iOS27: CADisplayLink for UIWindowScene

CADisplayLink Beta updates

Scheduled timers often come in handy. They are useful when you need to show a countdown, refresh some value every second, or make a periodic poll request.

But CADisplayLink has always stood on higher ground.

That is because Timer is time-based, while CADisplayLink is frame-based. A timer fires according to a run loop schedule. A display link fires in sync with the display refresh cycle. When the work you are doing is visual, that difference matters a lot.

During years I’ve faced CADisplayLink only a few times and it’s time to learn about it!

Another useful way to frame it is in Hz. A standard 60 Hz display can present up to 60 frames per second, while higher-refresh displays can go beyond that. CADisplayLink follows that rendering cadence, which is exactly why it feels more natural for animation and frame-driven UI work than a regular timer.

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What It Is

CADisplayLink is a QuartzCore object that lets your code run in step with screen updates.

That makes it a natural fit for:

• smooth custom animations

• progress that needs to track frames precisely

• rendering loops

• physics-like visual updates

• UI that should advance only when a new frame is about to be displayed

This is the key mental model: CADisplayLink is not just “a timer that fires fast.” It is display-synchronized work.

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Why It Differs From Timer

A Timer is about time intervals.

A CADisplayLink is about frame boundaries.

That one difference changes how they behave in practice.

Timer

Use Timer when you care about elapsed time more than visual smoothness.

Examples:

• a countdown label

• a polling request every few seconds

• refreshing cached data periodically

• lightweight background scheduling tied to a run loop

CADisplayLink

Use CADisplayLink when your update is visually tied to the screen.

Examples:

• moving or redrawing something every frame

• tracking animation progress manually

• synchronizing custom rendering with the display

• driving a game-like loop in UIKit

If you try to drive visual motion with a regular timer, it can work, but it is usually less precise and less naturally aligned with what the display is doing.

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Basic Setup

A common setup looks like this:

import QuartzCore
import UIKit

final class DisplayLinkViewController: UIViewController {
    private var displayLink: CADisplayLink?

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        startDisplayLink()
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        stopDisplayLink()
    }

    private func startDisplayLink() {
        let link = CADisplayLink(target: self, selector: #selector(step))
        link.add(to: .main, forMode: .common)
        displayLink = link
    }

    private func stopDisplayLink() {
        displayLink?.invalidate()
        displayLink = nil
    }

    @objc private func step(_ link: CADisplayLink) {
        print("Frame timestamp: \(link.timestamp)")
    }
}

This already shows one important difference from Timer: you usually treat display link lifetime much more carefully, because it is easy to keep doing work every frame when you no longer need to.

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SwiftUI Example

Even in SwiftUI, CADisplayLink can still make sense when you need frame-driven updates rather than interval-based ones.

One practical way is to keep the display link inside a small helper object and push the visible value back into SwiftUI state.

import SwiftUI
import QuartzCore

final class DisplayLinkDriver {
    var onFrame: ((CFTimeInterval) -> Void)?
    private var displayLink: CADisplayLink?

    func start() {
        guard displayLink == nil else { return }

        let link = CADisplayLink(target: self, selector: #selector(step))
        link.add(to: .main, forMode: .common)
        displayLink = link
    }

    func stop() {
        displayLink?.invalidate()
        displayLink = nil
    }

    @objc private func step(_ link: CADisplayLink) {
        onFrame?(link.targetTimestamp)
    }

    deinit {
        stop()
    }
}

struct DisplayLinkDemoView: View {
    @State private var progress: CGFloat = 0
    @State private var direction: CGFloat = 1
    private let driver = DisplayLinkDriver()

    var body: some View {
        VStack(spacing: 24) {
            Text("Frame-driven progress")

            Capsule()
                .fill(.gray.opacity(0.2))
                .frame(height: 12)
                .overlay(alignment: .leading) {
                    Capsule()
                        .fill(.blue)
                        .frame(width: max(12, progress * 240), height: 12)
                }
                .frame(width: 240)
        }
        .onAppear {
            driver.onFrame = { _ in
                let next = progress + (0.01 * direction)

                if next >= 1 {
                    progress = 1
                    direction = -1
                } else if next <= 0 {
                    progress = 0
                    direction = 1
                } else {
                    progress = next
                }
            }

            driver.start()
        }
        .onDisappear {
            driver.stop()
        }
    }
}

The idea is the same as in UIKit: use CADisplayLink when the update should move with frames, not just with elapsed time.

