Question

I need to Drag and Drop multiple items inside my Tlistbox.
The code which I am referring to is

var 
   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; 

from here.
It works fine but what I need to achieve is like this
.

Why I want this is because there is certain order that corresponds to these listbox items.
So instead of just having manually select each item and drag drop it,I want to enable multiple drag drop.

Any views on how can I achieve this is appreciated.
Also can suggest the use of other components if the following is possible using the same.

Answer 1

This is surprisingly tricky to do well (see my first revision of this answer for an example of how to get it wrong).

Here’s a rather easy to understand approach that solves the problem by:

1. Removing all selected items from the list and storing them in a temporary string list.
2. Re-adds the items to the list starting at the target index.
3. Re-selects each re-added item.

 

procedure TForm1.ListBox1DragDrop(Sender, Source: TObject; X, Y: Integer);
var
  ListBox: TListBox;
  i, TargetIndex: Integer;
  SelectedItems: TStringList;
begin
  Assert(Source=Sender);
  ListBox := Sender as TListBox;
  TargetIndex := ListBox.ItemAtPos(Point(X, Y), False);
  if TargetIndex<>-1 then
  begin
    SelectedItems := TStringList.Create;
    try
      ListBox.Items.BeginUpdate;
      try
        for i := ListBox.Items.Count-1 downto 0 do
        begin
          if ListBox.Selected[i] then
          begin
            SelectedItems.AddObject(ListBox.Items[i], ListBox.Items.Objects[i]);
            ListBox.Items.Delete(i);
            if i<TargetIndex then
              dec(TargetIndex);
          end;
        end;

        for i := SelectedItems.Count-1 downto 0 do
        begin
          ListBox.Items.InsertObject(TargetIndex, SelectedItems[i], SelectedItems.Objects[i]);
          ListBox.Selected[TargetIndex] := True;
          inc(TargetIndex);
        end;
      finally
        ListBox.Items.EndUpdate;
      end;
    finally
      SelectedItems.Free;
    end;
  end;
end;

Now, it is possible to do this with a series of calls to Move but it’s hard to get it right. Each time you make a move all the indices of the selected items change. The approach I give above is my preferred method for solving this problem. Incidentally I recently worked on exactly the same problem in the context of a tree view and it’s quite tricky there too!