Custom animation timing in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the visible effect of this animation code?

Consider this Swift code snippet for a UIView animation with a custom timing curve. What will the user see when this animation runs?

iOS Swift

UIView.animate(withDuration: 2.0, delay: 0, options: [.curveEaseInOut], animations: {
  view.alpha = 0
}, completion: nil)

A The view fades out instantly without any animation.

B The view fades out at a constant speed over 2 seconds.

C The view fades out quickly at first, then slows down towards the end.

D The view fades out slowly at first, speeds up in the middle, then slows down before disappearing.

💡 Hint

Think about what the .curveEaseInOut option means for animation speed over time.

The .curveEaseInOut option makes the animation start slowly, speed up in the middle, and slow down again at the end, creating a smooth fade out effect.

📝 Syntax

intermediate

Which option correctly creates a custom timing curve animation?

You want to animate a view's position with a custom cubic timing curve. Which code snippet correctly uses UIViewPropertyAnimator with a custom timing curve?

A

let timing = UICubicTimingParameters(controlPoint1: CGPoint(x: 0.1, y: 0.9), controlPoint2: CGPoint(x: 0.2, y: 0.8))
let animator = UIViewPropertyAnimator(duration: 1.0, timingParameters: timing)
animator.addAnimations {
  view.frame.origin.x += 100
}
animator.startAnimation()

B

let animator = UIViewPropertyAnimator(duration: 1.0, timingParameters: UICubicTimingParameters(controlPoint1: CGPoint(x: 0.0, y: 0.0), controlPoint2: CGPoint(x: 1.0, y: 1.0)))
animator.addAnimations {
  view.frame.origin.x += 100
}
animator.startAnimation()

C

UIView.animate(withDuration: 1.0, delay: 0, options: [.curveLinear], animations: {
  view.frame.origin.x += 100
}, completion: nil)

D

let animator = UIViewPropertyAnimator(duration: 1.0, curve: .easeInOut) {
  view.frame.origin.x += 100
}
animator.startAnimation()

💡 Hint

Look for the option that uses UICubicTimingParameters with control points.

Option A correctly creates a UICubicTimingParameters object with custom control points and uses it to initialize UIViewPropertyAnimator, then adds animations and starts it.

❓ lifecycle

advanced

What happens if you call startAnimation() twice on the same UIViewPropertyAnimator?

Given a UIViewPropertyAnimator instance, what is the effect of calling startAnimation() two times in a row without stopping or pausing?

iOS Swift

let animator = UIViewPropertyAnimator(duration: 1.0, curve: .easeInOut) {
  view.alpha = 0
}
animator.startAnimation()
animator.startAnimation()

A The second call has no effect because the animation is already running.

B The animation runs twice sequentially, fading out the view two times.

C The animation restarts from the beginning on the second call.

D The app crashes due to invalid animation state.

💡 Hint

Think about how UIViewPropertyAnimator manages its running state.

Calling startAnimation() on an already running animator does nothing; the animation continues without restarting or crashing.

❓ navigation

advanced

How to synchronize a custom animation with a navigation controller transition?

You want to add a custom timing animation to a view during a UINavigationController push transition. Which approach correctly synchronizes your animation with the navigation transition?

A Use UIView.animate inside viewWillAppear of the pushed view controller without any coordination.

B Start your animation in viewDidLoad of the pushed view controller with a fixed delay matching the navigation animation duration.

C Implement UIViewControllerAnimatedTransitioning and use the transitionContext to run your custom animation synchronized with the navigation transition.

D Add your animation in viewDidAppear of the pushed view controller after the navigation transition completes.

💡 Hint

Custom navigation animations require coordination with the transition system.

Implementing UIViewControllerAnimatedTransitioning and using the transitionContext allows your animation to run in sync with the navigation controller's transition.

🔧 Debug

expert

Why does this custom timing animation freeze halfway?

This code uses UIViewPropertyAnimator with a custom timing curve, but the animation freezes halfway and never completes. What is the most likely cause?

iOS Swift

let timing = UICubicTimingParameters(controlPoint1: CGPoint(x: 1.5, y: -0.5), controlPoint2: CGPoint(x: 0.5, y: 1.5))
let animator = UIViewPropertyAnimator(duration: 2.0, timingParameters: timing)
animator.addAnimations {
  view.alpha = 0
}
animator.startAnimation()

A The view's alpha property cannot be animated with a custom timing curve.

B The control points are outside the valid range, causing the animation to behave unpredictably and freeze.

C The animation duration is too long, causing it to appear frozen.

D The animator was not retained and got deallocated immediately.

💡 Hint

Check the valid range for cubic Bezier control points in timing curves.

Control points for UICubicTimingParameters must be within [0,1] range for x and y. Points outside this range can cause the animation to freeze or behave incorrectly.

Practice

1. What does the .curveEaseIn animation option do in UIView animations?

easy

A. Starts the animation quickly and then slows down.

B. Starts the animation slowly and then speeds up.

C. Keeps the animation speed constant throughout.

D. Reverses the animation direction automatically.

Solution

1. Step 1: Understand UIView animation curves

   UIView animation curves control how the animation speed changes over time.

2. Step 2: Identify the effect of .curveEaseIn

   .curveEaseIn means the animation starts slowly and accelerates towards the end.

