Design Patterns

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CHAPTER 14 ■ THE PROXY PATTERN
if(!this.initialized) {
return;
}
return this.directory.showPage(page);
}
};
Blocking calls to showPage is easy; you simply query the initialized attribute. If it is true,
you can invoke the real subject’s method. Showing a warning message while the object loads is
a little trickier. How can you know when the class is truly finished loading? It is possible for the
real subject to create a custom event that the proxy can subscribe to, but in this example, we
used a simpler technique. The currentPage attribute is only set after the data has loaded. Simply
query that attribute every 100 milliseconds until you find that it has been set. At that point,
remove the loading message and mark the object as initialized.
The virtual proxy is now complete. This is a very simple example of how this type of proxy
can work. A more complex version could implement more robust initialization checks and
more accurate triggers. Each proxy will vary depending on the exact user interaction you are
expecting. Next we will cover a dynamic virtual proxy that can be used as a template for creating
your own proxy in the future.
General Pattern for Creating a Virtual Proxy
JavaScript is an enormously flexible language. Because of this, you can create a dynamic virtual
proxy that will examine the interface of the class given to it, create its own corresponding methods,
and defer instantiation of that class until some set condition is reached. To create this
dynamic proxy, first create the shell of the class and the _initialize and _checkInitialization
methods. This class is abstract; it will need to be subclassed and configured to work properly:
/* DynamicProxy abstract class, incomplete. */
var DynamicProxy = function() {
this.args = arguments;
this.initialized = false;
};
DynamicProxy.prototype = {
_initialize: function() {
this.subject = {}; // Instantiate the class.
this.class.apply(this.subject, this.args);
this.subject.__proto__ = this.class.prototype;
var that = this;
this.interval = setInterval(function() { that._checkInitialization(); }, 100);
},
_checkInitialization: function() {
if(this._isInitialized()) {
clearInterval(this.interval);
this.initialized = true;
}