UIKit Animation Debugging

Overview

CAAnimation issues manifest as missing completion handlers, wrong timing, or jank under specific conditions. Core principle 90% of CAAnimation problems are CATransaction timing, layer state, or frame rate assumptions, not Core Animation bugs.

Red Flags — Suspect CAAnimation Issue

If you see ANY of these, suspect animation logic not device behavior:

• Completion handler fires on simulator but not device
• Animation duration (0.5s) doesn't match visual duration (1.2s)
• Spring animation looks correct on iPhone 15 Pro but janky on older devices
• Gesture + animation together causes stuttering (fine separately)
• [weak self] in completion handler and you're not sure why
• ❌ FORBIDDEN Hardcoding duration/values to "match what actually happens"
  • This ships device-specific bugs to users on different hardware
  • Do not rationalize this as a "temporary fix" or "good enough"

Critical distinction Simulator often hides timing issues (60Hz only, no throttling). Real devices expose them (variable frame rate, CPU throttling, background pressure). MANDATORY: Test on real device (oldest supported model) before shipping.

Mandatory First Steps

ALWAYS run these FIRST (before changing code):

// 1. Check if completion is firing at all
animation.completion = { [weak self] finished in
    print("🔥 COMPLETION FIRED: finished=\(finished)")
    guard let self = self else {
        print("🔥 SELF WAS NIL")
        return
    }
    // original code
}

// 2. Check actual duration vs declared
let startTime = Date()
let anim = CABasicAnimation(keyPath: "position.x")
anim.duration = 0.5  // Declared
layer.add(anim, forKey: "test")

DispatchQueue.main.asyncAfter(deadline: .now() + 0.51) {
    print("Elapsed: \(Date().timeIntervalSince(startTime))")  // Actual
}

// 3. Check what animations are active
if let keys = layer.animationKeys() {
    print("Active animations: \(keys)")
    for key in keys {
        if let anim = layer.animation(forKey: key) {
            print("\(key): duration=\(anim.duration), removed=\(anim.isRemovedOnCompletion)")
        }
    }
}

// 4. Check layer state
print("Layer speed: \(layer.speed)")  // != 1.0 means timing is scaled
print("Layer timeOffset: \(layer.timeOffset)")  // != 0 means animation is offset

What this tells you

• Completion print appears → Handler fires, issue is in callback code
• Completion print missing → Handler not firing, check CATransaction/layer state
• Elapsed time == declared → Duration is correct, visual jank is from frames
• Elapsed time != declared → CATransaction wrapping is changing duration
• layer.speed != 1.0 → Something is slowing animation
• Active animations list is long → Multiple animations competing

MANDATORY INTERPRETATION

Before changing ANY code, you must identify which ONE diagnostic is the root cause:

1. If completion fires but elapsed time != declared duration → Apply Pattern 2 (CATransaction)
2. If completion doesn't fire AND isRemovedOnCompletion is true → Apply Pattern 3
3. If completion fires but visual is janky → MUST profile with Instruments first
   • You cannot guess "it's probably frames" - prove it with data
   • Profile > Core Animation instrument shows frame drops with certainty
   • If you skip Instruments, you're guessing

If diagnostics are contradictory or unclear

• STOP. Do NOT proceed to patterns yet
• Add more print statements to narrow the cause
• Ask: "The diagnostics show X and Y but Z doesn't match. What am I missing?"
• Profile with Instruments > Core Animation if unsure

