NSImageView image aspect fill?

You may find it much easier to subclass NSView and provide a CALayer that does the aspect fill for you. Here is what the init might look like for this NSView subclass.

- (id)initWithFrame:(NSRect)frame andImage:(NSImage*)image
{
  self = [super initWithFrame:frame];
  if (self) {
    self.layer = [[CALayer alloc] init];
    self.layer.contentsGravity = kCAGravityResizeAspectFill;
    self.layer.contents = image;
    self.wantsLayer = YES;
  }
  return self;
}

Note that the order of setting the layer, then settings wantsLayer is very important (if you set wantsLayer first, you'll get a default backing layer instead).

You could have a setImage method that simply updates the contents of the layer.

Here is what I'm using, written with Swift. This approach works well with storyboards - just use a normal NSImageView, then replace the name NSImageView in the Class box, with MyAspectFillImageNSImageView ...

open class MyAspectFillImageNSImageView : NSImageView {
  
  open override var image: NSImage? {
    set {
      self.layer = CALayer()
      self.layer?.contentsGravity = kCAGravityResizeAspectFill
      self.layer?.contents = newValue
      self.wantsLayer = true
      
      super.image = newValue
    }
    
    get {
      return super.image
    }
  }

    public override init(frame frameRect: NSRect) {
        super.init(frame: frameRect)
    }
    
    //the image setter isn't called when loading from a storyboard
    //manually set the image if it is already set
    required public init?(coder: NSCoder) {
        super.init(coder: coder)
        
        if let theImage = image {
            self.image = theImage
        }
    }

}