All Manuals > CAPI User Guide > 16 Drag and Drop > 16.3 Dropping


16.3.1 The drop callback

To implement dropping in list-panel or tree-view or output-pane , supply the :drop-callback initarg.

You can also supply :drop-callback for an interface . When the user drags an object over a window, the system first tries to call the drop-callback of any pane under the mouse and otherwise calls the drop-callback of the top-level interface, if supplied.

The drop-callback receives as arguments a drop-object which is used to communicate information about the dropping operation and stage which is a keyword. The drop-callback is called at several stages: when the pane is displayed; when the user drags over the pane; and when the user drops over the pane. Various functions are provided which you can use to query the drop-object and set attributes appropriately.

You will use set-drop-object-supported-formats to specify the data formats that it wants to receive. The :string format can be used to receive a string from another application and the :filename-list format can be used to receive a list of filenames from another application such as the Macintosh Finder or the Windows Explorer. Any other keyword in formats is assumed to be a private format that can only be used to receive objects from with the same Lisp image.

You can use drop-object-provides-format to query whether a given data format is actually available, and then you can call (setf drop-object-drop-effect) to modify the effect of the dropping operation .

Finally, at the :drop stage, you will use drop-object-get-object to retrieve (for each data format) the object which was returned by the drag-callback , and then do something with this object, typically copying or moving it to the pane in some way.

CAPI User Guide (Unix version) - 30 Aug 2011