您可以在NSWorkspace.sharedWorkspace().notificationCenter观察NSWorkspaceDidActivateApplicationNotification钥匙时添加观察者。您将选择器指向您的一种方法，然后从userInfo字典中获取信息。

AppDelegate中的简单示例：

斯威夫特2.2

func applicationDidFinishLaunching(notification: NSNotification) {
    NSWorkspace.sharedWorkspace().notificationCenter.addObserver(self,
                                                        selector: #selector(activated),
                                                        name: NSWorkspaceDidActivateApplicationNotification,
                                                        object: nil)
}

func activated(notification: NSNotification) {
    if let info = notification.userInfo,
        app = info[NSWorkspaceApplicationKey],
        name = app.localizedName {
            print(name)
    }
}

迅捷3

func applicationDidFinishLaunching(_ aNotification: Notification) {
    NSWorkspace.shared().notificationCenter.addObserver(self,
                                                        selector: #selector(activated(_:)),
                                                        name: NSNotification.Name.NSWorkspaceDidActivateApplication,
                                                        object: nil)
}

func activated(_ notification: NSNotification) {
    if let info = notification.userInfo,
        let app = info[NSWorkspaceApplicationKey] as? NSRunningApplication,
        let name = app.localizedName 
    {
        print(name)
    }
}