Today I learned that something that has been annoying me significantly when working in Swift, has a very simple solution, if only I knew my tools better.

This is a class. I have written a bunch of classes similar to this one:

That self in the constructor, annoys me to no end. I like how self is explicit in Swift, I think it makes the code more readable, less verbose, simpler. So, having it in the constructor, again, annoys me.

The solution is quite simple. Functions can have not only parameter names, but also argument labels.

Each function parameter has both an argument label and a parameter name. The argument label is used when calling the function; each argument is written in the function call with its argument label before it. The parameter name is used in the implementation of the function. By default, parameters use their parameter name as their argument label.

Adding an argument label:

And client code would remain unchanged:

I guess the lesson is, as always, know your tools.

Fair warning: Coming up with an example relevant enough to illustrate the topic, but simple enough to fit in about 1500 words is challenging. Take this code as a starting point, as a way to illustrate a discussion, and not as a drop-in solution ready to apply to a codebase.

Also, this solution can be applied to collection views. But again, for brevity, I will only discuss table views.

The previous post in this very blog was about going from the abstract and generic to the concrete and specific. It was, in a way, about specialising something abstract and generic, by subclassing it and providing a specific implementation for a specific type.

The sample code I used was more or less copied and pasted (with some info removed to protect the innocent) from the codebase I am currently maintaining. At the time I posted it it was holding well, but soon after that post I started having the feeling that there had to be a better way.

The problem

The problem I am trying to solve is a fairly common problem when it comes to iOS development: presenting lists of data without having an a lot of boilerplate code repeated again and again.

The usual way to present a list would be having a UIViewController that implements some or all of the methods in UITableViewDataSource.

That could be fine if there was only one data collection to render on screen, but usually, iOS apps are a sequence of screens that present data collections. Not only that, but usually, those screens are not that different from each other.

Sometimes, what changes from one data screen to another is the way a data item is rendered on screen (different UITableViewCells), some other times what changes from one screen to another is the presence or absence of section headers, or section footers, or both headers and footers.

So, if we approach the problem trying to abstract it as much as possible, what we would have would be a core behaviour (presenting data) that we might want to customise (i.e. adding headers or footers). And to make things more interesting, we want to do that in a way that reduces boilerplate and duplication and makes the solution testable.

The solution I have been exploring

The first step would be finding a way to factor out data management from view controllers. First, because it makes sense (a view controller should control a view, not manage a collection of data), and second, because by doing so we can remove a lot of boilerplate and duplication.

Shameless self promotion here: some time ago I published a framework to do precisely that.

There are other solutions, but I am sticking to mine for now, not because I think it’s better, but because, so far, it works for me. Basically, the implementation of the UITableViewDataSource protocol does not populate cells outlets, but provides cells with a model object, and trusts cells to know how to render that model object themselves. Tell, don’t ask!

So let’s start there. Let’s assume we are working on a task manager. We have a Task:

And we have a class to manage tasks as flat arrays. In real life, this data manager should be able to handle sections, but, again, for brevity, this one will be all flat:

Now, let’s move to the other end of the problem: the UI. As mentioned, cells will know what to do to render themselves when they are provided with data. We can model that assumption with a protocol that indicates that a cell will accept a model object, and a concrete implementation of that protocol:

A class implementing the required methods in UITableViewDataSource will look like this:

We provide it with a data manager, and a cell type. Notice how the implementation of the cellForRowAtIndexPath method only dequeues the cell, and passes it the model object that corresponds to its index.

So far, none of this solves the actual problem: customising data listings. But it send the foundation for a solution.

What was the problem again?

Let’s say that we have to render two different task lists, one of them without any section headers, and another one with a single section header. Let’s say also that we need a third listing with a section footer.

If we go all abstract, what we want is our datasource to behave slightly differently in different situations, while maintaining a core behaviour unchanged. Or, in other words, what we want is to add behaviour to an individual object without affecting the behaviour of other objects of the same class.

The Decorator Pattern

And that is, literally, the definition of the Decorator Pattern. So, let’s explore the Decorator Pattern a little bit more, and see if it can actually be a good solution to our problem.

(UML taken from wikipedia)

The pattern’s intent is clear: extending the behaviour of a certain object, independently of other instances of the same class. That will fit our case: we want some behaviour to remain unchanged but we also want to be able to customise some other behaviours, but only for specific cases.

There is another subtle detail here: the core behaviour, what we do not want to change, happens to be what is provided by the required methods in the UITableViewDataSource protocol, and what we want to customise, happens to be declared in optional methods.

That fits the pattern quite well. The ConcreteComponent could provide the behaviour we don’t want to change, provided by the require methods in the UITableViewDataSource protocol, and we could add ConcreteDecorators for each behaviour we wanted to add or customise.

So, just to recap, there seems to be a one to one relationship between the elements of the pattern and our use case: the Component would be the UITableViewDataSource protocol, the ConcreteComponent would be the Required class, and we can provide ConcreteDecorators that also implement the UITableViewDataSourceProtocol.

Let’s get to it!

This is the starting point:

We would also need an implementation of the UITableViewDataSource protocol that aggregates an instance of the UITableViewDataSource type, implements all the methods in the protocol and forwards them to the instance it aggregates:

And now we can start encapsulating the different behaviours we want to provide in subclasses of that TableDecorator. For example, if we need to provide a header title for a single section, a footer title for a single section, and header titles for multiple sections:

The beauty of this solution is that now we can add behaviours to the core behaviour of our datasource, in a composable way. Each behaviour would be well encapsulated, which in turn means that it can be easily tested, and it could also be reused eventually.

And what is even better is that we can mix and match behaviours without modifying the code in the core datasource, and without adding any conditional logic.

