ON DRAG_OVER block

When the ON DROP control block is defined, the ON DRAG_OVER block will be executed after ON DRAG_ENTER, when the mouse cursor is moving over the drop target, or when the drag & drop operation has changed (toggling copy/move).

ON DRAG_OVER will be called only once per row, even if the mouse cursor moves over the row.

In the ON DRAG_OVER block, the method ui.DragDrop.getLocationRow() returns the index of the row in the target array, and can be used to allow or disallow the drop. When using a tree-view, you must also check the index returned by the ui.DragDrop.getLocationParent() method to detect if the object was dropped as a sibling or as a child node, and allow/disallow the drop operation accordingly.

The program can change the drop operation at any execution of the ON DRAG_OVER block. You can disallow or allow a specific drop operation with a call to ui.DragDrop.setOperation(); passing a NULL to the method will disallow the drop.

The current operation (returned by ui.DragDrop.getOperation()) is the value set in previous ON DRAG_ENTER or ON DRAG_OVER events, or the operation selected by the end user, if it can toggle between copy and move. Thus, ON DRAG_OVER can occur even if the mouse position has not changed.

If dropping has been prevented with ui.DragDrop.setOperation(NULL) in the previous ON DRAG_OVER event, the program can reset the operation to allow a drop with a call to ui.DragDrop.setOperation() with the operation parameter "move" or "copy".

ON DRAG_OVER will not be called if drop has been disabled in ON DRAG_ENTER with ui.DragDrop.setOperation(NULL)

ON DRAG_OVER is optional, and must only be defined if the operation or the acceptance of the drag object depends on the target row of the drop target.

DEFINE dnd ui.DragDrop 
...
DISPLAY ARRAY arr TO sr.* ...
  ...
  ON DRAG_ENTER (dnd)
    ...
  ON DRAG_OVER (dnd)
    IF arr[dnd.getLocationRow()].acceptsCopy THEN
      CALL dnd.setOperation("copy")
    ELSE
      CALL dnd.setOperation(NULL)
    END IF
  ON DROP (dnd)
    ...
END DISPLAY

During a drag & drop process, the end user (or the target application) can decide to modify the type of the operation, to indicate whether the dragged object has to be copied or moved from the source to the target. For example, in a typical file explorer, by default files are moved when doing a drag & drop on the same disk. To make a copy of a file, you must press the Ctrl key while doing the drag & drop with the mouse.

In the drop target dialog, you can detect such operation changes in the ON DRAG_OVER trigger and query the ui.DragDrop object for the current operation with ui.DragDrop.getOperation(). In the drag source dialog, you typically check ui.DragDrop.getOperation() in the ON DRAG_FINISHED trigger to know what type of operation occurred, to keep ("copy" operation) or delete ("move" operation) the original dragged object.

This example tests the current operation in the drop target list and displays a message accordingly:

DEFINE dnd ui.DragDrop 
...
DISPLAY ARRAY arr TO sr.* ...
  ...
  ON DRAG_ENTER (dnd)
    ...
  ON DRAG_OVER (dnd)
    CASE dnd.getOperation()
    WHEN "move"
      MESSAGE "The object will be moved to row ", dnd.getLocationRow()
    WHEN "copy"
      MESSAGE "The object will be copied to row ", dnd.getLocationRow()
    END CASE
    ...
  ON DROP (dnd)
    ...
END DISPLAY