Event Aggregator v Composite Command

Topics: Prism v2 - WPF 4
Aug 9, 2010 at 12:36 PM
Edited Aug 9, 2010 at 12:39 PM

Can someone please clarify the difference in approach between using the event aggregator and a composite command.

I think, in my mind, it comes down to threading/processing but I'm not sure if that's all I need to consider or if there's more to it.

I have an (shell) exe and a bunch of modules.
From my shell I want to be able to initiate a 'save' action. Unlike many of the samples, my application is not an MDI type app.
So when I initiate a 'save' action I'm not looking at a 'save all' ie. several order views performing a Save Order.
What I am looking for is for each module to persist it's state in an way that appropriate to the given module.
The customer module would save the state of the customer, the vehicle module would save the state of the vehicle etc etc

My dilemna...

I could use the event aggregator and publish a 'save' event.
Each module (that needed to persist state) could then register for that event and when it gets raised persist it's state in the appropriate way.

Alternatively, could I use a composite command to call a save method for each module?
(As you can probably tell - I don't have much experience in composite commands).
I'm assuming as long as each module's save command/method had the same signature I could wrap them all up into a composite command.

If my assumption is correct, I think the significant difference in the 2 approaches is to do with threading/processing.
If I wanted to do something immediately after the save (and only once the save of each module had finished) am I right in thinking that composite command is the way to go?

My thoughts here are that simply publishing an event is a bit like 'fire and forget' ie. you won't get any indication of when each subscriber to the event has finished handling the event.
Where as the composite command is a more synchronous approach and will execute each individual save command before continuing on to the next instruction.

So is that all there is to consider, or is there something I'm overlooking?

Thanks in advance,


Aug 9, 2010 at 4:49 PM
From what I understand: Commands are synchronous and will execute on the UI thread. With the event you have three options: Publisher thread, UI thread and, Background thread. If you receive the events on the publisher thread, that I believe will make it synchronous like a command. With the composite command your UI wont be responsive until all saves are complete.