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.