Create all the different sized iOS App Icons from a single source image

mkappicons.swift

#!/usr/bin/xcrun swift

import Cocoa
import AppKit
import CoreGraphics

class Spec {
	let pointSize: Int;
	let scales: [Int]
	
	init(pointSize: Int, atScale scales: [Int]){
		self.pointSize = pointSize;
		self.scales = scales;
	}
	
	convenience init(pointSize: Int){
		self.init(pointSize: pointSize, atScale: [1, 2, 3])
	}
	
	func edgeLen(atScale scale: Int)-> Int {
		return pointSize*scale
	}
}

var specs = [
	Spec(pointSize: 29),
	Spec(pointSize: 40),
	Spec(pointSize: 60),
	Spec(pointSize: 76, atScale: [1,2]),
]

var inputFilePath = Process.arguments[1]
var inputImage = NSImage(byReferencingFile: inputFilePath)
var inputImageBitmapRepOrNil:NSBitmapImageRep?
for rep in inputImage?.representations as [NSImageRep] {
	if let candidate = rep as? NSBitmapImageRep {
		inputImageBitmapRepOrNil = candidate
	}
}

if let inputImageBitmapRep = inputImageBitmapRepOrNil? {
	
	var baseOutputFilePath = inputFilePath.stringByDeletingPathExtension
	
	for spec in specs {
		
		for scale in spec.scales {
			
			var dim = spec.edgeLen(atScale: scale)
			var fdim = CGFloat(dim)
			var size = CGSizeMake(fdim, fdim)
			var rect = CGRectMake(0.0, 0.0, fdim, fdim)
			
			var outputBitmapRep = NSBitmapImageRep(
				bitmapDataPlanes: nil,
				pixelsWide: dim,
				pixelsHigh: dim,
				bitsPerSample: inputImageBitmapRep.bitsPerSample,
				samplesPerPixel: inputImageBitmapRep.samplesPerPixel,
				hasAlpha: inputImageBitmapRep.alpha,
				isPlanar: inputImageBitmapRep.planar,
				colorSpaceName: inputImageBitmapRep.colorSpaceName,
				bitmapFormat: inputImageBitmapRep.bitmapFormat,
				bytesPerRow: 0,
				bitsPerPixel: inputImageBitmapRep.bitsPerPixel
			)
			
			var graphicsContext = NSGraphicsContext(bitmapImageRep: outputBitmapRep!)
			
			if let c = graphicsContext?.CGContext {
				
				var cgImage = inputImage?.CGImageForProposedRect(nil, context: graphicsContext, hints: nil)?.takeUnretainedValue()
				CGContextDrawImage(c, rect, cgImage)
				
				var data = outputBitmapRep!.representationUsingType(NSBitmapImageFileType.NSPNGFileType, properties:[:])
				
				var outputFilePath = "\(baseOutputFilePath)\(spec.pointSize)pt@\(scale)x.png"
				data!.writeToFile(outputFilePath, atomically:false)
			}
		}
	}
}

Run

mkappicons.swift MyAppIcon.png 

To get

MyAppIcon29pt@1x.png
MyAppIcon29pt@2x.png
MyAppIcon29pt@3x.png
MyAppIcon40pt@1x.png
MyAppIcon40pt@2x.png
MyAppIcon40pt@3x.png
MyAppIcon60pt@1x.png
MyAppIcon60pt@2x.png
MyAppIcon60pt@3x.png
MyAppIcon76pt@1x.png
MyAppIcon76pt@2x.png