How do I implement drag and drop between two list boxes on a form?
Dragging items from one list box and dropping them into another is, with Delphi,
amazingly easy to implement. I'm going to show you a couple of very simple routines to
handle two types of dragging and dropping. The first involves moving items from one list
into another, and the second involves dragging items within the same list box.
Dragging Between Two List Boxes
To perform dragging between two list boxes, you first have to make the items in the
source list box "drag-able." The easiest way to do this is to set the DragMode
property of the list box to dmAutomatic. (I realize there are those of you who
prefer to have more precise control over dragging operations by programmatically starting
and ending drag operations with BeginDrag and EndDrag. But since this discussion is mainly
geared towards the novice, I'm going to skip that topic in favor of getting the user up
and running right away).
The second thing to do is to set the MultiSelect property to True. This is
purely optional, but it's quite useful for transferring multiple items between lists.
The next thing you have to do is let the target list accept items being dropped.
The way you typically do this is with the target list's OnDragOver event. Here's a simple
example:
procedure TMainForm.lstStudyTracersDragOver(Sender, Source: TObject; X,
Y: Integer; State: TDragState; var Accept: Boolean);
begin
{Only let another TListBox drop items}
if Source is TListBox then
Accept := True
else
Accept := False;
end;
The conditional above checks to see if the source is a list box. If it is, the target
sets its Accept property to true. The method that controls dragging and dropping is
the target list box's OnDragDrop method. But instead of writing code in the method to
handle the dropped items directly, I use a generalize method instead:
procedure TransferItemsNoDups(Sender, Source : TObject);
var
I, N : Integer;
Found : Boolean;
begin
with (Source AS TListBox) do begin
for I := 0 to Items.Count - 1 do
if Selected[I] then begin
Found := False;
for N := 0 to (Sender AS TListBox).Items.Count - 1 do
if (Sender AS TListBox).Items[N] = Items[I] then
Found := True;
if NOT Found then
(Sender AS TListBox).Items.Add(Items[I]);
end;
end;
end;
Notice the name of the procedure: TransferItemsNoDups. This method will iterate
through the items in the source list. For each selected item to be transferred, it checks
to see if the item exists in the target list. If it does, then the item is skipped;
otherwise, it's added to the end of the list. Since this came out of an application I
wrote, there's one bit of code I didn't insert that you might consider doing yourself;
that is, some code that will delete the selected items once they're transferred. In that
case, you'd probably write a procedure that looks like the following:
procedure TransferItems(Sender, Source : TObject);
var
I : Integer;
Found : Boolean;
begin
with (Source AS TListBox) do begin
for I := 0 to Items.Count - 1 do
if Selected[I] then
(Sender AS TListBox).Items.Add(Items[I]);
for I := 0 to Items.Count - 1 DownTo 0 do
if Selected[I] then
Items.Delete(I);
end;
end;
Notice that I didn't do any duplicate checking in the procedure above. That's because
it would be useless. This methodology is best used between two lists where you allow your
user to drag and drop between the two.
Moving Items Within a List Box
Moving items within a list box is a pretty simple thing to do. Look at the procedure
below:
procedure MoveItems(var Target : TObject; X, Y : Integer);
var
NPos : Integer;
begin
with Target AS TListBox do begin
NPos := ItemAtPos(Point(X, Y), False);
if (NPos >= Items.Count) then
Dec(NPos);
{Move selected item to the new position}
Items.Move(ItemIndex, NPos);
ItemIndex := NPos;
end;
end;
The first thing that happens in the code is the drop position is read into an integer
variable. Then the procedure checks the value of the position to determine if it's in the
range of the items of the list box. If not, its value is decremented to either 0 or to the
count of the items. After that, the Move method is invoked to move the currently
selected item to the new drop position, and the currently selected item is reselected in
the position it was dropped.
I didn't cover all the subtle nuances of drag and drop here. My purpose was to give you
something to start with. I suggest you pore over the Delphi user manual and online help
for more in-depth discussions of drag and drop functionality.
Copyright © 1997 Brendan V. Delumpa All Rights Reserved
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