19.8. Drag and Drop

Drag and drop is a bit more complicated. The basic mechanism is the same; you write data to a pasteboard when the drag operation starts and retrieve it at the end. This is complicated by the fact that drag operations are live—there is no intermediate storage—allowing a much greater interaction between the sender and the receiver. To further complicate matters, there are several possible operations that can be created by the user dragging an object:

• NSDragOperationMove is the most obvious kind of drag operation. This moves the object from the origin of the drag to the destination of the drop.

• NSDragOperationCopy is similar, but the source does not remove the object. This distinction is mainly important in intraprocess drag operations. If the source and destination are in different processes, then a copy occurs for both copy and move operations, but the sender deletes its copy after the drop. If they are in the same process, then they can just pass an object reference for a move, but not for a copy. OS X displays a + symbol next to the cursor while performing this kind of operation.

• NSDragOperationLink is used for operations where the source and destination should share the object. This is most often done for drag operations between windows in the same application, but can be used across applications when the object is a file.

• NSDragOperationDelete is used for items that can be dragged to the trash can. This kind of operation is ignored by the destination but should cause the source to delete the object. This is equivalent to a move operation with no destination.