Drawing to NSImage using Quartz in Swift

quartz-drawing.swift

import Cocoa

// helper function which takes a size and a drawing function.
// returns an image

public func drawItems(bounds: CGRect, block: (bounds: CGRect, context: CGContextRef) -> () ) -> NSImage? {
    var outputImage: NSImage? = nil
    let insetSize = CGSizeMake(-20, -20)
    let insetBounds = CGRectInset(bounds, insetSize.width, insetSize.height)
    let colorSpace = CGColorSpaceCreateDeviceRGB()
    let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedLast.rawValue)
    let bitmapContext = CGBitmapContextCreate(
        nil,
        Int(insetBounds.size.width),
        Int(insetBounds.size.height),
        8,
        0,
        colorSpace,
        bitmapInfo.rawValue)
    
    if let context = bitmapContext {
        CGContextConcatCTM(context, CGAffineTransformMakeTranslation(-insetSize.width, -insetSize.height))
        CGContextConcatCTM(context, CGAffineTransformMakeTranslation(-bounds.origin.x, -bounds.origin.y))
        
        block(bounds: bounds, context: context)
        
        if let image = CGBitmapContextCreateImage(context) {
            outputImage = NSImage(CGImage: image, size: insetBounds.size)
        }
    }
    
    return outputImage
}

// example drawing function
func drawSquare(bounds: CGRect, context: CGContextRef) {
    let path = CGPathCreateWithRect(bounds, nil)
    CGContextAddPath(context, path)
    
    CGContextSetStrokeColorWithColor(context, NSColor.redColor().CGColor)
    CGContextSetLineWidth(context, 20)
    CGContextSetLineJoin(context, CGLineJoin.Round)
    CGContextSetLineCap(context, CGLineCap.Round)
    
    let dashArray:[CGFloat] = [16, 32]
    
    CGContextSetLineDash(context, 0, dashArray, 2)
    CGContextReplacePathWithStrokedPath(context)

    CGContextSetFillColorWithColor(context, NSColor.redColor().CGColor)
    CGContextFillPath(context)
}

let bounds = CGRectMake(0, 0, 128, 128)
let image = drawItems(bounds, block: drawSquare)