panGesture.swift

    func onPanGesture(sender: UIPanGestureRecognizer) {
        let translation = sender.translationInView(view)
        let velocity = sender.velocityInView(view)
        
        if sender.state == UIGestureRecognizerState.Began {
        
        }
        else if sender.state == UIGestureRecognizerState.Changed {
            contentView.snp_updateConstraints(closure: { (make) -> Void in
                make.left.equalTo(view).offset(translation.x)
            })
        }
        else if sender.state == UIGestureRecognizerState.Ended {
            UIView.animateWithDuration(0.3, animations: {
                if velocity.x > 0 {
                    self.contentView.snp_updateConstraints(closure: { (make) -> Void in
                        make.left.equalTo(self.view).offset(TWMenuWidth)
                    })
                }
                else {
                    self.contentView.snp_updateConstraints(closure: { (make) -> Void in
                        make.left.equalTo(self.view).offset(0)
                    })
                }
            })
        }
    }