Exercises in Programming Style–Bulletin Board

NOTE : read the rest of the series, or check out the source code.

If you enjoy read­ing these exer­cises then please buy Crista’s book to sup­port her work.

Fol­low­ing on from the last post, we will look at the Bulletin Board style today.

 

Style 15 – Bulletin Board

Constraints

• Larger problem is decomposed into entities using some form of abstraction
• The entities are never called on directly for actions
• Existence of an infrastructure for publishing and subscribing to events, aka the bulletin board
• Entities post events subscriptions to the bulletin board, and publish events to the bulletin board. The bulletin board infrastructure does all the event management and distribution

 

If you’ve done any WPF work in the past (or in the present) then you might recognize this style as the Weak Event pattern.

Whilst similar in design to the Hollywood style, this style further decouples the entities with an EventManager that sits between them all.

So let’s start by defining what a handler and event manager looks like:

Unfortunately we have to forfeit some type safety here as we unify different events under the same umbrella. Here’s the implementation for the IEventManager interface:

The 3 entities – DataStorage, StopWordsFilter and WordFrequencyCounter – are similar to those in the Hollywood style, except they now subscribe and publish events through the aforementioned IEventManager abstraction.

Oh, and we’ll stick with the RunArgs type we created last time:

Finally, to put everything together:

 

You can find the source code for this exer­cise here.