在NSMenuItem中添加NSSubmenu项

// 创建父菜单项
let parentMenuItem = NSMenuItem(title: "父菜单项", action: nil, keyEquivalent: "")

// 创建子菜单项
let submenu = NSMenu()

// 创建子菜单项的内容
let item1 = NSMenuItem(title: "子菜单项1", action: #selector(submenuItemClicked(_:)), keyEquivalent: "")
let item2 = NSMenuItem(title: "子菜单项2", action: #selector(submenuItemClicked(_:)), keyEquivalent: "")
let item3 = NSMenuItem(title: "子菜单项3", action: #selector(submenuItemClicked(_:)), keyEquivalent: "")

// 将子菜单项的内容添加到子菜单中
submenu.addItem(item1)
submenu.addItem(item2)
submenu.addItem(item3)

// 将子菜单设置为父菜单项的子菜单
parentMenuItem.submenu = submenu

// 将父菜单项添加到菜单栏中的某个菜单
// 这里假设已经有一个名为menu的NSMenu对象
menu.addItem(parentMenuItem)