The public API of Common Widgets drag and drop includes the following classes, along with some methods in CwWidget and in callback data classes.

CwDragDropObject hierarchy

The CwDragDropObject class implements drag and drop objects. CwDragDropObject is an abstract class that provides resource and proc methods to its subclasses. The hierarchy looks like the following:
CwDragDropObject
CwDragContext
CwDropSite
CwDropTransfer

The CwDropTransferEntry class is used in the data transfer process. Instances of CwDropTransferEntry contain a targetType and a transferClientData object.