Create menu in mac Catalyst via UIMenuBuilder (beta 4) - (void)buildMenuWithBuilder:(id<UIMenuBuilder>)builder;

AppDelegate.swift

// AppDelegate.swift:

    // macCatalyst: Create menu
    override func buildMenu(with builder: UIMenuBuilder) {
        guard builder.system == .main else { return }
        
        // The format menu doesn't make sense
        builder.remove(menu: .format)
        
        // Add Open command
        let openKeyCommand = PSVMenuBuilder.keyCommand(withTitle: "Open...", image:nil, action: #selector(openFile), input: "o", modifierFlags: [UIKeyModifierFlags.command], propertyList: nil)

        // This results in an infinite loop inside init
        // FB6790921: Catalyst: Creating UIKeyCommand in Swift results in an endless loop
        //let openKeyCommand = UIKeyCommand(title: "Open...", image:nil, action: #selector(openFile), input: "o", modifierFlags: [UIKeyModifierFlags.command], propertyList: nil)

        let identifier = UIMenu.Identifier("open_file")
        let menu = UIMenu.init(title: "Open", image: nil, identifier: identifier, options: [.displayInline], children: [openKeyCommand]);

        builder.insertSibling(menu, afterMenu: .newScene)
    }

PSVMenuBuilder.h

// Put that into a separate file and add to your bridging headers. (ObjC)

#import "PSVMenuBuilder.h"

@implementation PSVMenuBuilder

// This is a simple wrapper to work around a bug:
// // FB6790921: Catalyst: Creating UIKeyCommand in Swift results in an endless loop
+ (UIKeyCommand *)keyCommandWithTitle:(NSString *)title
                                image:(nullable UIImage *)image
                               action:(SEL)action
                                input:(NSString *)input
                        modifierFlags:(UIKeyModifierFlags)modifierFlags
                         propertyList:(nullable id)propertyList {
    return [UIKeyCommand commandWithTitle:title image:image action:action input:input modifierFlags:modifierFlags propertyList:propertyList];
}

// This is a simple wrapper to work around a bug:
// // FB6790921: Catalyst: Creating UIKeyCommand in Swift results in an endless loop
+ (UICommand *)commandWithTitle:(NSString *)title
                          image:(nullable UIImage *)image
                         action:(SEL)action
                   propertyList:(nullable id)propertyList {
    return [UICommand commandWithTitle: title image:image action:action propertyList:propertyList];
}

@end

PSVMenuBuilder.m

@implementation PSVMenuBuilder

+ (UIKeyCommand *)keyCommandWithTitle:(NSString *)title
                                image:(nullable UIImage *)image
                               action:(SEL)action
                                input:(NSString *)input
                        modifierFlags:(UIKeyModifierFlags)modifierFlags
                         propertyList:(nullable id)propertyList {
    return [UIKeyCommand commandWithTitle:title image:image action:action input:input modifierFlags:modifierFlags propertyList:propertyList];
}

+ (UICommand *)commandWithTitle:(NSString *)title
                          image:(nullable UIImage *)image
                         action:(SEL)action
                   propertyList:(nullable id)propertyList {
    return [UICommand commandWithTitle: title image:image action:action propertyList:propertyList];
}

@end

@bmueller first I was like "I already did that and it didn't work" but then I was like, "this must be right, try harder" and the answer was... you're right. Some confusing things are that the IDs have to be unique otherwise the menus collapse, you must use insertSibling at some point, and... not sure.

THANKS for the comment here, it worked great for me to get separators in menus.

THANKS @steipete for the code, it's nice. I'm doing it all in Swift now (2022) so I think that's all sorted.