Design Patterns

CHAPTER 12 ■ THE DECORATOR PATTERN 171
throw new Error('Decorator ' + options[i].name + ' not found.');
}
var argument = options[i].arg;
bicycle = new decorator(bicycle, argument);
}
// Check the interface and return the finished object.
Interface.ensureImplements(bicycle, Bicycle);
return bicycle;
};
// Model name to class name mapping.
AcmeBicycleShop.models = {
'The Speedster': AcmeSpeedster,
'The Lowrider': AcmeLowrider,
'The Flatlander': AcmeFlatlander,
'The Comfort Cruiser': AcmeComfortCruiser
};
// Option name to decorator class name mapping.
AcmeBicycleShop.options = {
'headlight': HeadlightDecorator,
'taillight': TaillightDecorator,
'bell': BellDecorator,
'basket': BasketDecorator,
'color': FrameColorDecorator,
'lifetime warranty': LifetimeWarrantyDecorator,
'timed warranty': TimedWarrantyDecorator
};
If order is important, you can add code to sort the options array before you use it to
instantiate the decorators.
Using a factory to instantiate the bicycle object provides a couple of important benefits.
First, you don’t have to keep track of all of the different class names for the bicycles and decorators;
all of that is encapsulated in the AcmeBicycleShop class. This means you can add
bicycle models and options very easily just by adding them to the AcmeBicycleShop.models
or AcmeBicycleShop.options arrays. To illustrate this, let’s take a look at two different ways of
creating a decorated bicycle object. The first way will not use the factory:
var myBicycle = new AcmeSpeedster();
myBicycle = new FrameColorDecorator(myBicycle, 'blue');
myBicycle = new HeadlightDecorator(myBicycle);
myBicycle = new TaillightDecorator(myBicycle);
myBicycle = new TimedWarrantyDecorator(myBicycle, 2);
By instantiating the objects directly, you are now tightly coupled to no less than five
separate classes. The second way uses the factory and is coupled to only one class, the
factory itself: