Here is a project you can look at:
https://github.com/foundry/drawingtest
It's a little demo I made as I was trying to understand the relationship between the rects in this method:
- (void)drawInRect:(NSRect)dstRect
fromRect:(NSRect)srcRect
operation:(NSCompositingOperation)op
fraction:(CGFloat)delta
Note that the older compositeToPoint:
methods are deprecated and should not be used for this sort of thing.
srcRect
is the portion of the original image (in it's own coordinates) that you want to keep.
dstRect
is the rect that you want that cropped area to draw into.
JMRect
in the project is an NSObject representation of an NSRect - so that we can use cocoa bindings to tie the interface controls together.
For your UI, the cropping rectangle could just be a transparent subview view with a border that you push around and resize over the image you want to crop.
This is by no means a complete solution to your question, but it's something you can poke around with - it might help you to get started.