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Important Values

A few CADisplayLink properties are especially useful.

timestamp

This is the timestamp of the last displayed frame.

targetTimestamp

This gives you the timestamp for the next frame. It is often more useful when calculating frame progress.

duration

This is the interval between screen refreshes as reported by the link.

preferredFrameRateRange

This gives you more control over the cadence you want, instead of always blindly running at the highest possible rate.

That matters more now, because “every frame” is not always the same thing across devices and refresh rates. On a 60 Hz display, one frame is typically about 16.67 ms. On a 120 Hz display, it is about 8.33 ms. That is a meaningful difference when you are calculating motion, progress, or per-frame work.

Refresh based on Hz

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Common Caveats

CADisplayLink is powerful, but it is also easy to misuse.

1. It Is Not A General Replacement For Timer

If you only need to refresh a countdown every second, Timer is usually the simpler and cheaper tool.

Using CADisplayLink for that would mean waking up every frame just to update something that only changes once a second.

2. It Can Waste Work Fast

A display link can fire very often. If your callback is heavy, you will feel it quickly in dropped frames, CPU usage, or battery cost.

3. You Must Manage Lifetime

Invalidate it when you are done.

Forgetting to stop a display link is one of the easiest ways to keep a controller alive longer than expected or keep doing frame work off-screen.

4. Run Loop Mode Matters

Adding it with .common is often the practical choice in UI code, because default run loop mode alone may not behave the way you expect during interactions.

5. It Is Best For Visual Work

This is the big one.

CADisplayLink shines when your logic is tied to drawing or visible animation. For networking, clocks, or ordinary periodic tasks, Timer is often still the better fit.

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A Good Comparison Example

Here is a simple rule of thumb.

If you are building:

• a countdown to an event → use Timer

• a polling request every 30 seconds → use Timer

• a custom animation progress loop → use CADisplayLink

• a frame-by-frame canvas update → use CADisplayLink

That keeps the tools aligned with the kind of work they were meant to do.

Timer vs CADisplayLink

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The New iOS 27 Direction

The interesting part in iOS 27 is that Apple is pushing CADisplayLink in a more scene-aware direction.

It is also worth calling out that iOS 27 is still in beta, so the exact API shape and naming may still change before final release.

That is a meaningful shift.

Until now, display link setup was usually thought about in terms of a screen or a run loop, but not as explicitly in terms of a specific window scene. With the new API direction, you can now create it in a way that is linked to a UIWindowScene.

import UIKit

final class DemoViewController: UIViewController {
    private var displayLink: CADisplayLink?

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        if let scene = view.window?.windowScene {
            //iOS 27 Beta
            displayLink = scene.displayLink { link in
                print(link.targetTimestamp - link.timestamp)
            }
            displayLink?.add(to: .main, forMode: .common)
        }
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        displayLink?.invalidate()
        displayLink = nil
    }
}

That is a better fit for modern UIKit apps, especially on iPad and other environments where scenes matter more.

Why does that matter?

Because visual work is often not just “device-wide.” It belongs to a particular window, a particular scene, and a particular visible UI context. As UIKit becomes more scene-oriented, display-driven work benefits from becoming scene-aware too.

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Why Scene Linking Makes Sense

A scene-linked display link fits modern app architecture better in a few ways.

Better Ownership

If rendering or animation belongs to one window scene, it makes sense for the display link to belong there too.

Better Multiwindow Behavior

On iPad or multiwindow environments, scene ownership is much clearer than treating display updates as one global concern.

Better Mental Model

This is probably the biggest improvement.

Instead of thinking:

“Here is a display link somewhere in the app.”

you can think:

“Here is display-synchronized work attached to this scene.”

That is simply easier to reason about.

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Practical Takeaway

You still should not reach for CADisplayLink automatically.

Use it when:

• the work is visual

• smoothness matters

• frame alignment matters

• the update naturally belongs to visible rendering

Use Timer when:

• the work is interval-based

• exact frame boundaries do not matter

• you are scheduling ordinary periodic tasks

And if you are targeting iOS 27 and newer, the new scene-linked creation path is worth paying attention to, because it makes display-driven code feel much more aligned with UIKit’s scene model.

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Final Thoughts

Timers are useful because they are simple and practical.

But CADisplayLink has always been the more specialized tool for UI work that needs to move with the display itself. That is why it has always stood on higher ground.

The new scene-aware direction in iOS 27 makes that story even stronger. It suggests Apple wants frame-driven work to be modeled closer to where it actually belongs: not just on a run loop, but inside the lifetime and ownership of a scene.

That is a small API direction change, but a meaningful architectural one.

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References

• Apple Developer Documentation: CADisplayLink

• Apple Developer Documentation: UIWindowScene.displayLink(action:)