Scroll TTreeView while dragging over/near the edges - delphi

Question

I have a TTreeView that can have lots of nodes, when a lot of nodes are expanded the tree uses a lot of screen space.
Now suppose I want to drag a node that is near the bottom of the TreeView to the top, I can't physically see the top part of the TreeView because the node I am selecting is at the bottom. When dragging the node to the top of the TreeView I would like the TreeView to automatically scroll with me when dragging, by default this does not seem to happen.
A perfect example of this behaviour is seen in Windows Explorer. If you try to drag a file or folder, when you hover the dragged item (node) it automatically scrolls up or down depending on cursor position.
Hope that makes sense.
PS, I already know how to drag nodes, I want the TreeView to scroll with me when dragging if hovering near the top or bottom of the TreeView.
Thanks.

Answer 1

This is the code I use. It will work for any TWinControl descendent: list box, tree view, list view etc.
type
TAutoScrollTimer = class(TTimer)
private
FControl: TWinControl;
FScrollCount: Integer;
procedure InitialiseTimer;
procedure Timer(Sender: TObject);
public
constructor Create(Control: TWinControl);
end;
{ TAutoScrollTimer }
constructor TAutoScrollTimer.Create(Control: TWinControl);
begin
inherited Create(Control);
FControl := Control;
InitialiseTimer;
end;
procedure TAutoScrollTimer.InitialiseTimer;
begin
FScrollCount := 0;
Interval := 250;
Enabled := True;
OnTimer := Timer;
end;
procedure TAutoScrollTimer.Timer(Sender: TObject);
procedure DoScroll;
var
WindowEdgeTolerance: Integer;
Pos: TPoint;
begin
WindowEdgeTolerance := Min(25, FControl.Height div 4);
GetCursorPos(Pos);
Pos := FControl.ScreenToClient(Pos);
if not InRange(Pos.X, 0, FControl.Width) then begin
exit;
end;
if Pos.Y<WindowEdgeTolerance then begin
SendMessage(FControl.Handle, WM_VSCROLL, SB_LINEUP, 0);
end else if Pos.Y>FControl.Height-WindowEdgeTolerance then begin
SendMessage(FControl.Handle, WM_VSCROLL, SB_LINEDOWN, 0);
end else begin
InitialiseTimer;
exit;
end;
if FScrollCount<50 then begin
inc(FScrollCount);
if FScrollCount mod 5=0 then begin
//speed up the scrolling by reducing the timer interval
Interval := MulDiv(Interval, 3, 4);
end;
end;
if Win32MajorVersion<6 then begin
//in XP we need to clear up transient "fluff"; results in flickering so only do it in XP where it is needed
FControl.Invalidate;
end;
end;
begin
if Mouse.IsDragging then begin
DoScroll;
end else begin
Free;
end;
end;
Then to use it you add an OnStartDrag event handler for the control and implement it like this:
procedure TMyForm.SomeControlStartDrag(Sender: TObject; var DragObject: TDragObject);
begin
TAutoScrollTimer.Create(Sender as TWinControl);
end;

Answer 2

Here's an alternative based on the fact that the selected node always automatically scrolls in view.
type
TForm1 = class(TForm)
TreeView1: TTreeView;
TreeView2: TTreeView;
procedure TreeViewDragOver(Sender, Source: TObject; X, Y: Integer;
State: TDragState; var Accept: Boolean);
procedure TreeViewEndDrag(Sender, Target: TObject; X, Y: Integer);
procedure TreeViewMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
private
FDragNode: TTreeNode;
FNodeHeight: Integer;
end;
...
procedure TForm1.TreeViewMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
with TTreeView(Sender) do
begin
FDragNode := GetNodeAt(X, Y);
if FDragNode <> nil then
begin
Selected := FDragNode;
with FDragNode.DisplayRect(False) do
FNodeHeight := Bottom - Top;
BeginDrag(False, Mouse.DragThreshold);
end;
end;
end;
procedure TForm1.TreeViewDragOver(Sender, Source: TObject; X, Y: Integer;
State: TDragState; var Accept: Boolean);
var
Pt: TPoint;
DropNode: TTreeNode;
begin
Accept := Source is TTreeView;
if Accept then
with TTreeView(Source) do
begin
if Sender <> Source then
Pt := ScreenToClient(Mouse.CursorPos)
else
Pt := Point(X, Y);
if Pt.Y < FNodeHeight then
DropNode := Selected.GetPrevVisible
else if Pt.Y > (ClientHeight - FNodeHeight) then
DropNode := Selected.GetNextVisible
else
DropNode := GetNodeAt(Pt.X, Pt.Y);
if DropNode <> nil then
Selected := DropNode;
end;
end;
procedure TForm1.TreeViewEndDrag(Sender, Target: TObject; X, Y: Integer);
var
DropNode: TTreeNode;
begin
with TTreeView(Sender) do
if Target <> nil then
begin
DropNode := Selected;
DropNode := Items.Insert(DropNode, '');
DropNode.Assign(FDragNode);
Selected := DropNode;
Items.Delete(FDragNode);
end
else
Selected := FDragNode;
end;
You may want to link the OnDragOver event handler to the parent of the TreeView too, which results in scrolling ánd dropping when the mouse is outside the TreeView. If you dó want the scrolling, but not the dropping when the mouse is outside the TreeView, then check if Target = Sender in the OnEndDrag event handler.

Answer 3

Just to be complete, workarounds like in the other answers are not required anymore. Later versions have an option for this:
TreeOptions.AutoOptions.toAutoScroll := True