A proc is like a callback. It can be hooked to a drag and drop object to notify an application of various events in the drag and drop process. A proc is specific to the type of object and is represented by an instance of CwCallbackRec. When called, the third parameter sent to a proc is a callData object created by Common Widgets drag and drop.

Unlike callbacks, only one of each type of proc can be hooked to a drag and drop object at any given time, and most procs are mandatory. A proc must be hooked in the create argBlock using a proc resource setter. The first parameter sent to a proc is a drag and drop object or a widget, depending on the proc.