nwebb

Just to let you know that I will be giving a talk at the LFPUG next month on Cairngorm for beginners. If you want to come along and laugh at how nervous I get please feel free ;]

Here are the details:

Talk: Cairngorm for beginners – “Making A Molehill Out Of A Mountain”
Date: Thursday 30th August 2007
Venue: London Flash Platform User Group – Cosmo Bar, London (subject to change – see site)
Description: If you intend to build enterprise-level Flex applications and work with multiple developers then you will want to keep your code base well structured, promote code reuse and avoid the risk of different developers solving the same problems in different ways – enter Cairngorm. In this session Neil will walk you through the Cairngorm architectural framework, giving an explanation of what it is, when you may wish to use it and what you need to know in order to get up and running.

Hopefully see you there!

The use of ViewHelpers in an architectural framework seems to be a bit of a grey area/hotly debated topic ( Jesse Warden summed up both of these things in quite an amusing way some time ago here). I don’t have the breadth of knowledge to take one side or another, but I can explain how (and why) we are using them in our current project.

From what I can gather, many people see ViewHelpers as a way to locate, keep track of and simplify the use of Views. The rationale goes like this: when you work on a large project with quite a few people, and the team are creating lots of views, it becomes harder to keep track of the views and what they all do – the ViewLocator makes it easy to locate a particular view (using a descriptive String) and ViewHelpers themselves only expose public methods, thus simplifying working with views for people unfamiliar with the internals of that View.

That’s not how we use them here.

Using ViewHelpers To Manipulate Existing Views (Without Updating The Model):
In a simplified view of Cairngorm, the Command class retrieves some info from a DataBase and updates the Model. The View is then updated through bindings to the Model. The Command should not know about the View.

So what happens when you want to do something to the View but you don’t want to store anything in the Model? What if you simply wanted to make-visible some text in the View to say that all client-side data was up-to-date? You could set some flag/property in the Model, update that property and get the View to respond, but is that necessarily the right thing to do? Perhaps my simplified example could be better, but there are times where it doesn’t feel right to be cluttering up the Model with properties that you don’t feel are truly needed or rightly belong in the Model.

Sometimes it seems as if the Command should be able to manipulate the View, yet the Command should know nothing about the View, and you can achieve this with a ViewHelper. The ViewHelper sits in-between the two classes so that the Command and View are no-longer tightly coupled. The Command uses the ViewLocator ( a Singleton) to find the ViewHelper, and calls methods on it which in turn call methods on the View.

Note: To me, the notes in the Cairngorm ViewHelper class sound like they are backing up this as a valid use of ViewHelpers.

Using ViewHelpers When Creating Popups:
Let’s say that a user needs to see some information (from a database) in order to make a decision about how many “widgets” to create. You are interested in the user’s input (i.e. how many widgets he/she wants to create) but you don’t want to store all the retrieved information in your application’s Model – you are happy to discard it.

In other words, you want to generate a View (a pop-up window) from a Command. Here is an overview:
The Command would run, retrieve some info from the database and generate a pop-up window. The user would use the data to make a decision about how many widgets to create and enter a value in some input field. When he/she presses the “submit” button on the pop-up window, you remove the pop-up and return the input-value back to the Command. The Command then updates the Model with this value and the main View is updated accordingly via bindings.

Once again, this can be achieved using ViewHelpers without breaking the rule about letting the Command know about the View ( in much the same way as described above).

Using ViewHelpers When Adding Listeners To Popups:
Supposing for a moment that you did let your Command directly create your View… another problem with pop-ups that you may encounter, would be that they probably don’t exist at the time you want to add any event-Listeners to it. You can try creating the pop-up first and adding the listeners upon creation, but that’s not great.

The ViewHelper is a great place for putting an event-proxy. Extend the class and add the proxy functionality.
Now, rather than the Command listening to the View (which may not even exist, in which case you’ll get a “null” error), you listen for events as if they were dispatched from the ViewHelper:
i.e. myViewHelper.addListener (“evtName”, someFunc);

One way of coding this could be that when the Command tries to add a listener to the ViewHelper, you take all that info, wrap it up in an object and store it within an array on the ViewHelper.
As soon as the ViewHelper has created the View, it uses that information to add event listeners (between itself and the View). When an event is dispatched from the View it calls the method on the ViewHelper, which in turn calls the corresponding method on the Command class.
Okay, well there are my thoughts on ViewHelpers.
It would be great to hear your opinions (and alternative suggestions) for achieving these types of interactions within a Cairngorm app. Please feel free to leave feedback and add to the already raging debate about ViewHelpers :)

I still get emailed a lot of XML/Flash questions as the result of some Flash tutorials I wrote a few years back. I just wanted to post a quick overview and list of links that I regularly give out, so that I can point people to it in the future.

The info in the post is aimed at people using Flash 8 or lower, and who are new to XML. You’ll want to skip it if you’re familiar with XML or using e4x already :]

Okay here goes…
The thing about XML is there are multiple ways to achieve what you want, and your options depend somewhat on what version of Flash you are using.

XML is intended to be human-readable. It should be pretty easy to grasp the basics, but it can get fairly advanced too (namespaces, XSLT, SAX parsers, schemas, DTDs [insert more acronyms here!]). Luckily you don’t need to know all of that stuff to start using XML in Flash, but how to deal with XML in Flash is also not something that can be summed up in a single email (so please stop asking ;)). There is no shortcut to knuckling down and doing a bit of reading.

If you’re using Flash 9 or higher then you should be taking advantage of e4x – and the Flash documentation is your first port of call. However, if you are using Flash 8 or lower, the best place to start is probably here:
http://www.kirupa.com/web/xml/index.htm

Senocular has always written fantastic tutorials and these are no exception.

The most basic way to use XML (and the way that is demonstrated in a lot of the Flash docs) is to “walk down the hierarchy”. This is okay as an XML101, but can break easily if the order of the XML changes (if the XML is out of your direct control then I strongly recommend not to use such methods).

Other ways include looping through the XML using a series of nested loops (you may want to see one of my old tutorials on Nested Loops here to give you an idea of the general technique) . An alternative but similar solution is recursion – this is where you are calling the same function repeatedly but with different parameters.

You can also use XPATH, which can really make dealing with XML a lot easier. There is some limited direct support for XPATH in later versions of Flash but not all of them ( I think it is only Flash 8 ) . However, there is a full external XPATH library here: http://www.xfactorstudio.com/

You will need to hunt around for instructions on how to use it – if anyone can provide me with a link I would be grateful.

Finally, if you don’t want to use XPATH then you may wish to look at Seporith‘s XML2OBJECT class
http://www.sephiroth.it/file_detail.php?id=129

As I mentioned previously, if you are using Flash 9 you should really be using e4x, which is quite similar to xpath syntactically.