Hi Casey, the decoupling of the event emitters and receivers is what I find the most interesting in a pub/sub model. The publisher raises an event (in its semantic domain) and does not know what subscribers are going to receive it or what they're going to do with it. One of the advantage of this decoupling is that you can extend a system without modifying existing parts.
Practically, I also encountered systems where a pub/sub model helped reducing the amount of code. A lot of objects needed to be aware of events happening in the system and instead of having to call all these objects every time something interesting was happening in the system, we raised an event for each one of them and had the objects subscribe to these events. The other advantage I also felt while using pub/sub model is that it helped me create better object-oriented design: - because you can't send direct messages, you don't use your objects as data structures - and consequently, all the code that changes the state of your object is located in the object which makes the system easier to understand The issue I have with the event systems I encountered so far is that you gain the decoupling but lose the expressiveness of message passing. The code in the subscribers often looks like this: cause => effect cause => effect cause => effect If you want to implement a complex behavior, you need to maintain state and that becomes quickly harder to understand. That's why I was wondering if you could interleave the "causes" and "effects" in a grammar to describe the behavior of an object in a more readable way, be able to reuse rules, having higher-order rules... Benoit On Mon, Mar 19, 2012 at 3:35 PM, Casey Ransberger <casey.obrie...@gmail.com>wrote: > Here's the real naive question... > > I'm fuzzy about why objects should receive messages but not send them. I > think I can see the mechanics of how it might work, I just don't grok why > it's important. > > What motivates? Are we trying to eliminate the overhead of ST-style > message passing? Is publish/subscribe easier to understand? Does it lead to > simpler artifacts? Looser coupling? Does it simplify matters of concurrency? > > I feel like I'm still missing a pretty important concept, but I have a > feeling that once I've grabbed at it, several things might suddenly fit and > make sense. > _______________________________________________ > fonc mailing list > fonc@vpri.org > http://vpri.org/mailman/listinfo/fonc >
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