Most developers have heard of the Single Responsibility Principle (SRP). It is one of the principle pillars of SOLID and frequently touted as a core principle that all developers should strive for when designing their code. It is also one of the least understood principles I’ve ever seen. Most developers will agree that it’s really important, but these same developer will have a hard time defining SRP means in concrete terms. They have an even harder time translating the amorphous concept into their code.

This became painfully clear to me when we had a particularly bright ¹ developer create a Rest API Client for our app ². He was an enthusiastic proponent of SRP and sought to employ the principle in all of his code. Unfortunately, he left soon thereafter for a better job, because to this day, I do not understand what he wrote. His client spanned across 10 different classes and so fully conformed to the SRP principle that no one in our team ever managed to understand how all the pieces came together. Whenever it came to modifying the client to fix a bug or add functionality, that task became one of the few tasks no developer wanted to take up ³. In the end, I recreated the API client as a single class that contained one fifth the original code ⁴. The experience was eye opening to me. I thought I was a proponent of SRP until he showed exactly how far I hadn’t taken the concept.

Often, complicating a class is simpler than abstracting it out. Except, of course, when it’s not.

Over the past few years, I’ve seen an increasing interest in software architecture as an important, perhaps now even an integral, part of software development. While this may seem obvious now, it’s still surprising to me just how much architecture is considered as an afterthought by many developers. At least in the mobile space, it’s taken a long time for architecture to enter the foreground of the conversation for software development. But even as the discussion about software architecture grows, I’ve noticed that the conversation tends to center on individual architectures as a solution to specific problems inherent in software development. You can see detailed descriptions of VIPER, MVVM (and it’s variants), the ubiquitous MVC as well as a scattering of others for app development. While each of these solutions do address specific set of problems ¹, what they do not do is teach developers how to create their own architectures to address problems specific to their own development. As such, many developers will try to use an existing architecture to solve the wrong problem, or else failing that, to forget about architecture altogether and hobble along as best as they can.

While I have been pleased to see the conversations on software architecture, I think to a certain extent they are the wrong discussion. We’ve been discussing the benefits/drawbacks of individual architectures someone else created. What we should be talking about are the principle of architecture that allow us to create the architectures we need.

Now that I’ve spent quite a bit more time implementing a “handler” based development approach, I’m finding that there’s quite a bit I like about it. I still have some frustrations with it, and some unresolved questions, but overall I think this approach has some serious merits that warrant consideration as an Architecture approach to handling complex dependencies.

To give some perspective, I’ve been using this approach to construct a transport layer client for a messaging service. The service is not simple, with concepts of multiple messaging types, rooms, user, orgs, members, a “mentioning” system, markdown-like parsing, etc. It’s responsible for communicating with a TL layer (via a socket), persisting data, managing sync, updates, etc. As you can imagine, there is a lot of interdependencies doing on here, many of which can only resolve asynchronously via network calls. For example, A Member is a User and and an Org. So before you can have one, you must ensure you have the others. This results in a lot of what I call “asynchronous interdependent behavior” among the various machinations in the code. The reason I turned to a handler-based approach was that I found that these interdependencies were so numerous and varied that attempting a protocol based approach would end in a proliferation of single-use protocols. Moreover, Swift has limitation on protocols that I’m not quite pleased with. This is not to say I don’t use protocols. The TL client exposes itself to the rest of the app (it’s a module) via several protocols. But internally, that was not the right approach.

What I’ve realized is that this approach is extremely useful to a specific set of requirements, but the approach is not useful in all situations. To me, this is a new “tool” I’ve learned… a conceptual model and practical approach useful to a circumstances and requirements I’ve commonly come into contact with. It’s an architectural model useful to solve certain problems but definitely not all problems.

Lately I’ve been writing a Swift module for a project at work. I decided to branch out a little bit by utilizing a different design philosophy than I’ve used in the past. While Apple sits around and pretends that protocol based development is actually something new and exciting, I’ve been much more interested in the functional abilities that Swift seems to endow us with. To that end, I decided to escew protocols and instead define the dependencies in my objects using handlers¹ instead. This means instead of defining a protocol and requiring an object that conforms to this protocol, I simply require the specific behavior itself. An example of a simple handler might be:

	typealias GetUserForID = (userID: String) -> User

A protocol, instead, might look something like this:

	protocol UserRetrieval {
		func getUserFor(id: String) -> User
	}

At first glance these really look similar, except perhaps the the protocol is a little more verbose, which might not actually be a bad thing. I’ve noticed a trend with Swift devs (myself included) to be overly concise with their code which disturbs me because in the end it simply creates short code that’s impossible to read.

If you read my Architecting Complexity post, you know I feel pretty strongly about the need for Software Architecture ¹ in software development in general, but especially in “app” development. So far, I’ve not gone in to very much detail into my approach to applying Software Architecture to software development. To be fair, I’m going to focus only on app development, but many of the principles here apply much more broadly. It just happens that developing apps is something I do a lot, so I’ve developed a consistent strategy regarding it.

I can’t put all this in one post, so I’ll be splitting it out into several posts.

When I first started developing software, I made very limited use of protocols. I would occasionally model my software from what I’d seen Apple do with delegates, but once blocks came out I mostly abandoned delegates for callbacks, which I much prefer despite the inherent risks of reference cycles and the weak dance. Because really, why would I want to write my interface in a completely separate file when I already have a convenient “header” file right here. And yet, as I’ve continued to build software and especially as I learn more about software architecture, I find my eschewing interface files and depending on protocols more and more. Let’s just say I’ve become a convert to “protocol based software development”. What exactly do I mean by this?

Well, obviously, it means you use protocols, and I mean use them more than for just delegates. What I mean is that you abstract your interface away from your actual class definition, avoid putting anything in your interface except that it adheres to protocol x, y, etc. Essentially, you construct your interface by conforming to one or more protocols. Here are some “rules”¹ for protocol based software development:

As a contractor, I come across a lot of other people’s projects and may I just say, there’s a lot of crap out there. And I mean that. Not just “Oh, this developer could have done better here and there”, but more alone the lines of “Sorry, I can’t work with this. Your best option is to rewrite the whole damn thing.” To be clear, I’m not necessarily talking about a person’s coding skills. Certainly, I’ve come across some questionable code and downright stupid approaches to solving problems, but this is not what I’m talking about. I’m talking about a complete lack of any kind of architecture to the codebase. Too often I come across projects that seem to be trying to typify the “anti-design pattern”, usually by stuffing quite literally everything into the View Controllers.

So I want to explain here why Software Architecture is important and, if you’re a developer, why you should be learning it or, if you’re a client, why you want it in your project.