3. Final Answer:

   Starts the animation slowly and then speeds up. -> Option B

4. Quick Check:

   .curveEaseIn = slow start, speed up [OK]

Hint: EaseIn means slow start, speed up later [OK]

Common Mistakes:

• Confusing .curveEaseIn with .curveEaseOut
• Thinking it keeps constant speed
• Assuming it reverses animation

2. Which of the following is the correct syntax to animate a view's alpha to 0.5 over 0.3 seconds with ease-out timing?

easy

A. UIView.animate(duration: 0.3, options: .easeOut) { view.alpha = 0.5 }

B. UIView.animate(withDuration: 0.3, options: .curveEaseIn) { view.alpha = 0.5 }

C. UIView.animate(withDuration: 0.3, delay: 0, options: .curveEaseOut) { view.alpha = 0.5 }

D. UIView.animate(withDuration: 0.3) { view.alpha = 0.5, options: .curveEaseOut }

Solution

1. Step 1: Recall UIView.animate syntax

   The correct method signature is UIView.animate(withDuration:delay:options:animations:completion:). Options must be passed as a parameter.

2. Step 2: Check each option for syntax correctness

   UIView.animate(withDuration: 0.3, delay: 0, options: .curveEaseOut) { view.alpha = 0.5 } uses correct method name, parameters, and .curveEaseOut option. Others have wrong method names or misplaced options.

3. Final Answer:

   UIView.animate(withDuration: 0.3, delay: 0, options: .curveEaseOut) { view.alpha = 0.5 } -> Option C

4. Quick Check:

   Correct UIView.animate syntax = UIView.animate(withDuration: 0.3, delay: 0, options: .curveEaseOut) { view.alpha = 0.5 } [OK]

Hint: Use withDuration and options parameters correctly [OK]

Common Mistakes:

• Using wrong method name like animate(duration:)
• Placing options inside animation block
• Forgetting delay parameter

3. What will be the visible effect of this code snippet?

UIView.animate(withDuration: 1.0, delay: 0, options: [.curveEaseInOut]) {
  view.frame.origin.x += 100
}

medium

A. The view moves 100 points to the right, starting fast and slowing down.

B. The view moves 100 points to the right at a constant speed.

C. The view jumps instantly 100 points to the right without animation.

D. The view moves 100 points to the right, starting slow, speeding up, then slowing down.

Solution

1. Step 1: Understand .curveEaseInOut timing

   .curveEaseInOut means the animation starts slowly, speeds up in the middle, then slows down at the end.

2. Step 2: Analyze the animation effect

   The view's x position increases by 100 points over 1 second with ease-in-out timing, so it moves smoothly with acceleration and deceleration.

3. Final Answer:

   The view moves 100 points to the right, starting slow, speeding up, then slowing down. -> Option D

4. Quick Check:

   .curveEaseInOut = slow start & end, fast middle [OK]

Hint: EaseInOut means slow start and end, fast middle [OK]

Common Mistakes:

• Thinking it moves at constant speed
• Confusing easeInOut with easeIn or easeOut
• Expecting instant jump without animation

4. Identify the error in this Core Animation timing code:

let animation = CABasicAnimation(keyPath: "opacity")
animation.duration = 1.0
animation.timingFunction = CAMediaTimingFunction(name: .easeIn)
animation.fromValue = 1.0
animation.toValue = 0.0
view.layer.add(animation, forKey: "fade")

medium

A. The animation is missing a fillMode to keep final state.

B. The timingFunction property expects a string, not CAMediaTimingFunction.

C. The keyPath "opacity" is invalid for CABasicAnimation.

D. The timing function name should be .easeInEaseOut, not .easeIn.

Solution

1. Step 1: Check timingFunction usage

   Using CAMediaTimingFunction(name: .easeIn) is correct and supported.

2. Step 2: Identify missing fillMode

   Without setting fillMode and isRemovedOnCompletion, the animation resets after finishing, so the view returns to original opacity.

3. Final Answer:

   The animation is missing a fillMode to keep final state. -> Option A

4. Quick Check:

   Missing fillMode causes animation to revert [OK]

Hint: Add fillMode and isRemovedOnCompletion to keep final state [OK]

Common Mistakes:

• Assuming .easeIn is invalid timing function name
• Thinking timingFunction needs a string
• Ignoring fillMode and completion behavior

5. You want to create a custom animation that moves a view along a path with a non-linear speed, speeding up then slowing down repeatedly. Which approach best achieves this?

hard

A. Use CAKeyframeAnimation with a custom timing function and path to control speed and position.

B. Use CABasicAnimation with a custom CAMediaTimingFunction using control points for the speed curve.

C. Use UIView.animate with linear curve and manually update position in a timer.

D. Use UIView.animate with .curveEaseInOut option repeatedly in a loop.

Solution

1. Step 1: Understand animation requirements

   The animation needs to follow a path and have complex speed changes (speeding up and slowing down repeatedly).

2. Step 2: Evaluate animation options

   CAKeyframeAnimation allows animating along a path and supports custom timing functions for complex speed control, making it ideal here.

3. Final Answer:

   Use CAKeyframeAnimation with a custom timing function and path to control speed and position. -> Option A

4. Quick Check:

   CAKeyframeAnimation + custom timing = best for complex path and speed [OK]

Hint: Use CAKeyframeAnimation for path + custom speed control [OK]

Common Mistakes:

• Using UIView.animate for complex path animations
• Assuming CABasicAnimation can follow a path
• Trying to manually update position with timers