Removing lagging latency in drawing UIBezierPath s

Question

The code below draws smooth curved lines by overriding touches, but there is noticeable lagging or latency. The code uses addCurveToPoint and calls setNeedsDisplay after every 4 touch points which causes a jumpy appearance as the drawing doesn't keep up with finger movements. To remove the lagging or perceived latency, touch points 1, 2, 3 (leading up to touch point 4) could be temporarily filled with addQuadCurveToPoint and addLineToPoint.

1. How can this actually be achieved in code to remove perceived lagging by using a temporary Line and QuadCurved line before displaying a final Curved line?

2. If the below class is attached to one UIView (e.g. viewOne or self), how do I make a copy of the drawing to another UIView outside the class (e.g. viewTwo) after touchesEnded?

    //  ViewController.swift
   
   import UIKit
   
   class drawSmoothCurvedLinesWithLagging: UIView {
   
       let path=UIBezierPath()
       var incrementalImage:UIImage?
   
       var points = [CGPoint?](count: 5, repeatedValue: nil)
   
       var counter:Int?
   
       var strokeColor:UIColor?
   
       required init?(coder aDecoder: NSCoder) {
           super.init(coder: aDecoder)
       }
   
       override func drawRect(rect: CGRect) {
           autoreleasepool {
               incrementalImage?.drawInRect(rect)
               strokeColor = UIColor.blueColor()
               strokeColor?.setStroke()
               path.lineWidth = 20
               path.lineCapStyle = CGLineCap.Round
               path.stroke()
           }
       }
   
       override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
           counter = 0
   
           let touch: AnyObject? = touches.first
           points[0] = touch!.locationInView(self)
       }
   
       override func touchesMoved(touches: Set<UITouch>, withEvent event: UIEvent?) {
           let touch: AnyObject? = touches.first
           let point = touch!.locationInView(self)
   
           counter = counter! + 1
           points[counter!] = point
   
   
           if counter == 2{
               //use path.addLineToPoint ?
               //use self.setNeedsDisplay() ?
           }
   
           if counter == 3{
               //use path.addQuadCurveToPoint ?
               //use self.setNeedsDisplay() ?
           }
   
           if counter == 4{
               points[3]! = CGPointMake((points[2]!.x + points[4]!.x)/2.0, (points[2]!.y + points[4]!.y)/2.0)
               path.moveToPoint(points[0]!)
               path.addCurveToPoint(points[3]!, controlPoint1: points[1]!, controlPoint2: points[2]!)
   
               self.setNeedsDisplay()
   
               points[0]! = points[3]!
               points[1]! = points[4]!
               counter = 1
           }
       }
   
       override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
           self.drawBitmap()
           self.setNeedsDisplay()
           path.removeAllPoints()
           counter = 0
       }
   
       override func touchesCancelled(touches: Set<UITouch>?, withEvent event: UIEvent?) {
           self.touchesEnded(touches!, withEvent: event)
       }
   
       func drawBitmap(){
           UIGraphicsBeginImageContextWithOptions(self.bounds.size, true, 0.0)
           strokeColor?.setStroke()
           if((incrementalImage) == nil){
               let rectPath:UIBezierPath = UIBezierPath(rect: self.bounds)
               UIColor.whiteColor().setFill()
               rectPath.fill()
           }
   
           incrementalImage?.drawAtPoint(CGPointZero)
           path.stroke()
           incrementalImage = UIGraphicsGetImageFromCurrentImageContext()
           UIGraphicsEndImageContext()
       }
   
   }
   
   class ViewController: UIViewController {
   
       override func viewDidLoad() {
           super.viewDidLoad()
           // Do any additional setup after loading the view, typically from a nib.
       }
   
       override func didReceiveMemoryWarning() {
           super.didReceiveMemoryWarning()
           // Dispose of any resources that can be recreated.
       }
   
   
   }
   

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.