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ColdFusion Developer's Journal Special "Frameworks" Focus Issue: Model-Glue

Separating business logic from the presentation layer

After unzipping the Blueprint Application and configuring my ColdFusion instance to get the application running as-is, I poked around the code to see how it was laid out. It was pretty straightforward: create an object of the component we need to access, call a method in the object, and display the results. This is pretty standard in most applications. Model-Glue (M-G) can easily handle this kind of architecture even though it's not in an MVC pattern.

That's a great strength of Model-Glue. While it facilitates MVC, your application doesn't have to use this pattern to use M-G. Since the task presented to me was to alter the Blueprint Application to use Model-Glue, I decided at this point to keep as much of the original architecture as possible and not re-architect the application in an MVC pattern.

To get started, I downloaded Joe Rinehart's Model-Glue from www.model-glue.com. I put the Model-Glue folder in my webroot, and mapped /modelglue in the ColdFusion Administrator. In the download there's a directory called ModelGlueApplicationTemplate. This folder should be copied to a new location and renamed to match your new application. This folder contains default message-listeners, event-handlers, etc. that Model-Glue will use during runtime for this specific application.

Before I get into the Pet Market application, let me explain exactly what Model-Glue is and how it works. Developed by Joe Rinehart, Model-Glue was created as a framework to help developers separate business logic from the presentation layer. Model-Glue uses a simple XML schema so the developer can define event-handlers and message-listeners. A visual representation of how M-G works is shown in Figure 1.

When a request is placed, an event is passed to the framework via a URL or FORM variable. In Model-Glue, FORM and URL variables end up in one Event object. From your view, you access this object as viewState. In a controller, you access this object as Event. The framework automatically pushes this event object to each controller as an argument (arguments.event). Now, as I said above, I could easily use M-G and architect this application using MVC. Each controller attached to the message-listeners would simply have to accept the Event argument. Anyone who's read up on MVC knows that controllers should contain no interaction with the data persistence layer. Only the Model should interact with your data persistence. Figure 2 shows how Model-Glue fits into the MVC pattern.

In the case of the Pet Market Blueprint Application, we could either use the existing architecture and have Model-Glue act as an intermediary and just call the appropriate view templates that, in turn, access the Model directly, or rewrite the application so Model-Glue calls the controller and returns the data to the view as an MVC patterned architecture should. I think that a full re-architecture is outside the scope of this article and will first get the application running using M-G, and second, re-architect one template to use M-G in an MVC pattern as an example that better shows the power of M-G.

Now that you have a basic understanding of Model-Glue and our plan of attack for this application, let's delve into the Pet Market Blueprint Application and set it up to use Model-Glue.

Before we begin converting our application, we have to determine where to put our files. I generally like to have an assets directory where I keep all items consumed by the application. This doesn't include display templates (view), but does include things like images, components that belong to my application's Model, stylesheets, etc. Model-Glue will work with any directory structure. I just like to keep my directory layout consistent across my applications so it's easy to determine where the items I need are located. Notice that the assets folder only contains components related to our Model. I've learned to keep all controller components in a controller folder under the application root. Once again, this is personal preference and M-G doesn't require any specific directory structure. As a framework, it's very forgiving.

We also need a place to hold our view templates. When using Model-Glue, I usually create a folder in the application root called view. Under the view directory, we can create folders to hold the views specific to our separate modules. While reviewing the existing Blueprint Application code, it seemed like the application views broke into four distinct modules: Search, Products, Checkout, and static templates that would fall into a more general group. In the root of view we can store our general view templates. These templates would include files such as about.cfm, legal.cfm, and other templates that fall outside our application's general functionality. I then created a directory for each of the other modules above (Search, Product, and Checkout).

I now copy over the existing files in the Pet Market Blueprint Application into the appropriate folders. When I'm done, my directory structure looks like Figure 3.

