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Here is the code that I'm using to scale images in iOS i.e. scale 500x500 image to 100x100 image and then store scaled copy:

+ (UIImage *)image:(UIImage *)originalImage scaledToSize:(CGSize)desiredSize {
    UIGraphicsBeginImageContextWithOptions(desiredSize, YES, [UIScreen mainScreen].scale);
    [originalImage drawInRect:CGRectMake(0, 0, desiredSize.width, desiredSize.height)];
    UIImage *finalImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext;

    return finalImage;
}

Now I need to implement the same functionality in my macOS app. How can I do that? I saw the question like this but I still can't understand the logic of doing this in macOS.

Alexandr Chekel
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  • Take a look at the CoreGraphics APIs. Specifically, you’ll want to use CGBitmapContextCreate, CGContextDrawImage, CGBitmapContextCreateImage. Note that these are CoreFoundation C APIs so you’ll need to CFRelease objects when you’re done with them. – TyR May 06 '18 at 03:17

1 Answers1

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After some search I found a question like mine but in Swift. So I translated it in Objective-C and that's it:

+ (NSImage *)image:(NSImage *)originalImage scaledToSize:(NSSize)desiredSize {
    NSImage *newImage = [[NSImage alloc] initWithSize:desiredSize];

    [newImage lockFocus];
    [originalImage drawInRect:NSMakeRect(0, 0, desiredSize.width, desiredSize.height) fromRect:NSMakeRect(0, 0, imageWidth, imageHeight) operation:NSCompositingOperationSourceOver fraction:1];
    [newImage unlockFocus];

    newImage.size = desiredSize;
    return [[NSImage alloc] initWithData:[newImage TIFFRepresentation]];
}

But there's still an issue: if desiredSize = NSMakeSize(50,50); it will return 50 by 50 pixels. It's should be something with screen scale i guess. There is Swift code that I translated:

Alexandr Chekel
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