Drawing to a PNG

MacOSDrawPNG.swift

func makePNG(_ size: NSSize, by drawing: (CGContext) -> ()) -> Data? {
    guard let bitmapImageRep = NSBitmapImageRep(bitmapDataPlanes: nil,
                                                pixelsWide: Int(size.width),
                                                pixelsHigh: Int(size.height),
                                                bitsPerSample: 8,
                                                samplesPerPixel: 4,
                                                hasAlpha: true,
                                                isPlanar: false,
                                                colorSpaceName: NSColorSpaceName.deviceRGB,
                                                bytesPerRow: 0, bitsPerPixel: 0) else {
        return nil
    }

    let graphicsCtx = NSGraphicsContext(bitmapImageRep: bitmapImageRep)!

    let ctx = graphicsCtx.cgContext
    drawing(ctx)

    return bitmapImageRep.representation(using: .png, properties: [:])
}

Usage.swift

let pngData = makePNG(rect.size) { ctx in
    ctx.beginPath()
    ctx.move(to: .zero)
    ctx.addLine(to: NSPoint(x: 100, y: 100))
    ctx.addLine(to: NSPoint(x: 100, y: 0))
    ctx.addLine(to: NSPoint(x: 0, y: 100))
    ctx.closePath()
    ctx.strokePath()
}!

try! pngData.write(to: URL(fileURLWithPath: "/tmp/out.png"))