ActionScript 3 Design Patterns

Aggregation Aggravation

In the first installment of this Flyweight Saga, I noted that the relationship between a Flyweight and Flyweight Factory class is one of aggregation. The initial example shows that the Retrieve method in the Factory class returns an instance of IFlyweight, meeting the requirement of the proper connection between the two classes. In looking at one example in Java, (even with modest Java skills), the program clearly did not have such a relationship between the Factory and Flyweight. In fact, it claimed, that an object’s extrinsic state can be shared by classes. Now, maybe that was unfortunate wording because the big feature of the Flyweight design pattern is that the Flyweight can be a shared object, but only the intrinsic state can be shared. (Maybe the author meant that extrinsic states can be shared between classes in a Flyweight design pattern, and that’s probably right but is not a key feature of the pattern.)

Anyway, after looking at several different descriptions of aggregation, including the one provided by the GoF, it’s clear that the concept is one with fuzzy borders and can slip into either general composition or acquaintance. It implies that the Flyweight Factory aggregates the Flyweight—no Flyweight, no Factory. As a result, the life of the aggregator (Flyweight Factory) depends on the life of the aggregatee (Flyweight). Like acquaintance, aggregation is implemented with references or pointers rather than defining variables of once class in another. (Apparently C++ is an exception and does set up aggregation by defining variables from the aggregatee class.)

By and large the issue has not been especially significant so far because all of the output was using trace statements, and so output was largely confined to built-in features that only work when the code is run in test mode. It’s great for debugging up to a point, but developing with trace statements that do not take into account how certain graphic elements, especially those that are accessed by extending the Sprite classes, can generate unusable structures for applications that employ graphics and other elements that require the import and extension of other classes. In this next Flyweight example, we leave the realm of trace and use the graphics property (from the Sprite class) to draw solid balls using fill methods. The ball class extends Sprite and implements the interface. That’s all fine and good, but the aggregation becomes problematic in even the simplest example. In taking the general structure from the examples examined up to this point (Parts I and II of the Flyweight Saga), we can begin to see the trouble.
Continue reading ‘The ActionScript 3.0 Flyweight Saga: Part III Aggregation Aggravation, Stuff on the Stage and the Intrinsic State’