Animate drawing of a circle

The easiest way to do this is to use the power of core animation to do most of the work for you. To do that, we’ll have to move your circle drawing code from your drawRect function to a CAShapeLayer. Then, we can use a CABasicAnimation to animate CAShapeLayer‘s strokeEnd property from 0.0 to 1.0. strokeEnd is a big part of the magic here; from the docs:

Combined with the strokeStart property, this property defines the
subregion of the path to stroke. The value in this property indicates
the relative point along the path at which to finish stroking while
the strokeStart property defines the starting point. A value of 0.0
represents the beginning of the path while a value of 1.0 represents
the end of the path. Values in between are interpreted linearly along
the path length.

If we set strokeEnd to 0.0, it won’t draw anything. If we set it to 1.0, it’ll draw a full circle. If we set it to 0.5, it’ll draw a half circle. etc.

So, to start, lets create a CAShapeLayer in your CircleView‘s init function and add that layer to the view’s sublayers (also be sure to remove the drawRect function since the layer will be drawing the circle now):

let circleLayer: CAShapeLayer!

override init(frame: CGRect) {
    super.init(frame: frame)
    self.backgroundColor = UIColor.clearColor()
    
    // Use UIBezierPath as an easy way to create the CGPath for the layer.
    // The path should be the entire circle.
    let circlePath = UIBezierPath(arcCenter: CGPoint(x: frame.size.width / 2.0, y: frame.size.height / 2.0), radius: (frame.size.width - 10)/2, startAngle: 0.0, endAngle: CGFloat(Double.pi * 2.0), clockwise: true)
    
    // Setup the CAShapeLayer with the path, colors, and line width
    circleLayer = CAShapeLayer()
    circleLayer.path = circlePath.CGPath
    circleLayer.fillColor = UIColor.clearColor().CGColor
    circleLayer.strokeColor = UIColor.redColor().CGColor
    circleLayer.lineWidth = 5.0;
    
    // Don't draw the circle initially
    circleLayer.strokeEnd = 0.0
    
    // Add the circleLayer to the view's layer's sublayers
    layer.addSublayer(circleLayer)
}

Note: We’re setting circleLayer.strokeEnd = 0.0 so that the circle isn’t drawn right away.

Now, lets add a function that we can call to trigger the circle animation:

func animateCircle(duration: NSTimeInterval) {
    // We want to animate the strokeEnd property of the circleLayer
    let animation = CABasicAnimation(keyPath: #keyPath(CAShapeLayer.strokeEnd))

    // Set the animation duration appropriately
    animation.duration = duration

    // Animate from 0 (no circle) to 1 (full circle)
    animation.fromValue = 0
    animation.toValue = 1

    // Do a linear animation (i.e. the speed of the animation stays the same)
    animation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.linear)

    // Set the circleLayer's strokeEnd property to 1.0 now so that it's the
    // right value when the animation ends.
    circleLayer.strokeEnd = 1.0

    // Do the actual animation
    circleLayer.add(animation, forKey: "animateCircle")
}

Then, all we need to do is change your addCircleView function so that it triggers the animation when you add the CircleView to its superview:

func addCircleView() {
    let diceRoll = CGFloat(Int(arc4random_uniform(7))*50)
     var circleWidth = CGFloat(200)
     var circleHeight = circleWidth

        // Create a new CircleView
     var circleView = CircleView(frame: CGRectMake(diceRoll, 0, circleWidth, circleHeight))

     view.addSubview(circleView)

     // Animate the drawing of the circle over the course of 1 second
     circleView.animateCircle(1.0)
}

All that put together should look something like this:

Note: It won’t repeat like that, it’ll stay a full circle after it animates.