Removing lagging latency in drawing UIBezierPath smooth lines in Swift

Question

The code below draws smooth curved lines by overriding touches, but there is noticeable lagging or latency. The code uses addCurveToPoint and calls setNee

Answer 1

1. Yes, adding a curve every few points will give it a stuttering lag. So, yes, you can reduce this affect by adding a line to points[1], adding a quad curve to points[2] and adding a cubic curve to points[3].

   As you said, make sure to add this to a separate path, though. So, in Swift 3/4:

   class SmoothCurvedLinesView: UIView {
       var strokeColor = UIColor.blue
       var lineWidth: CGFloat = 20
       var snapshotImage: UIImage?
   
       private var path: UIBezierPath?
       private var temporaryPath: UIBezierPath?
       private var points = [CGPoint]()
   
       override func draw(_ rect: CGRect) {
           snapshotImage?.draw(in: rect)
   
           strokeColor.setStroke()
   
           path?.stroke()
           temporaryPath?.stroke()
       }
   
       override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
           if let touch = touches.first {
               points = [touch.location(in: self)]
           }
       }
   
       override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
           guard let touch = touches.first else { return }
           let point = touch.location(in: self)
   
           points.append(point)
   
           updatePaths()
   
           setNeedsDisplay()
       }
   
       private func updatePaths() {
           // update main path
   
           while points.count > 4 {
               points[3] = CGPoint(x: (points[2].x + points[4].x)/2.0, y: (points[2].y + points[4].y)/2.0)
   
               if path == nil {
                   path = createPathStarting(at: points[0])
               }
   
               path?.addCurve(to: points[3], controlPoint1: points[1], controlPoint2: points[2])
   
               points.removeFirst(3)
   
               temporaryPath = nil
           }
   
           // build temporary path up to last touch point
   
           if points.count == 2 {
               temporaryPath = createPathStarting(at: points[0])
               temporaryPath?.addLine(to: points[1])
           } else if points.count == 3 {
               temporaryPath = createPathStarting(at: points[0])
               temporaryPath?.addQuadCurve(to: points[2], controlPoint: points[1])
           } else if points.count == 4 {
               temporaryPath = createPathStarting(at: points[0])
               temporaryPath?.addCurve(to: points[3], controlPoint1: points[1], controlPoint2: points[2])
           }
       }
   
       override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
           finishPath()
       }
   
       override func touchesCancelled(_ touches: Set<UITouch>?, with event: UIEvent?) {
           finishPath()
       }
   
       private func finishPath() {
           constructIncrementalImage()
           path = nil
           setNeedsDisplay()
       }
   
       private func createPathStarting(at point: CGPoint) -> UIBezierPath {
           let localPath = UIBezierPath()
   
           localPath.move(to: point)
   
           localPath.lineWidth = lineWidth
           localPath.lineCapStyle = .round
           localPath.lineJoinStyle = .round
   
           return localPath
       }
   
       private func constructIncrementalImage() {
           UIGraphicsBeginImageContextWithOptions(bounds.size, false, 0.0)
           strokeColor.setStroke()
           snapshotImage?.draw(at: .zero)
           path?.stroke()
           temporaryPath?.stroke()
           snapshotImage = UIGraphicsGetImageFromCurrentImageContext()
           UIGraphicsEndImageContext()
       }
   }
   

   You could even marry this with iOS 9 predictive touches (as I described in my other answer), which could reduce lag even further.

2. To take this resulting image and use it elsewhere, you can just grab the incrementalImage (which I renamed to snapshotImage, above), and drop it into an image view of the other view.

For Swift 2 rendition, see previous revision of this answer.