Save NSImage as png in Swift

Question

I'm trying to create and save a .png from raw pixel data. I create an array of UInt8 where each number is a rgba value, kinda like this: [r, g, b, a, r, g, b, a...]. I can use this array to create a CGImage just fine. I can even use the CGImage to create an NSImage. The NSImage displays how I expect it to display when its loaded into an NSImageView.

What I want to do is to save an NSImage to disk. I've tried calling TIFFRepresentation on the NSImage and saving the NSData to "~/Desktop", but no file is saved. Any thoughts?

var pixels = [UInt8]()

for wPixel in 0...width {
    for hPixel in 0...height {
        pixels.append(0xff)
        pixels.append(0xaa)
        pixels.append(UInt8(wPixel % 200))
        pixels.append(0x00)     
    }
}

let image = createImage(100, height:100, pixels: pixels)
let nsImage = NSImage(CGImage: image, size: CGSize(width: 100, height: 100))
NSBitmapImageRep(data: nsImage.TIFFRepresentation!)!.representationUsingType(.NSPNGFileType, properties: [:])!.writeToFile("~/Desktop/image.png", atomically: true)



func createImage(width: Int, height: Int, pixels:Array<UInt8>) -> CGImage{
    let componentsPerPixel: Int = 4; // rgba
    let provider: CGDataProviderRef = CGDataProviderCreateWithData(nil,
                                                                   pixels,
                                                                   width * height * componentsPerPixel,
                                                                   nil)!;

    let rgbColorSpace = CGColorSpaceCreateDeviceRGB()
    let bitmapInfo:CGBitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.None.rawValue)

    let bitsPerComponent = 8
    let bitsPerPixel = 32

    let bytesPerRow = (bitsPerComponent * width) ;

    let cgImage = CGImageCreate(
        width,
        height,
        bitsPerComponent,
        bitsPerPixel,
        bytesPerRow,
        rgbColorSpace,
        bitmapInfo,
        provider,
        nil,
        true,
        .RenderingIntentDefault
    )

    print(cgImage.debugDescription)
    return cgImage!;
}