Auto layout completes animation in much less than the time provided - ios

Question

I want to animate a container width in 10 seconds (for example). I have two views and I assign size constraints to both of them, and call the empty init() method. The problem I'm having - the "animation" is over before you know it, even though I have 10 seconds in for the duration. What gives?
This is a reproducible example of the issue.
class ContainerView: UIView {}
class ChildView: UIView {}
class ViewController: UIViewController {
var containerView: ContainerView!
var childView: ChildView!
var widthConstraint: NSLayoutConstraint!
override func viewDidLoad() {
super.viewDidLoad()
containerView = ContainerView()
containerView.backgroundColor = UIColor.blueColor()
childView = ChildView()
childView.backgroundColor = UIColor.redColor()
containerView.translatesAutoresizingMaskIntoConstraints = false
childView.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(childView)
view.addSubview(containerView)
// Constraints
let containerWidthNumber: CGFloat = 200
let childWidthNumber: CGFloat = 100
let containerWidth = NSLayoutConstraint(item: containerView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: containerWidthNumber)
let containerHeight = NSLayoutConstraint(item: containerView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: containerWidthNumber)
containerView.addConstraints([
containerWidth,
containerHeight,
NSLayoutConstraint(item: childView, attribute: .CenterX, relatedBy: .Equal, toItem: containerView, attribute: .CenterX, multiplier: 1, constant: 0),
NSLayoutConstraint(item: childView, attribute: .Top, relatedBy: .Equal, toItem: containerView, attribute: .Top, multiplier: 1, constant: 0)
])
widthConstraint = NSLayoutConstraint(item: childView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: childWidthNumber)
let childHeight = NSLayoutConstraint(item: childView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: childWidthNumber)
childView.addConstraints([
widthConstraint,
childHeight,
])
view.addConstraints([
NSLayoutConstraint(item: containerView, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1, constant: 0),
NSLayoutConstraint(item: containerView, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1, constant: 0),
])
}
override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
super.touchesBegan(touches, withEvent: event)
self.widthConstraint.constant = 50
UIView.animateWithDuration(10, animations: {
self.childView.layoutIfNeeded()
}, completion: { (success: Bool) in
if success {
print("good")
}
})
}
}