Decision Tree

CAAnimation problem?
├─ Completion handler never fires?
│  ├─ On simulator only?
│  │  └─ Simulator timing is different (60Hz). Test on real device.
│  ├─ On real device only?
│  │  ├─ Check: isRemovedOnCompletion and fillMode
│  │  ├─ Check: CATransaction wrapping
│  │  └─ Check: app goes to background during animation
│  └─ On both simulator and device?
│     ├─ Check: completion handler is set BEFORE adding animation
│     └─ Check: [weak self] is actually captured (not nil before completion)
│
├─ Duration mismatch (declared != visual)?
│  ├─ Is layer.speed != 1.0?
│  │  └─ Something scaled animation duration. Find and fix.
│  ├─ Is animation wrapped in CATransaction?
│  │  └─ CATransaction.setAnimationDuration() overrides animation.duration
│  └─ Is visual duration LONGER than declared?
│     └─ Simulator (60Hz) vs device frame rate (120Hz). Recalculate for real hardware.
│
├─ Spring physics wrong on device?
│  ├─ Are values hardcoded for one device?
│  │  └─ Use device performance class, not model
│  ├─ Are damping/stiffness values swapped with mass/stiffness?
│  │  └─ Check CASpringAnimation parameter meanings
│  └─ Does it work on simulator but not device?
│     └─ Simulator uses 60Hz. Device may use 120Hz. Recalculate.
│
└─ Gesture + animation jank?
   ├─ Are animations competing (same keyPath)?
   │  └─ Remove old animation before adding new
   ├─ Is gesture updating layer while animation runs?
   │  └─ Use CADisplayLink for synchronized updates
   └─ Is gesture blocking the main thread?
      └─ Profile with Instruments > Core Animation

Common Patterns

Pattern Selection Rules (MANDATORY)

Apply ONE pattern at a time, in this order

1. Always start with Pattern 1 (Completion Handler Basics)

   • If completion NEVER fires → Pattern 1
   • Verify completion is set BEFORE add() with print statement (line 33)
   • Only proceed to Pattern 2 if completion FIRES but timing is wrong

2. Then Pattern 2 (CATransaction duration mismatch)

   • Only if completion fires but elapsed time != declared duration
   • Check logs from Mandatory First Steps (line 40-47)

3. Then Pattern 3 (isRemovedOnCompletion)

   • Only if animation completes but visual state reverts

4. Patterns 4-7 Apply based on specific symptom (see Decision Tree line 91+)

FORBIDDEN

• ❌ Applying multiple patterns at once ("let me try Pattern 2 AND Pattern 4 together")
• ❌ Skipping Pattern 1 because "I already know it's not that"
• ❌ Combining patterns without understanding why each is needed
• ❌ Trying patterns randomly and hoping one works

Pattern 1: Completion Handler Basics

❌ WRONG (Handler set AFTER adding animation)

layer.add(animation, forKey: "myAnimation")
animation.completion = { finished in  // ❌ Too late!
    print("Done")
}

✅ CORRECT (Handler set BEFORE adding)

animation.completion = { [weak self] finished in
    print("🔥 Animation finished: \(finished)")
    guard let self = self else { return }
    self.doNextStep()
}
layer.add(animation, forKey: "myAnimation")

Why Completion handler must be set before animation is added to layer. Setting after does nothing.

Pattern 2: CATransaction vs animation.duration

❌ WRONG (CATransaction overrides animation duration)

CATransaction.begin()
CATransaction.setAnimationDuration(2.0)  // ❌ Overrides all animations!
let anim = CABasicAnimation(keyPath: "position")
anim.duration = 0.5  // This is ignored
layer.add(anim, forKey: nil)
CATransaction.commit()  // Animation takes 2.0 seconds, not 0.5

✅ CORRECT (Set duration on animation, not transaction)

let anim = CABasicAnimation(keyPath: "position")
anim.duration = 0.5
layer.add(anim, forKey: nil)
// No CATransaction wrapping

Why CATransaction.setAnimationDuration() affects ALL animations in the transaction block. Use it only if you want to change all animations uniformly.

Pattern 3: isRemovedOnCompletion & fillMode

❌ WRONG (Animation disappears after completion)

let anim = CABasicAnimation(keyPath: "opacity")
anim.fromValue = 1.0
anim.toValue = 0.0
anim.duration = 0.5
layer.add(anim, forKey: nil)