Author: Jonas Bilinkevicius

How do I do drag and drop in a TListbox?

Answer:

Adding Drag and Drop facilities to a listbox is a matter of checking to see if 
there is an item under the mouse pointer in the MouseDown event, and if so save the 
item text and the index number to variables. Then check the MouseUp event to see if 
there is a different item under the mouse. If so, delete the old item and insert a 
copy of it in the new position.

Firstly, add three variables to the private section:

1   { Private declarations }
2     Dragging: Boolean;
3     OldIndex: Integer;
4     TempStr: string;
5   
6   //Then add the following code to the MouseUp and MouseDown events:
7   
8   procedure TForm1.ListBox1MouseUp(Sender: TObject; Button: TMouseButton;
9       Shift: TShiftState; X, Y: Integer);
10  var
11      Index: integer;
12  begin
13      if Dragging then
14          begin
15            Index := ListBox1.ItemAtPos(point(x, y), true);
16          if(Index > -1) and (Index <> OldIndex) then
17              begin
18                ListBox1.Items.Delete(OldIndex);
19              ListBox1.Items.Insert(Index, TempStr);
20              ListBox1.ItemIndex := Index;
21            end;
22        end;
23      Dragging := false;
24  end;
25  
26  procedure TForm1.ListBox1MouseDown(Sender: TObject; Button: TMouseButton;
27      Shift: TShiftState; X, Y: Integer);
28  var
29      Index: integer;
30  begin
31      Index := ListBox1.ItemAtPos(point(x, y), true);
32      if Index > -1 then
33          begin
34            TempStr := ListBox1.Items[Index];
35          OldIndex := Index;
36          Dragging := true;
37        end;
38  end;