How to Move ListBox Items with the Mouse (Drag and Drop)

The TListBox Delphi component displays a collection of items in a scrollable list. Delphi makes it easy to program dragging and dropping into your applications.

Here's how to allow a user to rearrange (change item's position) the items of a ListBox using drag and drop:

1. Drop a TListBox (named ListBox1) on a form
2. Add several strings using the Items property
3. Set ListBox1's DragMode to dmAutomatic (in Form's OnCreate or using Object Inspector at design-time).
4. Handle LisBox-es MouseDown, DragOver and DragDrop events

var // form level
   StartingPoint : TPoint;

implementation

...

procedure TForm1.FormCreate(Sender: TObject) ;
begin
   ListBox1.DragMode := dmAutomatic;
end;

procedure TForm1.ListBox1DragDrop(Sender, Source: TObject; X, Y: Integer) ;
var
   DropPosition, StartPosition: Integer;
   DropPoint: TPoint;
begin
   DropPoint.X := X;
   DropPoint.Y := Y;
   with Source as TListBox do
   begin
     StartPosition := ItemAtPos(StartingPoint,True) ;
     DropPosition := ItemAtPos(DropPoint,True) ;

     Items.Move(StartPosition, DropPosition) ;
   end;
end;

procedure TForm1.ListBox1DragOver(Sender, Source: TObject; X, Y: Integer; State: TDragState; var Accept: Boolean) ;
begin
   Accept := Source = ListBox1;
end;

procedure TForm1.ListBox1MouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer) ;
begin
   StartingPoint.X := X;
   StartingPoint.Y := Y;
end;

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