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How to synchronize code to drawing using CADisplayLink

Swift version: 5.2

Paul Hudson    @twostraws   

Lots of beginners think Timer is a great way to handle running apps or games so that update code is executed every time the screen is redrawn. Their logic is simple: update the app every 60th of a second and you're perfectly placed for smooth redraws.

The problem is, they are forgetting that Timer doesn't offer precise firing and can drift earlier or later than requested updates, and also has no idea about screen redraws and so could happily fire 10ms after a screen redraw just happened – and when you're working to 16.666ms frames, 10ms is a long time!

A smarter and faster solution is the CADisplayLink class, which automatically calls a method you define as soon as a screen redraw happens, so you always have maximum time to execute your update code. It's extremely simple to use – here's an example to get you started:

let displayLink = CADisplayLink(target: self, selector: #selector(update))
displayLink.add(to: .current, forMode: .common)

That will call a method called update() every 60th of a second by default. You can see it in action with this method stub:

@objc func update() {
    print("Updating!")
}
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