Single function handling events or a function for each event? - swift

Question

What to prefer in which situation: Single function handling events or a function for each event?
Here is a basic code example:
Option 1
enum Notification {
case A
case B
case C
}
protocol One {
func consumer(consumer: Consumer, didReceiveNotification notification: Notification)
}
or
Option 2
protocol Two {
func consumerDidReceiveA(consumer: Consumer)
func consumerDidReceiveB(consumer: Consumer)
func consumerDidReceiveC(consumer: Consumer)
}
Background
Apple use both options. E.g. for NSStreamDelegate we have the first option, while in CoreBluetooth (e.g. CBCentralManagerDelegate) we see option two.
One big difference I see is that Swift does not support optional protocol methods nicely (via extension or #obj keyword).
What would you prefer? What's the (dis)advantages?

Answer 1

In terms of achieving the loosest form of coupling and highest degree of cohesion, naturally the choice would sway towards individual events, not this kind of multi-event bundle of responsibilities.
Yet there are a lot of practical concerns that might move you towards favoring the opposite, coarser way of dealing with events instead of a separate function per granular event.
Here are some possible ones (not listed in any specific order).
Boilerplate
While it's not the biggest thing to worry about, writing a bunch of functions tends to take a bit more effort than writing a bunch of if/else statements or switch cases within one. More importantly than this, however, is the code needed to connect/disconnect event-handling slots to event-handling signals. Avoiding the need to write that subscription/unsubscription kind of code for every single teeny event handled can save considerably on the amount of code to maintain.
Performance
It might seem counter-intuitive that performance can favor the coarser multi-event handler. After all, the granular event-handler requires less branching (one dynamic dispatch to get to the precise event handler), while the coarser one requires twice as much (one dynamic dispatch to get to a coarse event-handling site, and another local series of branches to get to the precise event-handling code).
Yet the cost of dynamic dispatch leans heavily on branch prediction. If you're branching into coarser event handlers, then often you're branching more often into the same set of instructions, and that can be an optimization strategy. To have two sets of more predictable branches can often produce more optimal results than one less predictable branch.
Moreover, coarser event-handling typically implies fewer aggregates, fewer lists of functions to call on the side of those triggering events. And that can translate to reduced memory usage and improved locality of reference.
On the flip side, to branch into coarser event handlers often means branching more often. For example, some site might only be interested in push kind of input events, not resize events. If we lump all these together into a coarse event handler and without some filtering mechanism on top, then typically we would have to pay the cost of dynamic dispatch even for a resize event that isn't even handled for a particular site.
Yet I've found that this is actually often better than I thought it would be to branch needlessly into the same coarse functions (most likely due to the branch predictor succeeding) as opposed to branching into a wide variety of disparate functions and only as needed.
So there's a balancing act here and even performance doesn't clearly side with one strategy over the another. It still varies case-by-case.
Nevertheless, lacking measurements and very detailed data about the critical code paths, it's typically safer from a performance perspective to err on the side of these coarser multi-event handlers. After all, even if that proves to be the wrong decision from a performance standpoint, it's easier to optimize from coarse to fine (we can even do so very non-intrusively by keeping the coarse and using fine-grained event-handling in cases that benefit most from it) than vice versa.
Event Subscription/Unsubscription
This can likewise swing one way or the other, but in my experience (from team settings), most of the human errors associated with event handling do not occur within the event-handling code, but outside. The most common source of errors I see relate to failing to subscribing to events and, most commonly, failing to unsubscribe when the events are no longer of interest.
When events are handled at a coarser level, there's typically less of that error-prone subscription/unsubscription code involved (this relates to the boilerplate concerns above, but this is unusual kind of boilerplate in that it can be quite error-prone and not merely tedious to write).
This is also very case-by-case. In the systems I've often been involved in, there was a common need for entities to continue to exist that unsubscribed from certain events prematurely. Those premature cases often required the code to unsubscribe from events to be written manually, as they could not be tied to an entity's lifetime. That may have pointed more to design issues elsewhere, but in that scenario, the number of mistakes made team-wide went down with coarser event handling.
Type Safety
While not shown in the examples here, typically with coarser event-handling is a need to squeeze more disparate types of data through more generic parameters. That might translate in an extreme scenario like in C to squeezing more data through void pointers and more dangerous pointer type casts. With that, compile-time type safety is obliterated and we could start seeing a whole new source of human error.
In higher-level languages, this might translate to more down casts or things of that sort when we cannot model the signature of a delegate to perfectly fit the parameters passed in when an event is triggered.
I've found typically that this isn't the biggest source of confusions and bugs provided that there is at least some form of runtime type safety when casting or unboxing these parameters. But it is a con on the side of coarser event-handling.
Intellectual Overhead
This might vary per individual but I tend to look at systems from a very administrative/overview kind of standpoint and specifically with respect to control flow. It's because I tend to work in lower-level portions of the system, including things like proprietary UI toolkits.
In those cases, when a button is pushed, what functions are called? It turns into a mystery in a large-scale codebase composed of hundreds of thousands of little functions without tracing into the code actually invoked when a button is pushed and seeing each and every function that is called.
That's an inevitability with an event-driven paradigm and something I never became 100% comfortable about, but I find it alleviates some of that explosive complexity that I perceive in my personal mental model (something resembling a very complex graph) when there's less code decentralization. With coarser event handlers comes fewer, more centralized functions to branch into throughout a system on such a button push, and that helps me increase my familiarity when there are fewer but bigger functions involved in my mental graph.
There is a very simple practical benefit here where if you want to find out when a specific entity responds to a series of events, we can simply put a breakpoint on this one coarse event-handling site (while still being able to drill down a specific event for that specific entity by putting a breakpoint in a local branch of code).
Of course, I might be an exception there working in these low-level systems that everyone uses. It seems a lot of people are comfortable with the idea of just subscribing to a button push event in their code without worrying about all the other subscribers to the same event.
From my kind of holistic control flow view of the system, it helps me to absorb the complexity more easily when there are fewer but coarser event-handling sites in the codebase even though I normally otherwise find monolithic functions to be a burden. Especially in a debugging context where I face a concern like, "What caused this to happen?", that combined with the event-handling concern of "What functions are actually going to be called when this happens?" can really multiply the complexity. With fewer potential target sites where events are handled, the latter concern is mitigated.
Conclusion
So these are some factors that might sway you to choose one design strategy over another. I find myself somewhere in the middle. I generally don't choose design as coarse as say, wndproc on Windows which wants to associate a single, ultra-coarse event handler for every single window event imaginable. Yet I might favor designing at a coarser event-handling level than some just to alleviate this kind of mental complexity, reduce code decentralization, possibly improve performance (always with a profiler in hand).
And then there are times when I choose to design at a very granular level when the complexity isn't that great (typically when the package triggering events isn't that central), when performance isn't a concern or performance actually favors this route, and for the improved type safety. It's all case-by-case.