Yet Another Way to Do Publish & Subscribe . . .

About 2,5 years ago I first came across the idea of combining dependency injection with loosly coupled publish & subscribe. This was when I inspected the source code of the Composite Application Block (CAB) with its underlying InversionOfControl-container ObjectBuilder from Microsoft. What I liked about publish & subscribe in CAB:

  • The automatic thread marshalling. (Basically you’re able to specify whether the subscription-callback will be handled on the UI-thread or the same thread as the publisher)
  • The wiring was done during the build process in the ObjectBuilder. Publishers and Subscribers are completely decoupled. Wow, this was pretty amazing for me when I discovered that late 2005 :-).

What I didn’t like about it:

  • It is very tightly coupled to a CAB internal data structure called WorkItem. (Not reusable outside the scope of CAB)
  • It uses standard .NET events / delegates. (The publisher needed to define an event and the subscriber needed to have a public method with a matching “object-sender-eventargs-e” - signature.)
  • It uses attributes and especially a string based topic identifier to correlate publishers for a topic and corresponding subscribers. (Maybe I’m narrow-minded on this, but I think the simplest way to specify a subscription is the best, which imho is expressing a subscription by implementing an interface.)

Many frameworks or libraries I looked into over time mostly followed a comparable approach using either events or some other delegate-based solution (the EventBroker in CAB, the EventBrokerFacility in Castle Windsor, the EventAggregator in PRISM, ….) (Please correct me if I overlooked or misunderstood something :-)).

The only framework I came across with something similar to what I had in mind is Caliburn. Unfortunately Caliburn is a WPF/ .NET 3.0 based framework, but most of the products of my current employer are limited to .NET 2.0 due to a minimum system requirement of Windows 2k.

Because of that I decided to implement something similar for our purposes which can be easily integrated into the InversionOfControl - container of our choice (which is Castle Windsor at the moment, by the way :-) ). I’ll be describing the implementation and publish the source code for anyone who is interested in one or more follow-up posts … .

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