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.

Shouldn’t it be TSubject?

—

Best regards, Ilia Frenkel.

Yes, I should change this

> 2

> fController: Pointer;

> should be:

> fController: IInterface;

<snip>

> 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

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.

thx again, like 2c more btw

But, look, if you can provide some more sofisticated examples it will be great.

Patterns is great stuff, but Delphi itself imply a lot of limitations. Just assume that your Observer must work in threading environment and you will see what i mean.

Some stuff in VCL might turn our life in nightmare
I know it’s not about patterns, but as far as we are talking about Delphi implementation it would be nice to see.

I changed the code as in my last comment. Then my test application compiled and run fine - with ONE Digital Clock on the form.

Then I added three more. And then I get an access violation when I close the application. With up to three Digital Clocks it works fine. On four - BUM!

It happens at a place after the Interfacelist starts to be cleared. After the "finally" in Classes file.

Can you explaine this?

Regards,

Lars

I have been waiting for your book to learn more about OOP, design patterns and MVP. Don’t hesitate to continue your work on this page. I can hardly wait.

As far as I can see, there is a small error:

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

should be:

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

Your text is correct - but the code seems wrong (compiling gives an error).

Regards,

Lars

Potential future revisions could take the chance to include the idea of Message parameters to the Notify/Update-methods.