Get distance from the edge in swift 3

Question

I've got a moveable image using UIPanGestureRecognizer and i need to make the image more transparent the closer it get's to the edge of the screen. Below is the code I'm using to move and rotate the image the further it get's away from the center.

Adding the UIPanGestureRecognizer to the UIView the image is in:

let moveImage = UIPanGestureRecognizer(target: self, action: #selector(self.detectPan))
moveImage.cancelsTouchesInView = false 
MainImageView.addGestureRecognizer(moveImage)

The function that get's called when the UIPanGestureRecognizer starts.

func detectPan(gesture: UIPanGestureRecognizer) {
    if gesture.state == UIGestureRecognizerState.began || gesture.state == UIGestureRecognizerState.changed {
        let translation  = gesture.translation(in: self.view)
        gesture.view!.center = CGPoint(x: gesture.view!.center.x + translation.x, y: gesture.view!.center.y)
        gesture.setTranslation(CGPoint(x: 0,y: 0), in: self.view)
        let newValue = CGFloat(((gesture.view!.center.x + translation.x) - (self.view.bounds.width * 0.50)) / 500)
        MainImageView.transform = MainImageView.transform.rotated(by: -lastValue)
        MainImageView.transform = MainImageView.transform.rotated(by: newValue)
        lastValue = newValue
    }
}

Answer 1

I'm assuming that " further it get's away from the center" is relative to the center of the view's parent (or if it's relative to something else, you can adjust my example accordingly).

Since you seem to only be moving the image horizontally, the distance should be a simple first degree calculation.

Something like:

let distanceToCenter = abs(gesture.view!.center.x - gesture.view!.parent!.center.x)

If you were moving both vertically and horizontally, you would need a second degree calculation to get the distance:

let deltaX = gesture.view!.center.x - gesture.view!.parent!.center.x  
let deltaY = gesture.view!.center.y - gesture.view!.parent!.center.y
let distanceToCenter = sqrt(deltaX*deltaX + deltaY*deltaY)

You could then compute the alpha for fading out as follows:

let alpha = 1 - 2 * distanceToCenter / gesture.view!.parent!.bounds.size.width

Note that Swift may need some type casting for these calculations (at least until it goes to math school and figures out number theory)