Delphi - start/stop processing

Have a TButton, name:=StartStopBtn, caption:='start';
Have a var Stop:boolean :=true;

procedure TForm1.StartStopBtnClick(..);
begin
 if stop then begin  //start processing
   StartStopBtn.caption:='stop';
   Stop:=false;
   while not stop do begin   // almost endless loop
     // do your processing here
     Eg : 
     image1.draw(processing_list.next);
     ..
     ..
     Application.processmessages; // let the user stop the process
    end; // while
   StartStopBtn.caption:='start';
  end  // 
 else begin // stop the processing
   StartStopBtn.caption:='start';  
   Stop:=true;
  end;
end;

notes

• The endless loop is terminated when the processing is complete.
• The code can be changed that the stopped processing can be resumed.
• The caption of the button can show a loop counter