iconToggle.swift

import Cocoa

class ViewController: NSViewController {
    
    var timer : Timer?
    override func viewDidLoad() {
        super.viewDidLoad()
        
        timer = Timer.scheduledTimer(withTimeInterval: 2.0, repeats: true) { timer in
            self.toggleImage()
        }
        
    }
    
    var toggle = true
    func toggleImage() {
        let image = NSImage(named: toggle ? "thing1" : "thing2")
        NSApplication.shared().applicationIconImage = image
        toggle = !toggle
    }
}