IconMaker.m

#import "IconMaker.h"

#define SIZES @[@16, @32, @128, @256, @512]

@implementation IconMaker {
    NSImage *actImage;
    NSString *iconSet;
}

- (NSString*)createIconFromImage:(NSImage*)image {
    actImage = image;

    /*
     * Create Iconset
     */

    [self createIconSet];

    /*
     * Resize Icons
     */

    NSImageRep *imageRep = [[image representations] objectAtIndex:0];
    NSSize imageSize = NSMakeSize(imageRep.pixelsWide, imageRep.pixelsHigh);

    //check what's the biggest possible size
    int biggestSize = [self getBiggestSize:imageSize];

    /*
     * Resize image
     */

    for (int i=0; i<=biggestSize; i++) {
        //create normal size
        int size = [SIZES[i] intValue];
        NSImage *resizedImage = [self resizeImage:size];
        [resizedImage lockFocus];
        NSBitmapImageRep *imageRep = [[NSBitmapImageRep alloc] initWithFocusedViewRect:NSMakeRect(0.0, 0.0, resizedImage.size.width, resizedImage.size.height)];
        [resizedImage unlockFocus];
        NSData *imageData = [imageRep representationUsingType:NSPNGFileType properties:nil];
        NSString *fileName = [iconSet stringByAppendingFormat:@"/icon_%ix%i.png", size, size];
        [imageData writeToFile:fileName atomically:NO];

        //create retina size
        //actually we don't need to ... ?! but iconutil will throw warnings
    }

    /*
     * Create icns
     */

    NSString *icns = [self createIcns];

    /*
     * Remove Iconset
     */

    [self removeIconSet];

    /*
     * return
     */
    return icns;
}

#pragma mark - Iconset

- (void)createIconSet {
    iconSet = [NSTemporaryDirectory() stringByAppendingString:@"IconMaker.iconset"];
    [[NSFileManager defaultManager] createDirectoryAtPath:iconSet withIntermediateDirectories:NO attributes:nil error:nil];
}

- (void)removeIconSet {
    [[NSFileManager defaultManager] removeItemAtPath:iconSet error:nil];
}

#pragma mark - Size

- (int)getBiggestSize:(NSSize)size {
    for (int i=(int)SIZES.count-1; i>0; i--) {
        if ((int)size.width >= [SIZES[i] intValue])
            return i;
    }
    return 0;
}

#pragma mark - Resizing

- (NSImage*)resizeImage:(int)size {
    //retina?
    CGFloat screenScale = [[NSScreen mainScreen] backingScaleFactor];
    size = size / screenScale;

    NSImage *sourceImage = actImage;

    NSSize newSize = NSMakeSize(size, size);
    NSImage *resizedImage = [[NSImage alloc] initWithSize:newSize];

    NSSize originalSize = [sourceImage size];

    [resizedImage lockFocus];
    [sourceImage drawInRect:NSMakeRect(0, 0, size, size) fromRect:NSMakeRect(0, 0, originalSize.width, originalSize.height) operation:NSCompositeSourceOver fraction:1.0];
    [resizedImage unlockFocus];

    return resizedImage;
}

#pragma mark - make icns

- (NSString*)createIcns {
    NSTask *task = [[NSTask alloc] init];
    task.launchPath = @"/usr/bin/iconutil";
    task.arguments = @[@"-c", @"icns", iconSet];
    task.standardError = [NSPipe pipe];
    [task launch];
    [task waitUntilExit];   //otherwise the above method will delete the iconset before finishing creating the .icns file

    return [[iconSet stringByDeletingPathExtension] stringByAppendingString:@".icns"];
}

@end