macOS 11-compatible Mac Catalyst PopUp Button (Private API)

PSTLMacOS11AwarePopUpButton.swift

class PSTLMacOS11AwarePopUpButton: UIButton {
	var selectedPopUpIndex = 0 {
		didSet {
			sendActions(for: .valueChanged)
		}
	}
	
	override var menu: UIMenu? {
		set {
			/* Button title must match first menu item */
			setTitle(newValue?.children.first?.title, for: .normal)
			super.menu = newValue
		}
		get {
			super.menu
		}
	}
	
	convenience init() {
		self.init(type: .system)
		
		showsMenuAsPrimaryAction = true
		
		if #available(iOS 15.0, *) {
			changesSelectionAsPrimaryAction = true
		}
	}
	
	override func willMove(toSuperview newSuperview: UIView?) {
		super.willMove(toSuperview: newSuperview)
		
		preparePopUpButtonForMacOS11()
	}

	func preparePopUpButtonForMacOS11() {
		guard ProcessInfo.processInfo.operatingSystemVersion.majorVersion < 12 else { return }
		
		if let NSPopupButton = value(forKeyPath: "_visualElement._button") as? NSObject {
			guard NSPopupButton.responds(to: NSSelectorFromString("setPullsDown:")) else { return }
			NSPopupButton.setValue(false, forKey: "pullsDown")
			
			if let NSMenu = NSPopupButton.value(forKeyPath: "menu") as? NSObject {
				NotificationCenter.default.addObserver(forName: NSNotification.Name("NSMenuDidSendActionNotification"), object: NSMenu, queue: nil) { note in
					
					guard let indexOfSelectedItem = NSPopupButton.value(forKeyPath: "indexOfSelectedItem") as? Int else { return }
					
					DispatchQueue.main.async {
						self.selectedPopUpIndex = indexOfSelectedItem
					}
				}
			}
		}
	}
}