[UIView animateWithDuration:animations:completion:] is great for simple animations. But when we need to build a sequence of animations, chaining together completion is verbose and difficult to adjust.

The keyframe animation methods provide a better alternative when building multi-part animations. Let's use them to make a "bounce" effect.

This is your goal:

You'll need UIKeyFrameAnimations

Note how Bon Jovi "bounces" when the button is tapped -- the image first shrinks, then expands outward (slightly past its final size), then comes to a rest at a height that fills the screen. When the image shrinks, it goes through the same process in reverse. This sort of multi-step animation is perfect for the keyframe animation methods in UIKit.

Implement this animation by adjusting constraints on the image view. Check out the storyboard -- there are currently no constraints controlling the height and y-position of the image view.

So, your process should be something like this:

1. Add constraints that will let you easily animate the image as desired. Think about your options -- you could pin down the image view's height and top, but would that be easy to animate in the desired way?

   Hint: does the center-y of the image view change across animations? Maybe you could pin it down, and just animate between two height constraints.

2. Make Bon Jovi grow in the expandButtonTapped: method (we'll handle the toggling later). You'll need to build a keyframe animation that implements the "bounce" effect. It will have steps close to this:

   • Shrink slightly below the starting height
   • Grow slightly beyond the height of the screen
   • Shrink back down to the exact height of the screen.

   The animation should work on phones of all sizes and orientations. Don't hard-code the screen's height!

3. Make the expandButtonTapped: method toggle between the full-height and normal sizes of the image view. You'll probably have to add a flag property so you know whether to expand or shrink. The shrink animation should be the exact opposite of the expansion one:

   • Grow slightly beyond the height of the screen
   • Shrink down to slightly below the final height of the image
   • Return to the exact starting height

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