，这就是苹果所做的——苹果菜单项是从IBCarbonMenuItem，如下所示：
/System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/Resources/English.lproj/StandardMenus.nib/objects.xib
不幸的是，64 位 Carbon API 似乎充满了错误和缺失的功能，这使得安装工作绘制处理程序比 32 位版本困难得多。这是我想出的一个 hacky 版本：
#import <Carbon/Carbon.h>
OSStatus eventHandler(EventHandlerCallRef inHandlerRef, EventRef inEvent, void *inUserData) {
OSStatus ret = 0;
if (GetEventClass(inEvent) == kEventClassMenu) {
if (GetEventKind(inEvent) == kEventMenuDrawItem) {
// draw the standard menu stuff
ret = CallNextEventHandler(inHandlerRef, inEvent);
MenuTrackingData tracking_data;
GetMenuTrackingData(menuRef, &tracking_data);
MenuItemIndex item_index;
GetEventParameter(inEvent, kEventParamMenuItemIndex, typeMenuItemIndex, nil, sizeof(item_index), nil, &item_index);
if (tracking_data.itemSelected == item_index) {
HIRect item_rect;
GetEventParameter(inEvent, kEventParamMenuItemBounds, typeHIRect, nil, sizeof(item_rect), nil, &item_rect);
CGContextRef context;
GetEventParameter(inEvent, kEventParamCGContextRef, typeCGContextRef, nil, sizeof(context), nil, &context);
// first REMOVE a state from the graphics stack, instead of pushing onto the stack
// this is to remove the clipping and translation values that are completely useless without the context height value
extern void *CGContextCopyTopGState(CGContextRef);
void *state = CGContextCopyTopGState(context);
CGContextRestoreGState(context);
// draw our content on top of the menu item
CGContextSetRGBFillColor(context, 0.0, 0.0, 0.0, 0.5);
CGContextFillRect(context, CGRectMake(0, item_rect.origin.y - tracking_data.virtualMenuTop, item_rect.size.width, item_rect.size.height));
// and push a dummy graphics state onto the stack so the calling function can pop it again and be none the wiser
CGContextSaveGState(context);
extern void CGContextReplaceTopGState(CGContextRef, void *);
CGContextReplaceTopGState(context, state);
extern void CGGStateRelease(void *);
CGGStateRelease(state);
}
}
}
}
- (void)beginTracking:(NSNotification *)notification {
// install a Carbon event handler to custom draw in the menu
if (menuRef == nil) {
extern MenuRef _NSGetCarbonMenu(NSMenu *);
extern EventTargetRef GetMenuEventTarget(MenuRef);
menuRef = _NSGetCarbonMenu(menu);
if (menuRef == nil) return;
EventTypeSpec events[1];
events[0].eventClass = kEventClassMenu;
events[0].eventKind = kEventMenuDrawItem;
InstallEventHandler(GetMenuEventTarget(menuRef), NewEventHandlerUPP(&eventHandler), GetEventTypeCount(events), events, nil, nil);
}
if (menuRef != nil) {
// set the kMenuItemAttrCustomDraw attrib on the menu item
// this attribute is needed in order to receive the kMenuEventDrawItem event in the Carbon event handler
extern OSStatus ChangeMenuItemAttributes(MenuRef, MenuItemIndex, MenuItemAttributes, MenuItemAttributes);
ChangeMenuItemAttributes(menuRef, item_index, kMenuItemAttrCustomDraw, 0);
}
}
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
menu = [[NSMenu alloc] initWithTitle:@""];
// register for the BeginTracking notification so we can install our Carbon event handler as soon as the menu is constructed
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(beginTracking:) name:NSMenuDidBeginTrackingNotification object:menu];
}
首先它注册一个 BeginTracking 通知，如下_NSGetCarbonMenu仅在菜单构建后返回有效句柄，并且在绘制菜单之前调用 BeginTracking。
然后它使用通知回调来获取 Carbon MenuRef 并将标准 Carbon 事件处理程序附加到菜单。
通常我们可以简单地采取kEventParamMenuContextHeight事件参数并翻转 CGContextRef 并开始绘制，但该参数仅在 32 位模式下可用。 Apple 文档建议在当前端口的高度不可用时使用该值，但这也仅在 32 位模式下可用。
所以既然给我们的图形状态没有用，就把它从堆栈中弹出并使用之前的图形状态。事实证明，这个新状态被转换为菜单的虚拟顶部，可以使用以下命令检索该菜单GetMenuTrackingData.virtualMenuTop. The kEventParamVirtualMenuTop值在 64 位模式下也不正确，因此必须使用GetMenuTrackingData.
这是很hacky和荒谬的，但它比使用setView并重新实现整个菜单项行为要好。 OS X 上的菜单 API 是bit一团糟。
