@IBDesignable для UILabel на круговом пути для Swift 2

Большое спасибо как для @Grimxn, так и для @ mark-moeykens за абсолютно убийственную работу. Я сделал небольшой рефактор по работе Марка, поэтому я мог использовать его в проекте, который не успел обновиться до Swift 3. Хотелось поделиться, так как предыдущие сообщения были настолько полезными.

Код Swift 2 для пользовательского UILabel

import UIKit

@IBDesignable
class ArcUILabel: UILabel
{
    // *******************************************************
    // DEFINITIONS (Because I'm not brilliant and I'll forget most this tomorrow.)
    // Radius: A straight line from the center to the circumference of a circle.
    // Circumference: The distance around the edge (outer line) the circle.
    // Arc: A part of the circumference of a circle. Like a length or section of the circumference.
    // Theta: A label or name that represents an angle.
    // Subtend: A letter has a width. If you put the letter on the circumference, the letter's width
    //          gives you an arc. So now that you have an arc (a length on the circumference) you can
    //          use that to get an angle. You get an angle when you draw a line from the center of the
    //          circle to each end point of your arc. So "subtend" means to get an angle from an arc.
    // Chord: A line segment connecting two points on a curve. If you have an arc then there is a
    //          start point and an end point. If you draw a straight line from start point to end point
    //          then you have a "chord".
    // sin: (Super simple/incomplete definition) Or "sine" takes an angle in degrees and gives you a number.
    // asin: Or "asine" takes a number and gives you an angle in degrees. Opposite of sine.
    //          More complete definition: http://www.mathsisfun.com/sine-cosine-tangent.html
    // cosine: Also takes an angle in degrees and gives you another number from using the two radiuses (radii).
    // *******************************************************

    @IBInspectable var angle: CGFloat = 1.6
    @IBInspectable var clockwise: Bool = true

    override func drawRect(rect: CGRect)
    {
        centreArcPerpendicular()
    }

    /**
     This draws the self.text around an arc of radius r,
     with the text centred at polar angle theta
     */
    func centreArcPerpendicular() {
        guard let context = UIGraphicsGetCurrentContext() else { return }
        let str = self.text ?? ""
        let size = self.bounds.size
        CGContextTranslateCTM(context, size.width / 2, size.height / 2)

        let radius = getRadiusForLabel()
        let l = str.characters.count
        let attributes: [String : AnyObject] = [NSFontAttributeName: self.font]

        let characters: [String] = str.characters.map { String($0) } // An array of single character strings, each character in str
        var arcs: [CGFloat] = [] // This will be the arcs subtended by each character
        var totalArc: CGFloat = 0 // ... and the total arc subtended by the string

        // Calculate the arc subtended by each letter and their total
        for i in 0 ..< l {
            arcs += [chordToArc(characters[i].sizeWithAttributes(attributes).width, radius: radius)]
            totalArc += arcs[i]
        }

        // Are we writing clockwise (right way up at 12 o'clock, upside down at 6 o'clock)
        // or anti-clockwise (right way up at 6 o'clock)?
        let direction: CGFloat = clockwise ? -1 : 1
        let slantCorrection = clockwise ? -CGFloat(M_PI_2) : CGFloat(M_PI_2)

        // The centre of the first character will then be at
        // thetaI = theta - totalArc / 2 + arcs[0] / 2
        // But we add the last term inside the loop
        var thetaI = angle - direction * totalArc / 2

        for i in 0 ..< l {
            thetaI += direction * arcs[i] / 2
            // Call centre with each character in turn.
            // Remember to add +/-90º to the slantAngle otherwise
            // the characters will "stack" round the arc rather than "text flow"
            centre(text: characters[i], context: context, radius: radius, angle: thetaI, slantAngle: thetaI + slantCorrection)
            // The centre of the next character will then be at
            // thetaI = thetaI + arcs[i] / 2 + arcs[i + 1] / 2
            // but again we leave the last term to the start of the next loop...
            thetaI += direction * arcs[i] / 2
        }
    }

    func chordToArc(_ chord: CGFloat, radius: CGFloat) -> CGFloat {
        // *******************************************************
        // Simple geometry
        // *******************************************************
        return 2 * asin(chord / (2 * radius))
    }

    /**
     This draws the String str centred at the position
     specified by the polar coordinates (r, theta)
     i.e. the x= r * cos(theta) y= r * sin(theta)
     and rotated by the angle slantAngle
     */
    func centre(text str: String, context: CGContext, radius r:CGFloat, angle theta: CGFloat, slantAngle: CGFloat) {
        // Set the text attributes
        let attributes = [NSForegroundColorAttributeName: self.textColor,
                          NSFontAttributeName: self.font] as [String : AnyObject]
        // Save the context
        CGContextSaveGState(context)
        // Move the origin to the centre of the text (negating the y-axis manually)
        CGContextTranslateCTM(context, r * cos(theta), -(r * sin(theta)))
        // Rotate the coordinate system
        CGContextRotateCTM(context, -slantAngle)

        // Calculate the width of the text
        let offset: CGSize = str.sizeWithAttributes(attributes)
        // Move the origin by half the size of the text
        CGContextTranslateCTM(context, -offset.width / 2, -offset.height / 2)

        // Draw the text
        let txtStr = NSString(string: str)
        txtStr.drawAtPoint(CGPoint(x: 0, y: 0), withAttributes: attributes)

        // Restore the context
        CGContextRestoreGState(context)
    }

    func getRadiusForLabel() -> CGFloat {
        // Imagine the bounds of this label will have a circle inside it.
        // The circle will be as big as the smallest width or height of this label.
        // But we need to fit the size of the font on the circle so make the circle a little
        // smaller so the text does not get drawn outside the bounds of the circle.
        let smallestWidthOrHeight = min(self.bounds.size.height, self.bounds.size.width)
        let heightOfFont = self.text?.sizeWithAttributes([NSFontAttributeName: self.font]).height ?? 0

        // Dividing the smallestWidthOrHeight by 2 gives us the radius for the circle.
        return (smallestWidthOrHeight/2) - heightOfFont + 5
    }
}