Stop and restore updating in TScrollBox?

Question

Since Vcl.Forms.TScrollBox does not have BeginUpdate / EndUpdate methods, I use this code to reduce flickering while loading a document in a viewer which is a child of TScrollBox container:

procedure MyLockWinControl(const WC: Vcl.Controls.TWinControl; ALock: Boolean);
begin
  if (not Assigned(WC)) or (WC.Handle = 0) then EXIT;
  if ALock then
    WC.Perform(WM_SETREDRAW, 0, 0)
  else
  begin
    WC.Perform(WM_SETREDRAW, 1, 0);
    RedrawWindow(WC.Handle, nil, 0, RDW_ERASE or RDW_FRAME or RDW_INVALIDATE or RDW_ALLCHILDREN);
  end;
end;

procedure TFormMain.LoadDocFile(const ADocFile: string);
begin
  Screen.Cursor := crHourGlass;
  try
    MyLockWinControl(ScrollBox, True);
    try
      DoLoadDocFile(ADocFile);
    finally
      MyLockWinControl(ScrollBox, False);
    end;
  finally
    Screen.Cursor := crDefault;
  end;
end;

This works well except in some rare cases where the application reproducibly crashes on MyLockWinControl(ScrollBox, False);

So my question is: Is there a bug in MyLockWinControl, or is there a better method to stop and restore updating in TScrollBox?

Answer 1

The first thing I'd try is replacing your call to RedrawWindow with a call to WC.Invalidate.

I have to suppose that TWinControl is doing some stuff in addition to the API redraw that isn't getting done reliably when you call RedrawWindow directly.

Answer 2

I have used the following code quite reliably (with a TScrollBox):

  SendMessage(scrollbox.Handle, WM_SETREDRAW, 0, 0);
  try
    //do some stuff with the scrollbox here
  finally
    SendMessage(scrollbox.Handle, WM_SETREDRAW, 1, 0);
    RedrawWindow(scrollbox.Handle, nil, 0, RDW_ERASE or RDW_INVALIDATE or RDW_FRAME or RDW_ALLCHILDREN);
  end;

Your code (via the MyLockWinControl procedure) seems to essentially do the same thing - except for the use of Perform (send a message directly to the control) instead of SendMessage (uses the Windows message queue). You have also mentioned that you use a component that is not TScrollBox, but a descendent - so the issue could very well lie there as well.

Only thing I can suggest is to put a try/except block around your code and catch/log the error. That may provide further insight into where the issue lies...