In Prototyping iOS Animations in Swift I introduced the a range of block-based functions that UIKit provides to create tweened animations and how a simple animation can be programmatically altered with some random variation to create more complex scenes.
This alone can create an interesting range of animations, but is still only the tip of the iceberg of what Apple provides as tools for creating animations.
This tutorial looks at some more animation functions that require a little bit more setting up to create, but once mastered opens up an even larger number of possibilities.