Question

maybe i dont know delphi all that well, however i wish to ask you:

at this site : http://blogs.teamb.com/joannacarter/2004/06/30/690 i found an implemetation of observer pattern based on iterface.

when doing attach , there is a call to this:

procedure TSubject.Attach(Observer: IObserver);
begin
    if fObservers = nil then
      fObservers := TInterfaceList.Create;
    fObservers.Add(AObserver);
    Notify;
end;

and in the detach it has the code

procedure TSubject.Detach(Observer: IObserver);
begin
   if fObservers <> nil then
    begin
      fObservers.Remove(AObserver);
      if fObservers.Count = 0 then
        fObservers := nil;
    end;
end;

should it be :

procedure TSubject.Detach(Observer: IObserver);
begin
   if fObservers <> nil then
    begin
      fObservers.Remove(AObserver);
      if fObservers.Count = 0 then begin
        fObservers.Free; 
        fObservers := nil;
      end;
    end;
end;

Answer 1

There is no need to add fObservers.Free; statement. IInterface will take care of adding and releasing the fObservers .

Delphi uses _AddRef and _Release to manage the lifetime of interfaced objects.

When you assign an interface reference to an interface variable, Delphi automatically calls _AddRef.

When the variable goes out of scope, Delphi automatically calls _Release.

For more info go through this link.