CAMetalLayer.nextDrawable takes much time, even more than 8ms - ios

Question

CAMetalLayer.nextDrawable() should not be a very time-comsuming method.But sometimes it often takes much time, even more than 8ms
Copy the code below
Follow the comment guide in viewDidLoad
See the log print
class TestVC: UIViewController {
let metalLayer:CAMetalLayer = CAMetalLayer()
var displayLink:CADisplayLink!
var thread:Thread!
var animationView:AnimationView!
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
self.displayLink.isPaused = true
}
override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
self.displayLink.isPaused = false
}
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
print("ahhhaa")
}
override func viewDidLoad() {
super.viewDidLoad()
self.view.backgroundColor = .red
let scale = UIScreen.main.scale
metalLayer.frame = self.view.bounds
metalLayer.drawableSize = CGSize(width: self.view.frame.width * scale, height: self.view.frame.height * scale)
self.view.layer.addSublayer(metalLayer)
self.addDisplayLinkInUITaskRunner()
// Wait screen loading complete,and next,You can:
// 1. Swipe down the control center, you will see a lot log print in console, and it will
// constantly exists.
// 2. Move the app to bg, and then move to fg, you will see a lot of log,
// it also will constantly exist for a long time.
// 3. Just don't do anything, you might see it the log print constantly....
// And when you tap the screen, it may disappear, and After a while, you may see log again....
}
func addDisplayLinkInUITaskRunner() {
self.thread = Thread(block: {
RunLoop.current.add(NSMachPort(), forMode: .common)
RunLoop.current.run()
})
self.thread.name = ""
self.thread.start()
self.perform(#selector(addDisplayLink), on: thread, with: nil, waitUntilDone: false)
}
#objc func addDisplayLink() {
self.displayLink = CADisplayLink(target: self, selector: #selector(onDisplayLink))
if #available(iOS 15.0, *) {
self.displayLink.preferredFrameRateRange = .init(minimum: 60, maximum: 120, preferred: 120)
} else {
self.displayLink.preferredFramesPerSecond = 120
}
self.displayLink.add(to: .current, forMode: .common)
}
#objc private func onDisplayLink() {
let startTime = CACurrentMediaTime()
let frameDrawable = metalLayer.nextDrawable()!
let timeUsed = CACurrentMediaTime() - startTime
// If time used to get next drawble over 3ms,
// we print it here to indicate this method take much time!
if (timeUsed > 0.005) {
print("CAMetalLayer.nextDrawable take much time!! -> \(String(format: "%.2f", timeUsed * 1000)) ms")
}
frameDrawable.present()
}
}

Answer 1

If you have displaySyncEnabled set to true (the default) it will wait for the next vsync to display the drawable. This means you can very quickly run out of drawables, and so nextDrawable will wait until one becomes available (or up to 1 second).
In other words, since you already have drawables queued to be presented, you can't get another one until one actually becomes available.
If you indeed want to present them as fast as possible - set displaySyncEabled to false. However, the current behavior may just be what you want, since you rarely gain anything from displaying frames faster than the display's refresh rate.