Scroll TTreeView while dragging over/near the edges

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;