UIViewPropertyAnimator crashes on dealloc after calling finishAnimation(at:) - ios

Question

When using UIViewPropertyAnimator and calling finishAnimation(at:) after stopAnimation(false):
You either get a crash log of:
- [UIViewPropertyAnimator dealloc]
Or if you catch the assertion in a debugger (by setting an breakpoint for Objective-C exceptions)
Assertion failure in -[UIViewPropertyAnimator finishAnimationAtPosition:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKitCore_Sim/UIKit-3900.12.16/UIViewPropertyAnimator.m:1981
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'finishAnimationAtPosition: should only be called on a stopped animator!'
If you manage to catch the assertion, (the crash log isn't that helpful unless you catch those at release runtime as well), you will see that it complains about finishAnimation being called on a non-stopped animator.
Yet my code shows the animator always being stopped before finishAnimation is called.
animator.stopAnimation(false)
animator.finishAnimation(at: .start)
What could be causing this crash, the state should always be stopped after calling stopAnimation?

Answer 1

The reason that this happened for me was a timing issue with completion blocks:
It means that UIViewPropertyAnimator.state (UIViewAnimatingState) is in an invalid state. Since this is just an Objective C enum it can have any raw value beyond what is defined in the cases. In the case of my crash the value was 5.
I was doing a bunch of chained animations that referenced each other and had something like this:
animator.addCompletion({ _ in
// Here I called a piece of code that eventually referenced this animator
// and attempted to call `stopAnimation(false)` and `finishAnimation(at:)`
// on it causing the crash.
//
// You will notice if you examine the `state` of the animator here, it will have
// an invalid `rawValue` of 5, at least up to iOS 13.2, as this is an implementation detail.
})
The solution is to not attempt any operations on the animator until the completion block is finished executing.
If you can not avoid it, a workaround that works with current UIKit implementations is to DispatchQueue.main.async out of that completion block which will allow it to finish. Of course, this is not a guarantee that the state will be valid inside of that async in future versions of iOS.
On Swift, to debug this you can use the following extension:
extension UIViewAnimatingState: CustomStringConvertible, CustomDebugStringConvertible {
var isValid: Bool {
switch self {
case .inactive, .active, .stopped:
return true
default:
return false
}
}
public var description: String {
switch self {
case .inactive:
return "inactive"
case .active:
return "active"
case .stopped:
return "stopped"
default:
return "\(rawValue)"
}
}
public var debugDescription: String {
return description
}
}
Therefore when you call stopAnimation(false) and finishAnimation(at:) you can at least avoid a crash and alert yourself to a possible logical issue with your animators:
// This is for debugging, the code below should be safe even if isValid is false
assert(animator.state.isValid, "animator state is not valid")
animator.stopAnimation(false)
if animator.state == .stopped {
// This call is the one that can assert and crash the app
animator.finishAnimation(at: .start)
}