Regarding the discussion of the design for StatusMessages...

On Tue, May 5, 2009 at 7:59 AM, Clint Popetz <cpopetz@gmail.com> wrote:

 I want to
point out that using a deployment type and @Specializes would seem to
place us in the same situation as Seam 2.x with respect to the view
layers co-existing in the same deployment, which
https://jira.jboss.org/jira/browse/JBSEAM-3645 was meant to address.

In other words, I'd rather that the choice of the StatusMessages bean
that will be activated isn't based on deployment type, but rather is
chosen at runtime based on the type of request, using the pattern in
the patch for the above jira issue.  Deployment types would of course
still be used to choose which implementations of things like
StatusMessages are available.

I have taken a first stab at solving this problem. I'm open to other approaches, but first let me explain what I've done. As Clint points out, from one request to the next, you may be dealing with different frameworks (Wicket vs JSF for instance). But within the request, you want to be able to take specific actions relating to the current framework. Clearly, this is not the appropriate scenario for a deployment type since that is design for a setting/environment that is fixed. So I did some brainstorming.

What I realized is that StatusMessages are really a generic repository that should be updated and consumed by the framework-specific activity. Therefore, it seems to me like there should be one and only one conversation-scoped StatusMessages. Then, if you want to have some extra methods for convenience (for instance in JSF to add a StatusMessage from an existing FacesMessage) you create a class which extends and wraps the StatusMessages component (StatusMessages would be injected into it and all overrides delegating to it). You could also override the onBeforeRender() method where you might put the logic for converting and transfering to the native message type and storage. The executor of that conversion would be implemented in the framework-specific listener.

For instance, 9 out of 10 times you simply inject the StatusMessages to use in your application:

@Current StatusMessages statusMessages;

For JSF, there is a FacesMessages bean which inherits from StatusMessages (to keep the API the same) and delegates calls to the StatusMessages instance. It also adds the logic to create FacesMessage objects in the onBeforeRender() method

public void onBeforeRender() {
    super.onBeforeRender();
    // create FacesMessage objects and register them here
}

The ConvertStatusMessagesListener (a JSF system event listener) executes the onBeforeRender() call.

manager.getInstanceByType(StatusMessages.class, new AnnotationLiteral<Faces>() {}).onBeforeRender();

There are also some convenience methods on the FacesMessages instance. If you want to use them, you inject as follows:

@Faces FacesMessages facesMessages;

I can't use @Current here because otherwise the resolution would be ambigous. I could just clone the StatusMessages API in the FacesMessages class to avoid having to use this binding type, so if you have a feeling/advice there, I would be glad to here it.

Okay, that's one approach. Now for something different. There is a similar specialization w/ the Expressions class (an EL convenience API). If working within the JSF request, we want to override some methods so that the bean uses the current JSF EL context. In this case, I decided to try Clint's approach described in the cited issue report. A producer method will locate beans that have the binding type @RuntimeSelected and will consult the isActive() method to determine which one is prepared to handle the current request. The isActive() method comes from the RuntimeSelectedBean interface. So the bean must have both the binding type and the interface. There is a second binding type, @Default, which indicates which implementation should be used when there are no @RuntimeSelected implementations present or active. It is mutually exclusive with @RuntimeSelected.

So you have:
public @Default class Expressions { ... }
public @RuntimeSelected class FacesExpressions extends Expressions, RuntimeSelectedBean {
    public boolean isActive()
    {
        return FacesContext.getCurrentInstance() != null;
    }
}

public @RuntimeSelected class WicketExpressions extends Expressions, RuntimeSelectedBean {
    public boolean isActive()
    {
        return Application.exists();
    }
}

The producer type for the Expressions bean is application-scoped and the producer method is dependent-scoped (or request-scoped?). The available instances are looked up when the producer is initialized and the instance is selected from this set each time the producer is resolved.

Of course, I could have used this strategy for the StatusMessages too, except in the case of StatusMessages it is logical to use the shared/generic StatusMessages bean. It's only when you need to convert the messages do you need the specialization and therefore I think we can avoid having to do the runtime selection.

I've committed all of this to the Seam trunk if you want to check it out. I'm open to suggestions...I'm just trying to get used WB and find a workable approach. The solutions we decide on are important because the define a standard practice we will follow as more cases arise.

-Dan

--
Dan Allen
Senior Software Engineer, Red Hat | Author of Seam in Action

http://mojavelinux.com
http://mojavelinux.com/seaminaction
http://in.relation.to/Bloggers/Dan

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