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The ModelLocator is an Action Script Class that is stored in the model directory.

This is a concept used in every Cairngorm application (see here for example) but you don't need the Cairngorm library in your project to use it. In fact, you don't need to know anything about Cairngorm to get started.

About the name

The ModelLocator holds instances of Model Objects, making them easy to locate and accessible from anywhere in the application.

Benefits of using the ModelLocator

The ModelLocator will make Flex development easier and more enjoyable. Here's how: All application data will be stored in one single place, accessible from anywhere in the application at any time. That one place is, of course, the ModelLocator.

Storing all application data in one place is a wonderful thing. Here's why: All user interface components can be bound to the data in the ModelLocator. This means that one change to data in the ModelLocator will update every single ui component that is bound to that data - anywhere in the application. You will be writing MXML and ActionScript for your views that looks like this:

Whenever the selectedSchool property of the ModelLocator is changed/updated, every control in the application that binds to that property will be updated as well.

Creating the ModelLocator

The ModelLocator is an Action Script Class generally named ModelLocator.as and is stored in the model directory.

In the code block above, the ActionScript comment reads "Get a reference to the one and only instance of the ModelLocator." If there were multiple instances of the ModelLocator, then multiple versions of our application data would exist. So there may be only one single instance of the ModelLocator class. An ActionScript Class that may be instantiated only once is called a Singleton, i.e. ModelLocator is a Singleton Class. (Code for a singleton class may be written many ways. The method that follows is the simplest example that I feel is appropriate. It is also perfectly acceptable for use in production applications, though arguably better solutions exist which I may write about at a later time.)

Let's start with the full code for ModelLocator.as:

Explaining the ModelLocator Code

As seen in the first code block, data in the ModelLocator is accessible from all parts of the application with the following code:

Notice that a new instance of the ModelLocator class is never created. Instead, a public static method method of the ModelLocator class named getInstance is called. The return value of that method is assigned to a variable in the view of the type ModelLocator. Examine the getInstance method:

The getInstance method returns a variable called instance. This variable is declared earlier in the class and holds the one and only instance of ModelLocator - aptly named instance. Find the instance variable in the ModelLocator.as code above. It looks like this:

Since an instance of ModelLocator resides in the variable instance and ModelLocator is a Singleton, no further instances may be created. If an attempt is made, the constructor will throw an error with the message, "Cannot create a new instance. Must use ModelLocator.getInstance()."

Storing Bindable Data

All application data will be stored as public variables which are bindable to components the application. An example of this is found in the ModelLocator.as code above and looks like:

Try it out - right click to view source

This simple application uses the concepts found in this and the previous two posts. It demonstrates setting data in the ModelLocator and binding to that data. In a later post I will present a much more elegant way of working with data in the ModelLocator.

Next ...

The next post in this series will be titled "Flex Part 04: Introducing Frameworks." This post will cover Cairngorm and other MVC frameworks conceptually - further posts will put those concepts into action.

Thank you for reading. Your comments are very much appreciated. Remember that these posts are living documents and will be updated over time as input is received and new ideas emerge. So, please, consider your comment a contribution.