type
   TMyTask = class
   private
      FCalculator     : TCalculator;
      FCalculatedFlag : boolean;
      
      procedure OnCalculated(Sender: TObject);      
      ...
   end;
   
   
//------------------------------------------------------------------------------
constructor TMyTask.Create();
begin
   FCalculatedFlag := false;
   
   FCalculator := TCalculator.Create();
   FCalculator.OnCalculated := OnCalculated;   
end;
//------------------------------------------------------------------------------
procedure MyProcessMessages();
var
   lpMsg    : TMsg;
begin
   while PeekMessage(lpMsg,0,0,0,PM_REMOVE) do
   begin
      TranslateMessage(lpMsg);
      DispatchMessage(lpMsg);
   end;
end;
//------------------------------------------------------------------------------
procedure TMyTask.Execute();
begin
   FCalculatedFlag := false;
   
   FCalculator.Calculate();
   
   while not FCalculatedFlag do
   begin
      sleep(10);
      MyProcessMessages();
   end;   
   
   //...
end;
//------------------------------------------------------------------------------
procedure TMyTask.OnCalculated(Sender: TObject);
begin
   ...
   FCalculatedFlag := true;
end;