Open Radar

 
Summary:
The UIViewPropertyAnimator does not update the view when `fractionComplete` is set while it's in an `inactive` state, or even in `active` state.

Steps to Reproduce:
Run this Swift Playground code.

let liveView = UIView(frame: CGRect(x: 0, y: 0, width: 400, height: 50))
liveView.backgroundColor = .white

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = liveView

let square = UIView(frame: CGRect(x: 0, y: 0, width: 50, height: 50))
square.backgroundColor = .red

liveView.addSubview(square)

let animator = UIViewPropertyAnimator.init(duration: 5, curve: .easeIn)

animator.addAnimations {

    square.frame.origin.x = 350
}

// State: inactive
// Does nothing, position is at 0.0
DispatchQueue.main.async {

    animator.fractionComplete = 0.5
}

// State: active
// Stil does nothing, position is at 0.0
DispatchQueue.main.async {

    animator.fractionComplete = 0.5
}

// Updates from 0.0 to 0.75 - Why now?
DispatchQueue.main.asyncAfter(deadline: .now() + 5) { 

    animator.fractionComplete = 0.75
}

Expected Results:
The view is expected to change its position on each call of `fractionComplete`.

Actual Results:
The animator does nothing for the first two calls, except changing the state to from `inactive` to `active`. The view changes it's position after the third call of `fractionComplete` after 5 seconds.

CURRENT WORKAROUND:
Put these two lines before the first call:

animator.startAnimation()
animator.pauseAnimation()


Configuration:
The issue was testen in Swift Playground, in iOS simulator and on and iPhone 6 Plus and had always the same results