Detect the drop on target event

To enable drop actions on a list, you must define the ON DROP block; otherwise the list will not accept drop actions.

The ON DROP block is executed after the end user has released the mouse button to drop the dragged object. ON DROP will not occur if drop has been denied in the previous ON DRAG_OVER event or in ON DRAG_ENTER with a call to ui.DragDrop.setOperation(NULL).

The program might also check the MIME type of the dragged object with ui.DragDrop.getSelectedMimeType(), and then call the ui.DragDrop.getBuffer() method to retrieve drag & drop data from external applications.

Ideally the drop operation should be accepted (no additional call to ui.DragDrop.setOperation()).

In this block, the ui.DragDrop.getLocationRow() method returns the index of the row in the target array, and can be used to execute the code to get the drop data / object into the row that has been chosen by the user.

DEFINE dnd ui.DragDrop 
...
DISPLAY ARRAY arr TO sr.* ...
  ...
  ON DROP (dnd)
    LET arr[dnd.getLocationRow()].capacity == dnd.getBuffer() 
    ...
END DISPLAY

If the drag & drop operations are local to the same list or tree-view controller, you can use the ui.DragDrop.dropInternal() method to simplify the code. This method implements the typical move of the dragged rows or tree-view node. This is especially useful in case of a tree-view, but is also the preferred way to move rows around in simple tables.

DEFINE dnd ui.DragDrop 
...
DISPLAY ARRAY arr_tree TO sr_tree.* ...
  ...
  ON DROP (dnd)
    CALL dnd.dropInternal()
    ...
END DISPLAY

If you want to implement by hand the code to drop a node in a tree-view, you must 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 execute the code corresponding to the drop operation: If the drop target row index returned by getLocationRow() is a child of the parent row index returned by getLocationParent() , the new row must be inserted before getLocationRow(); otherwise the new row must be added as a child of the parent node identified by getLocationParent().