Swift - Poor gradient quality in UIView size animation - ios

Question

I have the following structure in place:
main view contains subview1; subview1 is laid out using constraints
subview1 contains subview2; subview2 also has constraints and I need to animate the size of subview2
in subview2 I call CGContextDrawRadialGradient to draw a circle with a gradient
The code in subview1 is:
let circle = GradientCircle(frame: CGRect(origin: CGPointZero, size: frame.size), withColor: color)
addSubview(circle)
circle.centerXAnchor.constraintEqualToAnchor(centerXAnchor).active = true
circle.centerYAnchor.constraintEqualToAnchor(centerYAnchor).active = true
let widthConstraint = circle.widthAnchor.constraintEqualToConstant(frame.width * 3)
let heightConstraint = circle.heightAnchor.constraintEqualToConstant(frame.width * 3)
circle.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activateConstraints([heightConstraint, widthConstraint])
layoutIfNeeded()
UIView.animateWithDuration(3.0, delay: 0, options: [.Repeat], animations: {
widthConstraint.constant = 2
heightConstraint.constant = 2
self.layoutIfNeeded()
}, completion: nil)
Here's a gif to show what the problem is: http://i.stack.imgur.com/LWP7d.gif
Why does this drop in quality occur and how can I fix it? Obviously, I'm new in Swift and I don't fully understand the whole layout and animation thing, so please help if you can, it's slowly driving me insane :)
EDIT:
Here's the GradientCircle class (which is subview2 in the description above), maybe the problem is with how the gradient is drawn:
class GradientCircle : UIView {
var color: UIColor
init(frame: CGRect, withColor: UIColor) {
color = withColor
super.init(frame: frame)
backgroundColor = UIColor.clearColor()
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func drawRect(rect: CGRect) {
let context = UIGraphicsGetCurrentContext()
let locations: [CGFloat] = [0.0, 1.0]
let colorspace = CGColorSpaceCreateDeviceRGB()
let colors = [color.CGColor, GraphicsUtils.darkGrayColor().CGColor]
let gradient = CGGradientCreateWithColors(colorspace, colors, locations)
let startPoint = CGPoint(x: rect.width / 2, y: rect.height / 2)
let endPoint = CGPoint(x: rect.width / 2, y: rect.height / 2)
let startRadius: CGFloat = 0
let endRadius: CGFloat = rect.width / 2
CGContextClearRect(context, rect)
CGContextDrawRadialGradient(context, gradient, startPoint, startRadius, endPoint, endRadius, CGGradientDrawingOptions.DrawsAfterEndLocation)
}