1

Apple's Interface Guidelines suggest a Pan gesture over a Swipe if seeking to "track the movement of the user’s fingers onscreen ... use a pan gesture recognizer to drag objects around in your interface or update their appearance".

Drawing shapes I am looking to use the Pan gesture to increase or decrease the number of sides of a shape e.g. a hexagon (six sides) to a circle (infinite sides), of which the onscreen shapes should update in real time:

override func draw(_ rect: CGRect) {

            let path = UIBezierPath()
            let radius: Double = Double(rect.width) / 2 - 20

            let centre = CGPoint(x: rect.width / 2, y: rect.height / 3)
            
            path.move(to: CGPoint(x: centre.x + CGFloat(radius), y: centre.y))


            for i in stride(from: 0, to: 361.0, by: 45) {
                let radians = i * Double.pi / 180
                
                let x = Double(centre.x) + radius * cos(radians)
                let y = Double(centre.y) + radius * sin(radians)
                
                path.addLine(to: CGPoint(x: x, y: y))
            }
            path.stroke()
        }

I was thinking of using UIPanGestureRecognizer to generate a number for the Stride "by" value which would alter the shape on screen:

func panningRange(_ sender: UIPanGestureRecognizer) {
                if sender.state == .began || sender.state == .changed {
                    pan = sender.location(in: circleGenerator).y
                    setNeedsDisplay()
                }
            }

Issues remain however:

  1. Anything more than 45 and the shape drawn won't connect its lines (this is why the "to" value is 361.0)

  2. With a pinch gesture example a print statement showed float numbers with fourteen decimal places, I can round these up as integers to mimic a slider bar, but they are only single digits like "1" (a circle). I need the shapes to change within a short distance of movement (even on a smaller iPhone SE screen), so I will likely need to modify the counter logic to get higher integers e.g. 25, 30..

  3. It will be necessary to round up generated numbers to known values that will produce recognisable shapes e.g. 45 = nonagon (9 sided shape), otherwise the shapes can look unflattering during transition

  4. Transitioning smoothly between recognisable shapes should be smooth and immediate, I don't know if the UIBezierPath logic I have will be the optimal choice for what I am trying to achieve?

  5. Is a Pan gesture the right option to avoid a visible sliding UI bar?

2

361, unlike 360, works because stride returns a sequence from a starting value to, but not including, an end value. Also, it's always a good idea to close() the path once you're done drawing. You can do it instead of drawing the last side to ensure a clean connection:

...
let angle = (2 * .pi) / Double(numberOfSides)

for i in 1..<numberOfSides  {
  let radians = angle * Double(i)
                
  let x = Double(centre.x) + radius * cos(radians)
  let y = Double(centre.y) + radius * sin(radians)
                
  path.addLine(to: CGPoint(x: x, y: y))
}
path.close()
path.stroke()

UIBezierPath is a great choice. You can also draw using the methods on CGContext, there is no particular difference.
100 points is more than enough to transition from a triangle to a polygon that is visually indistinguishable from a circle. An iPhone SE screen should be at least 300 points wide.

In terms of gestures, It certainly is the most suitable one.