Hello,

Although it is clear that in we need to write an animatableData variable for SwiftUI to be able to interpolate between 2 different states in order to do a smooth animation, it is not clear to me why sometimes the animation can occur without doing this..?

My guess is that when you create a custom struct you need to include an animatableData variable, but when you use a built-in struct you do not (i.e. you do not have to write an extension anywhere).

I am pretty new and not very good at crawling through documentation, so if you have any resource tips via the apple developer site or through XCode that would be appricate!

Thanks!

You are basically correct. When you use a struct that is already included in SwiftUI, such as RoundedRectangle, you do not need to add an animatableData property to the struct, because it already exists in that struct's definition.

However, when you define your own struct, you do need to include animatableData as a property if you want your struct to be animatable. (That is, if you want your struct to conform to the Animatable protocol.)

If you look at this documentation page for the RoundedRectangle struct, and scroll down to the "Instance Properties" section, you will see that it does have a property named animatableData.

https://developer.apple.com/documentation/swiftui/roundedrectangle

Also, if you look at this page and scroll down to the "Conforming Types" section, you will see all of the types included in SwiftUI that conform to the Animatable protocol. All of those types would have a property named animatableData included in them.

https://developer.apple.com/documentation/swiftui/animatable

Thank you for that clarification!