Problem related to pan gesture flickers view - ios

Question

I am trying to move the bottom view up/ down using a pan gesture. It worked but has an issue of flickering and therefore it's not a smooth transition.
Here is the code
#IBOutlet weak var bottomViewHeight: NSLayoutConstraint!
#IBOutlet weak var bottomView: UIView!
var maxHeight: CGFloat = 297
var minHeight: CGFloat = 128
let panGest = PanVerticalScrolling(target: self, action: #selector(panAction(sender:)))
bottomView.addGestureRecognizer(panGest)
#objc func panAction(sender: UIPanGestureRecognizer) {
if sender.state == .changed {
let endPosition = sender.location(in: self.bottomView)
let differenceHeight = maxHeight - endPosition.y
if differenceHeight < maxHeight && differenceHeight > minHeight {
bottomViewHeight.constant = differenceHeight
self.bottomView.layoutIfNeeded()
}
}
}
Here is the gesture class
class PanVerticalScrolling : UIPanGestureRecognizer {
override init(target: Any?, action: Selector?) {
super.init(target: target, action: action)
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
super.touchesMoved(touches, with: event)
if state == .began {
let vel = velocity(in: self.view)
if abs(vel.x) > abs(vel.y) {
state = .cancelled
}
}
}
}
And here in the image, you can check the actual issue

Answer 1

The problem is that you are repeatedly getting the touch position in the bottom view ... and then changing bottom view size.
Since the touch Y is relative to the height of the view, it's bouncing around.
Try this...
Add a class property:
var initialY: CGFloat = -1.0
and change your panAction() to this:
#objc func panAction(sender: UIPanGestureRecognizer) {
if sender.state == .changed {
if initialY == -1.0 {
initialY = sender.location(in: self.view).y
}
let endPositionY = sender.location(in: self.view).y - initialY
let differenceHeight = maxHeight - endPositionY
if differenceHeight < maxHeight && differenceHeight > minHeight {
bottomViewHeight.constant = differenceHeight
self.bottomView.layoutIfNeeded()
}
}
}
You'll need to reset initialY to -1 each time you "end" the drag / resize process.
Edit
A better way - maintains current state:
// these will be used inside panAction()
var initialY: CGFloat = -1.0
var initialHeight: CGFloat = -1.0
#objc func panAction(sender: UIPanGestureRecognizer) {
if sender.state == .changed {
let endPositionY = sender.location(in: self.view).y - initialY
let differenceHeight = self.initialHeight - endPositionY
if differenceHeight < maxHeight && differenceHeight > minHeight {
bottomViewHeight.constant = differenceHeight
}
}
if sender.state == .began {
// reset position and height tracking properties
self.initialY = sender.location(in: self.view).y
self.initialHeight = bottomViewHeight.constant
}
}