UIKit Animation Debugging

Systematic CAAnimation diagnosis covering completion handlers, spring physics, timing mismatches, and gesture conflicts. CAAnimation behavior differs significantly between simulator and device.

When to Use

Use this skill when:

• CAAnimation completion handler never fires
• Spring animation looks wrong on real device (but fine in simulator)
• Animation duration doesn't match specified value
• Animation stutters during pan gesture
• Multiple animations interfere with each other

Example Prompts

• "My CAAnimation completion handler never fires"
• "Spring animation looks perfect in simulator but bounces wrong on iPhone"
• "Animation duration is 0.3 but actually takes 0.5 seconds"
• "When I pan a gesture and trigger animation, it's janky"

What This Skill Provides

Pattern 1: Completion Handler That Never Fires

CABasicAnimation has no completion property — use CATransaction:

CATransaction.begin()
CATransaction.setCompletionBlock {
    print("Animation finished")  // This fires reliably
}
let animation = CABasicAnimation(keyPath: "position")
// ... configure animation
view.layer.add(animation, forKey: "move")
CATransaction.commit()

Pattern 2: Spring Physics Device Differences

Simulator runs at 60 FPS, iPhone Pro at 120 FPS (ProMotion). Spring physics need 1.5-2x higher damping/stiffness for ProMotion:

// Tuned for ProMotion
spring.damping = 15    // Not 10
spring.stiffness = 150 // Not 100

Pattern 3: Duration Mismatch

Don't mix UIView.animate and CAAnimation durations:

CATransaction.begin()
CATransaction.setAnimationDuration(0.3)  // Single source of truth
// ... add animations
CATransaction.commit()

Pattern 4: Gesture + Animation Jank

Use CADisplayLink with .common mode for gesture-driven animation:

displayLink.add(to: .main, forMode: .common)  // Runs during gesture

Debugging Tools

view.layer.animationKeys()           // List active animations
view.layer.presentation()?.position  // Get animated value (not final)
view.layer.speed = 0.1               // Slow down 10x for debugging

Related

• swiftui-debugging – SwiftUI animation issues
• performance-profiling – Animation performance

Resources

Docs: /quartzcore