Comments for Joanna Carter

Comment on The Iterator Pattern by giulio

giulio — Tue, 08 Aug 2006 17:49:29 +0000

Why don’t to add an implementation template using C# enerics? Anyway… very good!

Bye,

Giulio

Comment on The Observer Pattern by Mike S

Mike S — Tue, 28 Mar 2006 00:19:14 +0000

Nice article and thanks for putting it together.

Do you have a working example available for download? It is much easier to start with something that works and I have been completely unsuccessful in splicing

together the various snippets.

Comment on OO Frameworks Masterclass by wl

wl — Mon, 12 Sep 2005 20:36:26 +0000

no comment

Comment on The Iterator Pattern by Jose Luis

Jose Luis — Sat, 20 Aug 2005 15:55:57 +0000

If I add the "friendly" class to be able to declare the iterator class into another unit separated from TCustomerList class, I would need to add into the TCustomerIterator unit the uses of the TCustomerList.

Then, when I would like to add the GetIterator into the TCustomerList to create the iterator from withing the list ifself, am I going to have a circular reference?

Comment on The Iterator Pattern by Shawn Stamps

Shawn Stamps — Sun, 10 Jul 2005 19:05:11 +0000

Yes, it is an infinite loop. The correct code would be:

Repeat

Result := inherited Next;

Until Not Result Or Result And CurrentItem.CreditStop;

I’ve coded that kind of error so many times that I usually catch it right after I type it in.

Comment on The Observer Pattern by Joanna Carter

Joanna Carter — Thu, 09 Jun 2005 13:45:00 +0000

No, using interface references for delegation is just as valid as objects as long as you are aware of the refcounting and QueryInterface implications.

Comment on The Observer Pattern by Ilia Frenkel

Ilia Frenkel — Thu, 09 Jun 2005 13:34:16 +0000

Great article!

I have some remark, though:

TClockTimer = class(TInterfacedObject, IClockTimer, ISubject)

private

fTimer: TTimer;

fInternalTime: TDateTime;

fSubject: –>ISubject<–;

function GetTime: TDateTime;

procedure Tick(Sender: TObject);

property Subject: –>ISubject<–

read fSubject

implements ISubject;

public

constructor Create;

end;

Shouldn’t it be TSubject?

—

Best regards, Ilia Frenkel.

Comment on The Iterator Pattern by Stephen Melnyk

Stephen Melnyk — Tue, 23 Nov 2004 21:30:54 +0000

Thanks for the article, Joanna. I appreciate you taking the time to write it.

Could you clarify one point for me? In the Skip Iterators section, you implement TBadCustomerIterator.Next with:

repeat
Result := inherited Next;
until Result and CurrentItem.CreditStop

Isn't this an infinite loop? The last item in the list is going to return False from the inherited Next.

Stephen

Comment on The Observer Pattern by Joanna Carter

Joanna Carter — Wed, 20 Oct 2004 00:18:26 +0000

> Great article! But I think there are some small

> problems in it.

> 1 just as Lars Christian Svane said,

> (DigitalClock1 as ISubject).Attach(fClockTimer as

> IObserver);

> should be:

> (fClockTimer as ISubject).Attach(DigitalClock1 as

> IObserver);

Yes, I should change this

> 2

> fController: Pointer;

> should be:

> fController: IInterface;

> Why?

> If fController is defined as a Pointer type, I

> think it is a pointer type cast and not a

> interface reference in code

> "fController := Pointer(Controller)". you have to

> add "Controller._AddRef;" to avoid a access

> violation.

The use of the Pointer is known as a ‘weak reference’ and is used to break the circular refcount problem where an outer class holds a reference to an inner class and an inner class holds a reference to the outer class. This then means that neither of the refcounts get to drop to zero thus causing a memory leak. Assigning to a Pointer means that the refcount of the Controller is not incremented from the one that is holding the outer (controller) object; therefore, when that ref falls out of scope it then drops to zero releasing the outer object which, in turn releases the inner object.

Try it

Joanna

Comment on The Observer Pattern by wango

wango — Tue, 19 Oct 2004 08:27:31 +0000

Great article! But I think there are some small problems in it.

1 just as Lars Christian Svane said,
(DigitalClock1 as ISubject).Attach(fClockTimer as IObserver);

should be:

(fClockTimer as ISubject).Attach(DigitalClock1 as IObserver);

2

fController: Pointer;

should be:

fController: IInterface;

constructor TSubject.Create(const Controller: IInterface);

begin

inherited Create;

fController := Pointer(Controller);

should be:

fController := Controller;

end;

procedure TSubject.Notify;

var

i: Integer;

begin

if fObservers <> nil then

for i := 0 to Pred(fObservers.Count) do

(fObservers[i]. as IObserver).Update(IInterface (fController));

should be:

(fObservers[i] as IObserver).Update(fController);

end;

Why?

If fController is defined as a Pointer type, I think it is a pointer type cast and not a interface reference in code "fController := Pointer(Controller)". you have to add "Controller._AddRef;" to avoid a access violation.