Cocoa Programming for OS X: The Big Nerd Ranch Guide

For the More Curious: Lazy Copying

An application can implement copying to a pasteboard in a lazy manner. For example, imagine a graphics application that copies large images to the pasteboard in several formats: PNG, TIFF, PDF, and so on. You can imagine that copying all these formats onto the pasteboard would be hard on the application and the pasteboard server. Instead, such an application might do a lazy copy. That is, the application will declare all the types that it could put on the pasteboard but will put off copying the data until another application asks for it.

Essentially, the application puts an “IOU” (instead of the data) on the pasteboard and gives an object that will provide the data when it is needed. When another ...