Observer pattern terminology

gistfile1.md

When it comes to the Observer pattern, there are a number of related, overlapping terms used by different codebases. Off the top of my head:

• Observer/Observable/Subject
• Listener/Listenable
• Event/Event source/Event sink/Event handler
• Hook/Callback
• Subscriber/Subscribable
• "Firing" vs. "Triggering" vs "Notifying" vs "Executing"

Which of these do consider to be synonymous?

Of terms that are synonymous, which do you prefer to use in your programs?

Of terms that are not synonymous, what semantic differences do the different terms imply to you?

Curious if subscriber/subscribable fits here as well?

@rubycoder1 yes! Added.

Does Command apply here?

@davidjbeveridge no, I'd say not.

'Observable' is called subject in the GOF pattern.

I think hooks and callbacks are different because you're observing individual messages (using a Proc), whereas observers / subscribers seem to mean an object that understands a richer protocol.

Brainstorming:

• Observer and Listener to me are synonymous in the behaviour but you listen for an event and observe an object.
• So there is a more strict semantic connection between the "subject object" and the "observer object" because all the concrete observers share the same type while the listener of an event can be very diverse.
• An event in general recalls me an async concept that decouple the time of execution of a procedural flow and can be listened and so handled.
• But Event Source reminds me the DDD Event Sourcing pattern that is very specific way of handling messages between two different bounded contexts.
• A callback is generated by a method call but it doesn't specify a relationship of two or more objects. It just happens when a message is sent.
• Publishing subscribing mechanism shifts the accent more on passing data asynchronously more than sending messaging decoupling the time axis.
• "Firing" an event will "Trigger" a listener that could "Notify" an observer that will "Execute" a query :) Jokes apart I think execute is the only not time decoupled verb here.

Does something I said makes sense to anyone? :D

• Hook/Callback is a just function that is triggered by some specific message arriving on an object (a method call). So the difference with the listener/object is that the callback is just involving the object that defines it

@mattwynne thanks! Updated.

The very last point is a typo. Sorry.

If i did - I didn't mean to stop the conv flow (!) and I'm very intersted in understanding what do you think about my brain dump and confront it with different opinions! So I'll keep on observing this subject or subscribing this publication as you wish :-)

"Firing" vs. "Triggering" vs "Notifying" vs "Executing"

The first thing that springs to mind when I read these is firing an event, triggering a state change or transition in a state machine, notifying a listener, and executing a method.

I've also seen codebases talk in terms of emitting events.

I tend to think an object would 'listen' to events generated (to use a neutral word) by another object, but 'subscribe' to a persistent message queue or topic (where "message" is not the OO kind of message, but the message bus kind of message).

Observer and Listener seem closely coupled with the source, compared to Publish/Subscribe and Notify, which both feel quite decoupled from the source (of events/notifications). So I will ATTACH a Listener directly to an object, but Subscribe to events/notifications from who-knows-where.

@kevinrutherford +1 this is a good point: in general you can publish/subscribe to a "topic/queue/board" that is a more general concept than listen/observer an object.

I've seen pub/sub as in — publish / subscribe — somewhere when developing Javascript a time ago.