Simple Design with Design Patterns

In the blog post Simple Design with Design Patterns, Cory Foy runs a thought experiment in which he explores the use of patterns in solving the Game of Life kata. Cory’s starting point is the intention behind the GoF patterns, coupled with the XP rules of Simple Design and Uncle Bob’s SOLID principles.

Cory stops his analysis when he has the following model:

|Cell| <------- |Status|
           |Alive|    |Dead|

At this point Cory says

“Certainly if we implemented a Cell with a Status object with an AliveStatus class and a DeadStatus class someone, somewhere, should slap us upside the head.”

However, I’d like to carry on from that point just a little further…

I like to teach the use of CRC for exploring design, and so I naturally looked at Cory’s objects from the point of view of their behaviours and responsibilities. Thus it struck me that if we don’t make the split suggested by Cory, we’d have a Cell that knows about its position/neighbours and knows what change to make when the clock ticks. That would violate the Single Responsibility Principle. So the above split into Cell and Status makes sense from the point of view of responsibilities, if we give the Status object the job of deciding how the Cell should change.

With those responsibilities in mind, a little mental CRC session suggests that, when the clock ticks, the Cell would tell its Status object how many neighbours it had, and ask for a new Status object in return.

class Cell
  def change
    @status = @status.update_based_on(@number_of_neighbours)

The Status objects become Strategy/Policy objects, and thus don’t seem to need any instance state. All of which means that the Status objects could be Flyweights, with only one extant instance of each type, referenced by every Cell that happens to be in the corresponding state.

I think this approach could well be worth a punt during the upcoming Global Day of Coderetreats

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