scale nsimageview inside nsscrollview

in macos cocoa how would you automatically scale different image sizes to fit inside the nsscrollview as shown below. i'm only interested in the scaling part of whats going on and not the transparent cropping.

imageScrollView.documentView?.setFrameSize(imageScrollView.visibleRect.size)
    selectedImage.setFrameSize((imageScrollView.documentView?.frame.size)!)