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

nscollectionviewitem-touchesmoved.swift

override func touchesMoved(with event: NSEvent) {
    if(!isTracking) { return }
    
    let currentTouches = event.touches(matching: .touching, in: view)
    if(currentTouches.count != 2) { return }
    
    currentTouchOne = nil
    currentTouchTwo = nil
    
    currentTouches.forEach { (touch: NSTouch) in
        if(touch.identity.isEqual(initialTouchOne?.identity)) {
            currentTouchOne = touch
        } else {
            currentTouchTwo = touch
        }
    }
    
    let initialXPoint = [
        initialTouchOne?.normalizedPosition.x ?? 0.0,
        initialTouchTwo?.normalizedPosition.x ?? 0.0
    ].min() ?? 0.0
    
    let currentXPoint = [
        currentTouchOne?.normalizedPosition.x ?? 0.0,
        currentTouchTwo?.normalizedPosition.x ?? 0.0
    ].min() ?? 0.0
    
    let deviceWidth = initialTouchOne?.deviceSize.width ?? 0.0
    let oldX = (initialXPoint * deviceWidth).rounded(.up)
    let newX = (currentXPoint * deviceWidth).rounded(.up)

    var delta: CGFloat = 0.0
    if(oldX > newX) { // Swiping left
        delta = (oldX - newX) * -1.0
    } else if(newX > oldX) { // Swiping right
        delta = newX - oldX
    }

    NSAnimationContext.runAnimationGroup { [weak self] (context: NSAnimationContext) in
        context.timingFunction = CAMediaTimingFunction(name: .easeIn)
        context.duration = 0.2
        context.allowsImplicitAnimation = true
        self?.leftAnchor?.animator().constant = delta
    }
}