setNeedsDisplay grows linear with drawRect:, rdar://31113056

setNeedsDisplayRadar.playground

import UIKit
import PlaygroundSupport

// This playground compares the time spend in `setNeedsDisplay` to the time spend in `draw(rect:)`
// From the documentation, `setNeedsDisplay` should return in constant time but as you see here
// it grows linear with the time needed in `draw(rect)`.
// to compare for yourself, observe the variables `timeInSetNeedsDisplay` and `timeInDrawRect` and start drawing points by panning in
// the live view

final class PointView: UIView {
	private var dataPoints: [CGPoint] = []
	
	override init(frame: CGRect) {
		super.init(frame: frame)
		dataPoints.reserveCapacity(1000)
		isOpaque = false
		self.backgroundColor = UIColor.clear
		contentMode = .redraw
	}
	
	required init(coder: NSCoder) {
		fatalError("NSCoding not supported")
	}
	
	public func addDataPoint(_ point: CGPoint){
		dataPoints.append(point)
		let s = CFAbsoluteTimeGetCurrent()
		self.setNeedsDisplay()
		
		// Time spend in `setNeedsDisplay`
		let timeInSetNeedsDisplay = CFAbsoluteTimeGetCurrent() - s
	}
	
	override func draw(_ rect: CGRect) {
		let time = CFAbsoluteTimeGetCurrent()
		drawPoints(rect)
		
		// Time spend in `draw(rect:)`
		let timeInDrawRect = CFAbsoluteTimeGetCurrent() - time
	}
	
	private func drawPoints(_ rect: CGRect){
		let context = UIGraphicsGetCurrentContext()!
		context.saveGState()
		
		let circle = UIBezierPath(ovalIn: CGRect(origin: .zero, size: CGSize(width: 3, height: 3)))
		circle.lineWidth = 0
		UIColor.red.setFill()
		
		for point in dataPoints {
			context.saveGState()
			context.translateBy(x: point.x, y: point.y)
			circle.fill()
			context.restoreGState()
		}
		
		context.restoreGState()
	}
}

final class GraphViewController: UIViewController {
	var pointView: PointView!
	
	override func viewDidLoad() {
		super.viewDidLoad()
		view.backgroundColor = UIColor.white
		pointView = PointView(frame: self.view.bounds)
		pointView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
		view.addSubview(pointView)
		
		let pan = UIPanGestureRecognizer(target: self, action: #selector(gestureRecognizerDidFire(gestureRecognizer:)))
		pointView.addGestureRecognizer(pan)
	}
	
	@objc private func gestureRecognizerDidFire(gestureRecognizer: UIPanGestureRecognizer){
		switch(gestureRecognizer.state){
		case .began, .changed:
			pointView.addDataPoint(gestureRecognizer.location(in: pointView))
		default: break
			
		}
	}
}

PlaygroundPage.current.liveView = GraphViewController()