CircleView.swift

class CircleView: UIView {
    
    override func drawRect(rect: CGRect) {
        // Draw this:
        // https://twitter.com/baarde/status/696716466037657600
        UIColor.blackColor().setFill()
        UIRectFill(rect)
        
        let center = CGPoint(x: bounds.midX, y: bounds.midY)
        let radius = CGFloat(120)
        let lineWidth = CGFloat(32)
        let startAngle = CGFloat(-M_PI_2)
        let endAngle = CGFloat(-M_PI_2 / 3)
        let clockwise = false
        
        // Blue path (start → end)
        UIColor(red: 0x35 / 255.0, green: 0xC5 / 255.0, blue: 0xD8 / 255.0, alpha: 1.0).setFill()
        UIBezierPath(roundedArcCenter: center, radius: radius, startAngle: startAngle, endAngle: endAngle, clockwise: clockwise, lineWidth: lineWidth).fill()
        
        // Blue path (end → start)
        UIColor(red: 0x71 / 255.0, green: 0x2D / 255.0, blue: 0xF6 / 255.0, alpha: 1.0).setFill()
        UIBezierPath(roundedArcCenter: center, radius: radius, startAngle: endAngle, endAngle: startAngle, clockwise: clockwise, lineWidth: lineWidth).fill()
    }
}

private extension UIBezierPath {
    
    convenience init(roundedArcCenter center: CGPoint, radius: CGFloat, startAngle: CGFloat, endAngle: CGFloat, clockwise: Bool, lineWidth: CGFloat) {
        self.init()
        let startPoint = CGPoint(x: center.x + cos(startAngle) * radius, y: center.y + sin(startAngle) * radius)
        let endPoint = CGPoint(x: center.x + cos(endAngle) * radius, y: center.y + sin(endAngle) * radius)
        // Outer path
        addArcWithCenter(center, radius: radius + lineWidth / 2, startAngle: startAngle, endAngle: endAngle, clockwise: clockwise)
        // Rounded end
        addArcWithCenter(endPoint, radius: lineWidth / 2, startAngle: endAngle, endAngle: endAngle + CGFloat(M_PI), clockwise: clockwise)
        // Inner path
        addArcWithCenter(center, radius: radius - lineWidth / 2, startAngle: endAngle, endAngle: startAngle, clockwise: !clockwise)
        // Rounded start
        addArcWithCenter(startPoint, radius: lineWidth / 2, startAngle: startAngle + CGFloat(M_PI), endAngle: startAngle, clockwise: !clockwise)
    }
}

Really cool. Pro Tip for others: You can test/view this simply in a Storyboard. All you need to see it is

CircleView(frame: CGRectMake(0, 0, 300, 300))