API question: Delagates/FPs or Interface?

[Daniel Keep]

Robert Fraser wrote:
> Hi all,
> 
> I'm working on a new NAT/firewall traversal library in D right now, and I have a question for you all: since D has enough expressive power to expose both function pointers and interfaces, which would you prefer for a set of "callback" functions. In particular, the library allows a client to start establishing a connection in a separate thread, and when the state of the connection changes, a function is called to inform the client.
> 
> I had two possible solutions in mind:
> 1. Create an interface with the required methods (such as onFailedState, onErrorState, onSuccessState, etc.) specified. The library takes a pointer to an implementing object and calls the methods through that.
> 2. Accept a set of function pointers or delegates (I'll overload the methods so either is acceptable), and call the functions through that. One advantage of this is that a single callback function is easily interchangeable without having to re-implement the entire interface, and it promotes code reuse because a client might want to implement, for example, only a single onFailedState method, but have a different onSuccessState for different types of connections.
> 
> If you think the first option would be better for something like this, there are a couple of other places with a single callback function is needed (for example, to deal with an incoming connection invitation) that don't fit into a set... Should these be interfaces as well?
> 
> Thanks for whatever input you have,
> All the best,
> Robert Fraser

My personal feelings would be to use delegate callbacks for singular
things, and interfaces for logically connected things.

For example, SAX works well using interfaces because if you're going to
have a "startElement" callback, odds are you're going to at least use an
"endElement" and "characters" callback as well.  Since these are
logically related to each other, you use an interface to keep them together.

On the other hand, if you've got a window with things like onClick,
onKeyPress, onLoseFocus, they're not really related to one another.
Using one of them doesn't really mean you'll use any of the others.  In
that case, use delegates and let the user attach a callback to them
individually.

Finally, I'd be inclined to err on the side of delegates; they're
slightly more flexible, and if you find you're always using a particular
set of delegates, you can always write a base class that handles
registering the delegates for you.

Anyway, my AU$0.02 :)

	-- Daniel

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