Animating size from within SwiftUI

[nicoreese]

My biggest pain point of using SwiftUI within UIKit is animation. Example:
I can't animate the height of a SwiftUI view from within SwiftUI when it's in a UIHostingController or your HostingView that's constrained to other views or in a UIStackView. SwiftUI tries to animate the change, but UIKit abruptly updates the layout. This leads to a less than ideal animation in total.

I've attached a video and a sample project. I've seen that you already do a lot of stuff for layout in your code, so you might be able to crack this issue and make SwiftUI much nicer to use in UIKit for all of us!

Simulator.Screen.Recording.-.iPhone.16.Pro.Max.-.2024-09-14.at.12.22.03.mp4

Test33.zip

[Jaesung-Jung]

@nicoreese

SwiftUI and UIKit have different animation systems. The demo code you provided doesn’t work because UIKit cannot detect state changes that occur in SwiftUI.

Unfortunately, after various tests, I couldn’t find a way to synchronize animations between SwiftUI and UIKit. This is because SwiftUI renders views differently from UIKit.

In cases like the demo code you attached, it seems appropriate to manage the visible/hidden state with a UIStackView and implement the arranged subview items using SwiftUI. (There should be no actions in SwiftUI that internally change its own size based on its internal state.)

When using SwiftUI partially within UIKit, SwiftUI should perform rendering based only on state. In other words, interactions should occur in UIKit.