Swift3でCocoaによる画像リサイズのサンプルコード。
以前作成したコードが3になって動かなくなっていたのでリファクタ。
# !/usr/bin/swift


import Cocoa


class fileMG {

    var dir:  String
    var file: String
    var path: String
    var extn: String
    var name: String

    init(p: String) {
        let ns: NSString = p as NSString
        path = p
        file = ns.lastPathComponent
        dir  = ns.deletingLastPathComponent
        extn = ns.pathExtension
        name = (file as NSString).deletingPathExtension
        print("path : \(path)")
        print("file : \(file)")
        print("dir  : \(dir)")
        print("extn : \(extn)")
        print("name : \(name)")
    }
}


class imgMG: fileMG {

    func resizeImg(w: Int, h: Int) -> String {

        if let img: NSImage = NSImage(contentsOfFile: path) {

            let orgBmpRep: NSBitmapImageRep = NSBitmapImageRep(data:img.tiffRepresentation!)!
            let orgRef:    CGImage          = orgBmpRep.cgImage!
            let orgW:      Int              = Int(orgRef.width)
            let orgH:      Int              = Int(orgRef.height)

            var resizeW: Int = 0, resizeH: Int = 0
            if (orgW < orgH) {
                resizeW  = w
                resizeH = orgH * resizeW / orgW
            } else {
                resizeH = h
                resizeW = orgW * resizeH / orgH
            }
            let resizeSize: NSSize = NSMakeSize(CGFloat(resizeW), CGFloat(resizeH))

            let bitsPerComponent: Int          = 8
            let bytesPerRow:      Int          = 4 * resizeW
            let colorSpace:       CGColorSpace = CGColorSpaceCreateDeviceRGB()
            let bitmapInfo:       UInt32       = CGImageAlphaInfo.premultipliedLast.rawValue

            let bitmapContext: CGContext = CGContext(data: nil, width: resizeW, height: resizeH, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: UInt32(bitmapInfo))!

            // 元画像 (image) をビットマップに書き出します。
            let bitmapRect: CGRect = NSMakeRect(0.0, 0.0, resizeSize.width, resizeSize.height)
            bitmapContext.draw(orgRef, in: bitmapRect)

            // ビットマップを NSImage に変換します。
            let newImageRef: CGImage = bitmapContext.makeImage()!
            let newImage:    NSImage = NSImage(cgImage: newImageRef, size: resizeSize)

            // 保存
            let newBmpRep:  NSBitmapImageRep    = NSBitmapImageRep(data:newImage.tiffRepresentation!)!
            let imageProps: [String: AnyObject] = [ NSImageCompressionFactor as String: 1.0 as AnyObject ]
            let newData:    NSData              = newBmpRep.representation(using: NSBitmapImageFileType.PNG, properties: imageProps)! as NSData
            let newPath:    String              = "\(dir)/\(name)_\(w).\(extn)"
            newData.write(toFile: newPath, atomically: true)
            return "finish!!\n\(newPath)"
        } else {
            return "not file : " + path

        }
    }
}


///
/// 引数取得
///
if (CommandLine.arguments.count <= 1) {
    print("画像ファイルを指定してください")
    exit(0)
}
var imgResize = imgMG(p: String(CommandLine.arguments[1]))
///
/// 3種類の画像出力
///
print(imgResize.resizeImg(w:  50, h:  50))
print(imgResize.resizeImg(w: 100, h: 100))
print(imgResize.resizeImg(w: 300, h: 300))