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
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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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Did you have a look at the Publish/Subscriber Pattern Sample at the Unity Source
Yes I did (in case you mean the SimpleEventBroker sample). It’s conceptually very close to the original CAB-implementation (Based on .NET-events which are marked with attributes containing string-based topic identifiers.) So most of the things I didn’t like about Publish & Subscribe in CAB apply to this sample, too.
Or is there something newer?