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

nscollectionviewitem-touchesended.swift

override func touchesEnded(with event: NSEvent) {
    if(self.isTracking) {
        self.endTracking(leftAnchor?.constant ?? 0)
    }
}

override func touchesCancelled(with event: NSEvent) {
    if(self.isTracking) {
        self.endTracking(leftAnchor?.constant ?? 0)
    }
}

func endTracking(_ delta: CGFloat) {
    initialTouchOne = nil
    initialTouchTwo = nil
    currentTouchOne = nil
    currentTouchTwo = nil
    isTracking = false
    
    let leftThreshold: CGFloat = 50.0
    let rightThreshold: CGFloat = -50.0
    var to: CGFloat = 0.0
    
    if(delta > leftThreshold) {
        to = leftThreshold
    } else if(delta < rightThreshold) {
        to = rightThreshold
    }
    
    NSAnimationContext.runAnimationGroup { [weak self] (context: NSAnimationContext) in
        context.timingFunction = CAMediaTimingFunction(name: .easeIn)
        context.duration = 0.5
        context.allowsImplicitAnimation = true
        self?.leftAnchor?.animator().constant = to
    }
}