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I use TMemo as a Log and I add lines to it every time an event has been called. Before I add a new line I use BeginUpdate and then EndUpdate and also have DoubleBuffered enabled. However, it seems like that the scrollbar(s) are not double buffered at all an keep flickering. Is there a way I can also set the scrollbars to DoubleBuffered := True?

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It seems like that the boarder is flickering too. Not sure if that's associated with the scrollbar(s). 

unit uMainWindow; interface uses Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.ExtCtrls, IdContext, IdBaseComponent, IDGlobal, IdComponent, IdCustomTCPServer, IdTCPServer, Vcl.ComCtrls, Winsock; type TMainWindow = class(TForm) TCPServer: TIdTCPServer; StatusBar: TStatusBar; PageControl: TPageControl; ConfigSheet: TTabSheet; StartButton: TButton; PortEdit: TLabeledEdit; LogSheet: TTabSheet; LogMemo: TMemo; LogEdit: TLabeledEdit; TCPLogSheet: TTabSheet; TCPLogEdit: TLabeledEdit; TCPLogMemo: TMemo; CheckBox1: TCheckBox; procedure StartButtonClick(Sender: TObject); private public end; // ============================= Public Vars =================================== var MainWindow : TMainWindow; hServer : TSocket; sAddr : TSockAddrIn; ListenerThread : TThread; // =============================== Threads ===================================== type TListenThread = class (TThread) private procedure WriteToTCPLog (Text : String); public Form : TMainWindow; procedure Execute; override; end; type TReceiveThread = class (TThread) private procedure WriteToTCPLog (Text : String); public Form : TMainWindow; hSocket : TSocket; IP : String; procedure Execute; override; end; implementation {$R *.dfm} // ================================= Uses ====================================== uses uTools, uCommonConstants; // ================================== TListenThread ============================ procedure TListenThread.WriteToTCPLog(Text: string); var MaxLines : Integer; begin if not(Form.CheckBox1.Checked) then exit; if GetCurrentThreadId = MainThreadID then begin Form.TCPLogMemo.Lines.BeginUpdate; MaxLines := StrToInt(Form.TCPLogEdit.Text); if Form.TCPLogMemo.Lines.Count >= MaxLines then begin repeat Form.TCPLogMemo.Lines.Delete(0); until Form.TCPLogMemo.Lines.Count < MaxLines; end; Form.TCPLogMemo.Lines.Add (Text); Form.TCPLogMemo.Lines.EndUpdate; end else begin Text := '[' + DateToStr (Now) + ' - ' + TimeToStr(Now) + '] ' + Text; Synchronize(procedure begin WriteToTCPLog(Text); end); end; end; procedure TListenThread.Execute; var iSize : Integer; hClient : TSocket; cAddr : TSockAddrIn; SynchIP : String; begin WriteToTCPLog ('Server started'); while not (terminated) do begin iSize := SizeOf(cAddr); hClient := Accept(hServer, @cAddr, @iSize); if (hClient <> INVALID_SOCKET) then begin SynchIP := inet_ntoa(cAddr.sin_addr); WriteToTCPLog(SynchIP + ' - connected.'); with TReceiveThread.Create (TRUE) do begin FreeOnTerminate := TRUE; hSocket := hClient; IP := SynchIP; Form := Self.Form; Resume; end; end else begin break; end; end; WriteToTCPLog('Server stopped.'); end; // ==================================== TReceiveThread ========================= procedure TReceiveThread.WriteToTCPLog(Text: string); var MaxLines : Integer; begin if not(Form.CheckBox1.Checked) then exit; if GetCurrentThreadId = MainThreadID then begin Form.TCPLogMemo.Lines.BeginUpdate; MaxLines := StrToInt(Form.TCPLogEdit.Text); if Form.TCPLogMemo.Lines.Count >= MaxLines then begin repeat Form.TCPLogMemo.Lines.Delete(0); until Form.TCPLogMemo.Lines.Count < MaxLines; end; Form.TCPLogMemo.Lines.Add (Text); Form.TCPLogMemo.Lines.EndUpdate; end else begin Text := '[' + DateToStr (Now) + ' - ' + TimeToStr(Now) + '] ' + Text; Synchronize(procedure begin WriteToTCPLog(Text); end); end; end; procedure TReceiveThread.Execute; var iRecv : Integer; bytBuf : Array[0..1023] of byte; begin iRecv := 0; while true do begin