Beginning Web Development, Silverlight, and ASP.NET AJAX

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CHAPTER 14 ■ JAVASCRIPT PROGRAMMING WITH ASP.NET AJAX
Going back to our boat sample, we can now build an interface for all powered vehicles,
which could be implemented by a motorcycle or an aircraft. If SpeedBoat implements
this interface, then the prototypes for the engine functions are available to it.
First of all, you declare an interface as a prototyped set of JavaScript functions that
you then register as interfaces using the registerInterface command. Here’s an example:
Vehicles.IPowered = function() {}
Vehicles.IPowered.Prototype = {
checkFuel: function(){}
}
Vehicles.IPowered.registerInterface('Vehicles.IPowered');
This creates and registers an interface called Vehicles.IPowered. So, if you want to
make SpeedBoat implement this interface, you use the registerClass call. Earlier, when
you registered the SpeedBoat class, you did it like this:
Vehicles.SpeedBoat.registerClass('Vehicles.SpeedBoat', Vehicles.Boat);
To change this to implement the interface, you use the interface name as the third
parameter, like this:
Vehicles.SpeedBoat.registerClass('Vehicles.SpeedBoat', Vehicles.Boat,
Vehicles.IPowered);
Now if you want to use the function, you must implement an override within the
SpeedBoat class, or just use the base declaration from the interface. Here’s an example of
overriding:
Vehicles.SpeedBoat.prototype = {
checkFuel: function(){
return "Yes, I use and need fuel, because I implement IPowered";
}, ...
}
As you can imagine, your code can get pretty complex if you are defining large hierarchies
of classes and using interfaces and inheritance to have coherent functionality
across the set. In many cases, you would need to have an object check to see if it implements
a certain interface or inherits from a certain class before invoking a method, in
case it isn’t implemented and you get an error. This is where reflection, the process of
examining the structure of a component at runtime, comes in handy.