To configure our new Model-Glue application, we edit config/Modelglue.xml. The Modelglue.xml file is broken into three distinct sections: config, controllers, and event-handlers. <Config> contains our application specific configuration settings. <Controllers> defines the location of our controller files and the message-listeners attached to each controller. Every broadcast defined in our event-handler will have one message-listener defined pointing to a method in a controller (refer back to Figure 1). The last section, <event-handlers>, listens for events broadcast during our request. The first event-handler we'll intercept is the event passed from our user (via the URL or a form). Each event handler can then broadcast its own messages that our controller message-listeners "hear" and call methods in our controllers. These controller methods interact with the Model (data persistence) and apply business logic. They then set data into the Event object and set an optional result. If a result's been set, it's interpreted by our event-handler and then the next message is broadcast or another event is fired or the view is displayed. Model-Glue is awesome in that it lets not only several methods be applied during a single request, but that the results set via our controllers can be interpreted by the event handler and another event can be broadcast, or one of several views can be called. Allowing the developer to use this conditional logic is unlike many other frameworks, and is very powerful.

When using Model-Glue, all requests are processed using the index.cfm page by default much like other frameworks. When a request is made from within your application, whether it be from a form action or a URL, it will have the format index.cfm?event=event_name. (Note: the event may also be passed as a form variable.) Event is the default variable name; however, you may customize M-G to listen for any variable name.

I now searched the wrapper.cfm file for all form actions and href links. I needed to have the request call index.cfm with the new event that I defined for each event-handler in ModelGlue.xml (see Listing 1 for an example event-handler). There were also two forms that onChange called submit() and posted back to the current page. I added hidden form fields named event and specified the event that it should post to. Since M-G looks in the Event Object for the event being called and URL and FORM variables are both compiled into this object, this still let me get the desired result. Model-Glue lets you specify which scope takes precedence (statePrecedence) in the <config> section of ModelGlue.xml (FORM or URL) in case both the FORM and URL scopes contain the same variable name.

Since I moved all the templates around and created a new directory structure, I had to point each template to the new location of the wrapper.cfm file. This was easy using a site-wide search and replace. Once the application was able to <cfimport> the wrapper, the application was up and running great, using the Model-Glue framework.

As promised, I then created a search controller to show how Model-Glue processes requests using MVC. The first thing to do is to set up ModelGlue.xml to broadcast a message to be intercepted by a controller, in this case SearchController.cfm (see Listing 2). In the sample application structure we copied over from Model-Glue, there's a template Controller you can use as an example. It includes three required methods by default. The init() method is called as a constructor. OnRequestStart() is invoked when the request first starts processing, and onRequestEnd() is the last method called for that request. I added a function called getSearchResults() to call our product object method search() and then set the results into the Event (see Listing 3). These results are now available in our viewState (see Listing 4). To see the complete controller and ModelGlue.xml, visit www.cfpetmarket.com and download the full code archive for this project.

One more thing I'd like to mention is the built-in Model-Glue debugging feature (see Figure 5). In the <config> section of ModelGlue.xml, there's a debug setting. When set to true, M-G caches important debugging information as your request is processed. This debug information is displayed at the bottom of your request (before the ColdFusion debugging information if you have debugging turned on in CFAdmin) regardless of whether or not an Exception is thrown. It shows information such as the time a specific action, such as an event-handler or message-listener, took to complete. The debug also displays results set in your controllers. In my opinion, the best debugging feature is the built-in trace method. In my controller I can trace any variable of any type by using the format arguments.event.trace("trace_name", variable). This traced value will then display in the Model-Glue debugging output. What a great feature to have at your disposal.

To close out this brief case study, let me say that integrating the Pet Market Blueprint Application into the Model-Glue framework took a total of 30 minutes. It was very simple and while this application didn't use many of M-G's powerful features, it clearly demonstrates that Model-Glue can handle applications of any size or complexity. Just how complex? I'm presently the team lead overseeing what is described as the largest production implementation of Model-Glue, and Model-Glue easily handles the load, reduced the learning curve for new team members learning MVC and OOP, and enabled us to keep the application code tidy. Thanks, Joe!

More Stories By Nic Tunney

Nicholas Tunney is a Macromedia Certified ColdFusion developer, and has been programming ColdFusion for over 7 years. He is currently Senior Software Architect for AboutWeb, a consulting firm located in Rockville, Maryland. To learn more about using objects in ColdFusion, visit Nic's Blog at http://www.nictunney.com.

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