Creates a series of images for an animation similar to UIActivityIndicatorView

SpinningWaitImageGenerator.swift

import UIKit

private extension CGPoint {
    /// Multiplies point by given value
    /// - parameter value: The x and y coordinates are multipled by this value.
    /// - returns: A new CGPoint whose coordinate components have been multiplied by `value`
    func multipliedBy(value: CGFloat) -> CGPoint {
        return CGPoint(x: self.x * value, y: self.y * value)
    }
}

/// Creates an array of images for a spinning wait indicator.
/// On iOS, we have UIActivityIndicatorView, but since this is unavailble on WatchOS,
/// we can create a similar-looking view by passing an animated UIImage to WKInterfaceImage.
/// - parameter radius: Radius of the spinning indicator.
/// - returns: An array of images to be used in a spinning wait indicator animation.
/// The width and height of the generated images will be 2x the given radius.
func generateSpinningWaitImagesWithRadius(radius: CGFloat) -> [UIImage] {
    let startRadius = radius * 0.5
    let lineWidth = radius / 5
    let endRadius = radius - lineWidth / 2
    let diameter = radius * 2
    
    let ticks = (0..<12).map { (tickIndex: Int) -> CGFloat in
        return CGFloat(Double(tickIndex) * (M_PI / 6))
    }
    
    return ticks.map { (angle: CGFloat) -> UIImage in
        UIGraphicsBeginImageContextWithOptions(CGSize(width: diameter, height: diameter), false, 0)
        defer {
            UIGraphicsEndImageContext()
        }
        
        for tick in ticks {
            UIColor(white: tick / ticks.last!, alpha: 1).setStroke()
            
            let unitPoint = CGPoint(x: cos(tick), y: sin(tick))
            
            let tickMark = UIBezierPath()
            tickMark.lineWidth = lineWidth
            tickMark.lineCapStyle = .Round
            tickMark.lineJoinStyle = .Round
            
            tickMark.moveToPoint(unitPoint.multipliedBy(startRadius))
            tickMark.addLineToPoint(unitPoint.multipliedBy(endRadius))
            
            tickMark.applyTransform(CGAffineTransformMakeRotation(angle))
            tickMark.applyTransform(CGAffineTransformMakeTranslation(radius, radius))
            
            tickMark.stroke()
        }
        
        return UIGraphicsGetImageFromCurrentImageContext()
    }
}