Design Pattern Principles for ActionScript 3.0: Single Responsibility Principle
The final principle we’re going to examine for the Lunch Bucket Rules series is the Single Responsibility Principle. Succinctly stated, it means that each class should have one and only one responsibility. At the base of this principle is the idea that if you make a change, you are less likely to run into a problem if each class has a single responsibility. The same principle is expressed as, each class should only have a single reason to change.
In our book, we discuss this principle briefly (pg. 443). The MVC design represents three clear areas of responsibility—Model, View, and Controller. Each responsibility is clearly spelled out, and each of the three elements in the framework sticks with its own responsibility. That’s not a bad place to start as far as a getting a general sense of what this principle means.
Simple Never Is
In searching around for information about the Single Responsibility Principle, I came across a short post by David Chelimsky. In that post he cites the following quote:
In procedural programming, a process is expressed in one place in which a series of instructions are coded in order. Whereas in OO, a process is expressed as a succession of messages across objects. One object sends a message to another, which does part of the process and then sends a new message off to another object, which handles part of the process, etc. You modify a process by reorganizing the succession of messages rather than changing a procedure.
Ooof! That quote knocked the wind out of me because of all of its implications. It’s saying that when you make a change, your main focus is on rearranging responsibilities encapsulated in classes instead of re-writing a procedure. This forces the developer to ask, What responsibility does each class have? If each class has a single responsibility, that makes life a lot simpler. On the other hand, classes with multiple responsibilities may be ripped up or cause problems elsewhere because they can be changed in more ways than one and do more than one thing, wrecking unpredictable havoc.
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An Enigma Wrapped in a Clear Casing


This post is going to be short and sweet. I worked up a little application that you can see
The first principle of design patterns is,
Gentle Reader: Now that we’ve worked through all of the design patterns in ActionScript 3.0 from GoF (well, Builder is still in the works, but that’ll be available soon), now would be a good time start going through the principles underlying design patterns. This will be the first in that series.
Bill Sanders
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