unit Unit1;

interface

uses
   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
   Dialogs, ExtCtrls, UThread;

type
   TForm1 = class(TForm)
     procedure FormCreate(Sender: TObject);
     procedure ThreadPaint(var msg: TMessage); message WM_THREADPAINT;
   private
   public
     { Public-Deklarationen }
   end;

var
   Form1: TForm1;
   Paintthread : TPaintThread;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  Paintthread := TPaintThread.Create(false, self.Handle);
end;

procedure TForm1.ThreadPaint(var msg: TMessage);
begin
  Paintthread.LockBMP;
  try
    self.Canvas.Draw(40, 40, PaintThread.BMP);
  finally
    Paintthread.UnlockBMP;
  end;
end;

end.

unit UThread;

interface

uses
   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
   Dialogs, ComCtrls, StdCtrls, ExtCtrls, SyncObjs;
     
type
  TPaintThread = class(TThread)
  private
    FBMP: TBitmap;
    FBMPCS: TCriticalSection;
    FHandle: THandle;
  protected
    procedure Execute; override;
  public
    constructor Create(CreateSuspended: Boolean; aMainformHandle: THandle);
    procedure LockBMP;
    procedure UnlockBMP;
    property BMP: TBitmap read FBMP;
  end;

const
  WM_THREADPAINT = WM_USER + 1234;

implementation

constructor TPaintThread.Create(CreateSuspended: Boolean; aMainformHandle: THandle);
begin
  inherited Create(CreateSuspended);
  FBMPCS := TCriticalSection.Create;
  FBMP := TBitmap.Create;
  FBMP.Width := 300;
  FBMP.Height := 300;
  FHandle := aMainformHandle;
end;

procedure TPaintThread.Execute;
begin
  while not Terminated do
  begin
    LockBMP;
    try
      FBMP.Canvas.MoveTo(Random(300), Random(300));
      FBMP.Canvas.LineTo(Random(300), Random(300));
    finally
      UnlockBMP;
    end;
    PostMessage(FHandle, WM_THREADPAINT, 0, 0);
    sleep(1);
  end;
end;

procedure TPaintThread.LockBMP;
begin
  FBMPCS.Enter;
  FBMP.Canvas.Lock;
end;

procedure TPaintThread.UnlockBMP;
begin
  FMP.Canvas.Unlock;
  FBMPCS.Leave;
end;

end.