This weekend I tweeted three simple rules for determining if a class's responsibility has been properly assigned. Class responsibilities were on my mind for a significant portion of the day on Saturday. It wasn't anything conscious or intentional. Rather I think it was a sort of run-off from a lot of responsibility talk over the course of the previous week.
Lately I find that whenever someone asks my opinion about a particular design decision, the question of responsibility is the first place I go. It's far and away the most effective and powerful thing to ask, as it easily gets the most guaranteed bang for the buck.
At both the class and the method level, the most important factor in deciding where and how to implement a particular bit of functionality is whose responsibility it is. Likely, the whole picture of the task is made up of several bits of responsibility that belong in several places. If your application is architected in such a way that responsibilities are well isolated and assigned to layers and clusters of classes, then breaking the task down into those constituent responsibilities gives you a draft blueprint for how to get the job done.
If your application is not fortunate enough to have such a clear and well articulated architectural scheme... well I'd advise you to make it a goal. Start by learning about onion architecture. But in the meantime, try to keep in mind the rules of thumb below and evaluate your classes against them as you work. If they don't measure up, see if they can be improved incrementally as you work. Leave them cleaner than you left them.
Now, without further ado, the rules.
- If you can't sum up a class's responsibility in 2 sentences, shift your level of granularity. It almost certainly has more than one responsibility, and you need to think hard about why you've combined them.
- If you can't sum up a class's responsibility in a single sentence, then you have lots of room for improvement. Don't be satisfied until you have a clear, concise statement of the class's responsibility.
- If you can sum up a class's responsibility in a single sentence, go a step further and think hard about whether it has another unstated or assumed responsibility. Unit tests can really help here.
The message here is not subtle. Every class needs to have a clear answer to the question "what is my responsibility". This is, in my experience, the most important factor in establishing clean, clear, composable, testable, maintainable, comprehensible application design and architecture. It's not the end of the road, but it's a darn important step on the way.