CAB Event publishers and subscribers allow your application to be designed in a very modular and decoupled way. That’s a good thing, but it can bite you if you’re unprepared. In this post, I want to describe a situation that recently snagged me while working on a CAB-based application project.
One of the advantages of using CAB, or other composite application frameworks for that matter, is the fact that your code becomes much more loosely coupled. It helps to isolate your classes allowing you to better unit test. It makes it possible to organize the development of your application functionality into discreet units. In order to support this modularization, CAB brings a number of important features to the table that allows your loosely coupled code to share information.
While this modularization and loose coupling is a big benefit in the big scheme of things, it also puts a burden on you to design your application modules with certain things in mind. In particular, each module will not have direct knowledge of other modules loaded at runtime. In our case, modules are organized into their own projects and do not have any references to one another. At most, they will share some references to common projects that provide some base functionality. If your modules are interested in sending or receiving information from each other, there are a number of possible ways to approach this. The most common way and the way that leverages the publish/subscribe pattern is the use of CAB events.
Using CAB events is very straightforward. In your publisher, simply declare the event that your class will raise. You add the CAB EventPublicationAttribute to the event. The constructor for this attribute takes two parameters: a topic string and an enum for the publication scope.
1: [EventPublication(ConstantsEvent.CurrentLeadSummaryChanged, PublicationScope.Global)]
2: public event EventHandler<LeadSummaryEventArgs> CurrentLeadSummaryChanged;
We define a set of string constants for our event topics. In the case above, the publication scope is defined as Global so that CAB notifies everyone that the selection of a lead has changed.
The subscribers to the event simply declare their event handler and apply the EventSubscriptionAttribute. This attribute has a couple of different constructors. One simply takes the string topic ID and the other takes the topic ID and a ThreadOption enum, which allows you to control marshaling of the event data. This is useful when your publisher raises their events from a different thread and you need to marshal it to the UI thread for instance.
1: [EventSubscription(ConstantsEvent.CurrentLeadSummaryChanged)]
2: public void CurrentLeadChanged(object sender, LeadSummaryEventArgs e)
At development time, you must take the initiative to make sure that the signatures of the two (publisher and subscriber) match. Your code will happily compile, even if the two have differing EventArg types. If you fail to make them match, you will be presented with an ArgumentException at runtime indicating that one cannot be converted to the other.
Admittedly, the fix for the situation that I just ran into is pretty straightforward, but I think it bears pointing out that your development methodology should take this into consideration. If you miss one, probably the easiest way to locate all of the places where the event is used is to simply search the solution by the topic ID. This will allow you to verify that the signatures match.
Long term, add tests to your integration tests to validate that both publisher and subscribers work together. This will help prevent future mismatches and validate that the communication between the two is working well.
Hope this helps if you find yourself in this situation…
