Drag and drop

To support dragging and dropping objects on a window, a complicated dance of callbacks and subprogram calls must be performed. Two procedures must be overridden, On_Drag and On_Drop.

Dragging

The first one is invoked when the user drag something over the window. Call procedure Set_Action in AWT.Drag_And_Drop in response to this event:

overriding
procedure On_Drag
  (Object : in out Test_Window;
   X, Y   : AWT.Inputs.Fixed)
is
   use all type AWT.Inputs.Action_Kind;
   use type AWT.Inputs.Fixed;
begin
   AWT.Drag_And_Drop.Set_Action (if X < 100.0 and Y < 100.0 then Copy else None);
end On_Drag;

The parameters X and Y indicate to where the user is trying to drag something. The procedure Set_Action in package AWT.Drag_And_Drop must be called to set the requested action that must happen when the user drops something on the current focused window. Valid values are Copy, Move, Ask, or None.

Dropping

Procedure On_Drop is invoked when the user finally drops something on the window. The function Valid_Action must be used to determine the final action that must occur. If the value is not equal to None, a data transfer can be initiated or the application may ask the user to make a choice between some actions.

overriding
procedure On_Drop
  (Object : in out Test_Window)
is
   use all type AWT.Inputs.Action_Kind;

   Action : constant AWT.Inputs.Action_Kind := AWT.Drag_And_Drop.Valid_Action;
begin
   if Action /= None then
      Object.Dnd_Signal.Set;
   end if;
end On_Drop;

A data transfer must not be initiated in the On_Drop callback, but in the task containing the event loop. In the example above, an object of the protected type Signal with a procedure Set and an entry Wait is used to set a signal.

In the event loop, wait for the signal and then perform the drag-and-drop by calling subprogram Get in package AWT.Drag_And_Drop. This can be a function that blocks until the data transfer has been completed, or it can be a procedure with a callback to a protected procedure with one parameter of the type AWT.SU.Unbounded_String. In both cases, the actual data transfer may take place in some background task.

After the data transfer has been completed, call procedure Finish to either accept or reject the drag-and-drop operation. To accept the operation, use Copy or Move. If the last set action was Ask, then the value may either be Copy or Move, otherwise it must match the last set action. To reject the operation, use None.

while not Window.Should_Close loop
   AWT.Process_Events (Interval);

   select
      Window.Dnd_Signal.Wait;

      declare
         Result : constant String := AWT.Drag_And_Drop.Get;
      begin
         Orka.OS.Put_Line ("value: '" & Orka.Strings.Strip_Line_Term (Result) & "'");
         AWT.Drag_And_Drop.Finish (AWT.Inputs.Copy);
      end;
   else
      null;
   end select;
end loop;