[webbeans-dev] removeObserver type

[David Allen]

On Wed, 2009-05-27 at 14:07 -0700, Gavin King wrote:
> On Wed, May 27, 2009 at 1:47 PM, Clint Popetz <cpopetz at gmail.com> wrote:
> 
> >> (2) adding a type variable doesn't help you, since it just gets erased
> >> at runtime
> >>
> >> You can't do T.class in Java :-)
> >
> > Good point.  Is the parameterization of addObserver just to ensure that
> > Observer<T> is registered for Class<T>?
> 
> yes, exactly.
> 
> So now I'm wondering (again) whether the TypeLiteral/Class/Type on
> addObserver() is really adding any value here.

I thought the TypeLiteral/Class/Type was to identify the event type that
the observer is interested in.  Like you said, the type variable does us
no good; however, we have to know somehow for an event type T that some
registered Observer<?> is interested in that event.

So where do we get T now that the signature has changed to not include
it?

> 
> It is there to accommodate two cases:
> 
> (1) where the event type that you are registering an observer for is a
> type variable, in which case you could potentially have a Class<T> or
> a TypeLiteral<T>, which hold an actual value for T at runtime, but an
> Observer<T> which is generic and does not.
> 
> (2) If Foo extends Bar, I was thinking that you could register an
> Observer<Bar> for an event of type Foo. However, I now notice that the
> type declarations don't actually support that (though this could by
> fixed easily).
> 
> I don't think (2) is a very important case, since it is trivial to
> wrap Observer<Bar> in Observer<Foo>.

Seems a little confusing.  Observer<Bar> would always see events of type
Foo anyhow with the current semantics in the RI (did it change in the
spec?).

> 
> Case (1) is a little more subtle, but it seems like I can't really
> think of a real usecase for it.
> 
> I think we could reasonably have a single addObserver() method as follows:
> 
>     public <T> void addObserver(Observer<T> observer, Annotation... bindings);
> 
> 
>