Gist for https://rymc.io/2018/12/03/swipeable-nscollectionview/

nscollectionviewitem-touchesbegan.swift

override func touchesBegan(with event: NSEvent) {
    if(isTracking) { return }

    let initialTouches = event.touches(matching: .touching, in: view)
    if(initialTouches.count != 2) { return }
    
    isTracking = true
    initialPoint = view.convert(event.locationInWindow, from: nil)

    let touches = Array(initialTouches)
    initialTouchOne = touches[0]
    initialTouchTwo = touches[1]
    currentTouchOne = touches[0]
    currentTouchTwo = touches[1]
}