ZeroMemory(@bytBuf[0], Length(bytBuf)); iRecv := Recv(hSocket, bytBuf, SizeOf(bytBuf), 0); if iRecv > 0 then begin WriteToTCPLog(IP + ' - data received (' + inttostr(iRecv) + ' bytes).'); end; if iRecv <= 0 then break; end; WriteToTCPLog(IP + ' - disconnected.'); closesocket(hSocket); end; // ================================= TMainWindow =============================== procedure TMainWindow.StartButtonClick(Sender: TObject); begin if StartButton.Caption = 'Start' then begin try hServer := Socket(AF_INET, SOCK_STREAM, 0); sAddr.sin_family := AF_INET; sAddr.sin_port := htons(StrToInt(PortEdit.Text)); sAddr.sin_addr.S_addr := INADDR_ANY; if Bind(hServer, sAddr, SizeOf(sAddr)) <> 0 then raise Exception.Create(''); if Listen(hServer, 3) <> 0 then raise Exception.Create(''); except OutputError (Self.Handle, 'Error','Port is already in use or blocked by a firewall.' + #13#10 + 'Please use another port.'); exit; end; ListenerThread := TListenThread.Create (TRUE); TListenThread(ListenerThread).Form := Self; TListenThread(ListenerThread).Resume; StartButton.Caption := 'Stop'; end else begin closesocket(hServer); ListenerThread.Free; StartButton.Caption := 'Start'; end; end; end.
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    Can you show some code or explain what you're trying to solve? I use a TMemo as a log similarly in multiple apps, don't use Begin/EndUpdate or DoubleBuffered, and I don't have any issues. Are you using Lines.Add()? Commented Oct 6, 2013 at 13:54
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    Try to not use BeginUpdate/EndUpdate function . Commented Oct 6, 2013 at 19:23
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    Please show your code. Something's not right there. Commented Oct 6, 2013 at 21:42
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    The only way I could get it to "flicker" was by removing the begin/endupdate because the delete moves the scroll button up and the add moves it down. I think this uses GDI. Perhaps your machine has GDI issues (is slow)? Maybe you could describe the flickering. Commented Oct 7, 2013 at 20:21
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    If the worker threads are putting the log information into a TThreadedQueue, and this queue is drained in the main thread inside a timer event, the strain of the GUI will ease and likely the flicker will not be of any problem anymore. All in all, this is what David is proposing. Commented Oct 7, 2013 at 20:23

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I doubt very much if double buffering will help you here. In fact, as a general rule I always recommend avoiding it. Modern operating systems do it automatically for you and adding more and more layers of buffering just hurts performance and changes nothing visually.

Your problem sounds very much as though you are updating the GUI too frequently. Instead of buffering the painting, buffer the text content of the GUI control.

  1. Create a text buffer, a string list, to hold new log messages.
  2. Add a timer with a refresh rate of, say 5Hz. Choose a different rate if you prefer.
  3. When you have new log information, add this to the buffer string list.
  4. When the timer fires, add the buffer to the GUI control, and flush the buffer list.

Perform all interaction with the buffer list on the main thread to avoid date races.

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5 Comments

The text content of the Memo does not flicker. Just the boarder and the scrollbar. Thanks for your answer tho.
I guess you need to give us a repro then. Because if the issue is not the update frequency then something else is the problem. How can we reproduce the problem?
I added some code. You will need a TCP Client that sends constantly data
The behaviour cannot be reproduced without TCP comms?
How about you make an SSCCE. With one of those, I'd investigate and solve the problem.

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