Agile Enterprise Application Development with Flex

Proxy

A proxy is an object that represents another object and controls access to it. Think of someone’s spokesperson or a secretary. If someone brings a package to a big shot, the package is taken by the secretary, who would inspect the contents and then either deliver the package to the boss or delegate its further processing to someone else (e.g., security personnel).

In object-oriented programming in general and in ActionScript specifically, you can wrap the class XYZ in mx.util.ObjectProxy, which will be a proxy that controls access to XYZ’s properties.

Let’s think of some concrete Flex examples that illustrate how proxies can control access to object properties by dispatching propertyChange events. As a matter of fact, your Flex programs that use data binding already implement a similar mechanism of event notifications under the hood.

Data binding is a very useful technique that substantially increases the productivity of Flex developers. If you start the declaration of a variable or a class with the meta tag [Bindable], all of a sudden the variable starts emitting events about all changes that can happen to it. The syntax to make this happen is very simple:

[Bindable]
var lastName:String;

How does this event notification mechanism get engaged by simply adding the magic word [Bindable]? You are all seasoned programmers and don’t believe in the tooth fairy. Someone has to write the code that will dispatch events when the value of the property lastName changes. The compiler does it ...