How to resize NSImage?

Thomas Johannesmeyer's answer using lockFocus doesn't work as you may intend on Retina/HiDPI screens: it resizes to the desired points in the screen's native scale, not pixels.

• If you're resizing for display on screen, use that method.
• If you're resizing for a file with exact pixel dimensions, it'll be twice as large when running on Retina (2x DPI) screens.

This method, cobbled together from various answers including some in this related question, resizes to the specified pixel dimensions regardless of current screen DPI:

+ (NSImage *)resizedImage:(NSImage *)sourceImage toPixelDimensions:(NSSize)newSize
{
    if (! sourceImage.isValid) return nil;

    NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]
              initWithBitmapDataPlanes:NULL
                            pixelsWide:newSize.width
                            pixelsHigh:newSize.height
                         bitsPerSample:8
                       samplesPerPixel:4
                              hasAlpha:YES
                              isPlanar:NO
                        colorSpaceName:NSCalibratedRGBColorSpace
                           bytesPerRow:0
                          bitsPerPixel:0];
    rep.size = newSize;

    [NSGraphicsContext saveGraphicsState];
    [NSGraphicsContext setCurrentContext:[NSGraphicsContext graphicsContextWithBitmapImageRep:rep]];
    [sourceImage drawInRect:NSMakeRect(0, 0, newSize.width, newSize.height) fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0];
    [NSGraphicsContext restoreGraphicsState];

    NSImage *newImage = [[NSImage alloc] initWithSize:newSize];
    [newImage addRepresentation:rep];
    return newImage;
}

Edit: Since this answer is still the accepted answer, but was written without Retina screens in mind, I will straight up link to a better solution further down the thread: Objective-C Swift 4

---

Because the method of Paresh is totally correct but deprecated since 10.8 I'll post the working 10.8 code below. All credit to Paresh's answer though.

- (NSImage *)imageResize:(NSImage*)anImage newSize:(NSSize)newSize {
    NSImage *sourceImage = anImage;
    [sourceImage setScalesWhenResized:YES];
    
    // Report an error if the source isn't a valid image
    if (![sourceImage isValid]){
        NSLog(@"Invalid Image");
    } else {
        NSImage *smallImage = [[NSImage alloc] initWithSize: newSize];
        [smallImage lockFocus];
        [sourceImage setSize: newSize];
        [[NSGraphicsContext currentContext] setImageInterpolation:NSImageInterpolationHigh];
        [sourceImage drawAtPoint:NSZeroPoint fromRect:CGRectMake(0, 0, newSize.width, newSize.height) operation:NSCompositingOperationCopy fraction:1.0];
        [smallImage unlockFocus];
        return smallImage;
    }
    return nil;
}