Understanding drag & drop

This is an introduction to drag & drop programming.

Drag & drop is a well-known feature of graphical applications, allowing the end user to use the mouse to drag an element of a window to another window in the same program or into an external application. The front-end platform/device must support this feature.

Drag & drop can be implemented in regular tables and tree-views controlled by a singular DISPLAY ARRAY or a DISPLAY ARRAY sub-dialog within a DIALOG instruction. Drag & drop is not supported in other dialog contexts, such as a singular INPUT, INPUT ARRAY or CONSTRUCT.

With drag & drop, end users can:

• Move drag-able objects between lists and tree-views in the same Genero form or program.
• Move drag-able objects between lists and tree-views in different Genero forms and programs.
• Move drag-able objects between other desktop applications and tables / tree-views in Genero programs.

Drag & drop control is implemented in a DISPLAY ARRAY with specific interaction blocks, to handle the events related to the drag and drop operation. These specific blocks will be triggered when drag and drop events arrive from the front-end.

• ON DRAG_START
• ON DRAG_FINISHED
• ON DRAG_ENTER
• ON DRAG_OVER
• ON DROP

Each of these interaction blocks takes an ui.DragDrop object as a parameter. A reference variable to that object must be declared before the dialog. In the interaction block, the ui.DragDrop object can be used to configure the drag & drop action to take. For example, a "drag enter" event can be refused.

The ON DRAG_START and ON DRAG_FINISHED triggers apply to the source of the drag & drop operation; the dialog where the object was dragged. The other triggers provide notification to the drop target dialog, used to inform the program when the different drop events occur and to let the target accept or reject the drop action.

DEFINE dnd ui.DragDrop
...
DISPLAY ARRAY arr TO sr.* ...
...
    ON DRAG_ENTER(dnd)
      IF ok_to_drop THEN
        CALL dnd.setOperation("move")
      ELSE
        CALL dnd.setOperation(NULL)
      END IF
...
END DISPLAY