For example, if we want a table with section header and no footer, we can do this:

But if we want a table with header and section header and footer we can do this:

And if for some reason we wanted a table with a section header that is a slight modification of the previous one, we could just add another decorator like this:

That is the way we can compose optional behaviours: by adding them one by one, as layers of an onion, to a core behaviour.

Final words

I have ambivalent feelings about Design Patterns. I do agree that these patterns tend to be overused, and I have found myself a few times at that stage where all you can see are design patterns everywhere, and you try to solve any possible problem with an implementation of the Visitor Pattern.

But if design patterns are taken as what I believe they are, which is just one more tool, being able to identify them, and being able to consider the trade-offs of implementing or not implementing them can make a huge difference in the quality of a codebase.

In this particular example, having a collection of highly cohesive, loosely coupled and highly testable behaviours, that can be applied to modify a core behaviour at will, seems like a win.

I am not a big fan of inheritance. I would never say something like “inheritance is evil”, because inheritance, like composition, is just one of the many tools we can reach for, but in my experience, designs based on inheritance tend to be very rigid.

But here is one particular use case where I think the combination of generics and inheritance can provide an interesting solution.

As usual, the example that tries to illustrate this post is not very good. But it is close to my real use case, so I am going to stick with it.

I try to avoid boilerplate as much as possible. That’s why I try to use a generic implementation of the datasource protocols (UITableViewDataSource and UICollectionViewDataSource) to feed all my listings. I even released those datasources as a framework!

The thing is, there is always going to be some particular listing that does not fit completely into that framework. In particular, those listings that require implementing one of the optional methods.

For example, let’s take a look at tableView:titleForHeaderInSection. Do we need a header in every single tableview? Not really, some table views might not even be sectioned.

That method does not seem something that would belong in a generic datasource, because, well, it is there to provide a solution to a very specific use case.

So we could have a generic implementation of the UITableViewDatasource protocol like this (bear in mind I have over simplified it, for brevity):

This is generic enough to cover most of our use cases. But know, what if we need to provide a header for a very specific table?

Here is where subclassing to specialise can be handy. Let’s say our “special” listing is the one that displays Appliances. We could do something like this:

And done. What is generic remains generic, what is specific to a particular data type remains encapsulated in one single class.

Of course, the next question would be “what happens if we want the specific case to apply to more than one type?”. Well, that would be a matter for a separate post…

Mutability is dangerous and I have been aware of the fact for a long time. However, mutability can be sneaky, and make its way into the code without anyone noticing. Sometimes, with disastrous consequences.

First, a little disclaimer. I am a big fan of dependency injection. I have touched the subject in this very blog maybe too many times. I know it is not a golden bullet, I know it is not the solution to every problem, but still, I believe injecting dependencies is, in general, good.

Once that’s out of the way, let’s move on to today’s story. Imagine a view controller that needs to display data fetched from a backend. The integration with said backend is well encapsulated in a class, abstracted by an interface.

A simplified version of this view controller could look like this:

I like this approach: the view controller is provided with everything it needs to work in the constructor, the origin of the data is well abstracted (it could be a remote server, it could be a local database, it could be core data, we don’t care, because we don’t need to know).

But…

This view controller belongs to a storyboard. Dum dum duuuuum.

Here is another disclaimer. I am not a big fan of storyboards for multiple reasons (and the more I use them, the less I like them). But that does not change the fact that this view controller belongs to a storyboard.

So, now, in order to provide it the data service, you have to do something like this:

As far as I know, there is no way to avoid the fact that data can not be assigned a value when the view controller is instantiated. No matter how had I tried to work around it, like making the data variable private and providing a public setter, declaring that setter in a protocol and enforcing compliance in an extension, the fact is that, as far as I know, dataService needs to be a variable, and can not be set when this class is instantiated.

And that is a problem. Because if, for example, you have a segue that leads to this view controller, and you forget to set the value in an override prepareforSegue method, data is going to be nil and DataService.allItems() is not going to be called.

And if you have two segues that lead to this view controller, then you have to make sure that you set the value of data in two different places.

And that’s exactly how my rear got bit. I didn’t notice that second segue, so it was possible to instantiate this view controller without data being set.

You might argue, and you wouldn’t be wrong, that it is my fault, that I am the one that was not setting the value of a variable that is necessary for a part of the system to work properly. Yes, but…

If this class was really immutable (like it is in the first sample), the compiler would enforce that for me. If I do not provide a non-nil value to this view controller’s initialiser, the compiler will not build the project.

And that’s important. Because I am not good at remembering details, in particular details that I somehow consider that I should not need to remember.

In any system, in any codebase of a significant size, special cases, exceptions to the rule, and small details that you have to remember, are a recipe for disaster.

Thanks to multiple painful experiences, I have learnt that this apply to everything. If I need to change a url manually before submitting an app for review, there is going to be a moment when I am going to forget, and submit with the wrong url. If an API needs a parameter named data, and returns data in a parameter named values, sooner or later I am going to mess those up. And if I code a class in a way that it can be initialised in an inconsistent state, by code is going to find a way to initialise that class in the most inconsistent state possible.

I won’t day that “mutability is evil”. There is a place for it. But, this week, I got confirmation, one again, that the best way to avoid problems is allowing the compiler to help me as much as possible.

This post is about one of the Design Patterns, the Template Method. Except for the fact that, well, it is actually about somethign competely different.

Some time ago, in a galaxy far far away, the team I was with was building an internal framework. Our code was supposed to be an amalgamation of rainbows and unicorns that the rest of the company would use as a starting point to build highly customised projects. Yes, it didn’t end well.

But something today made reminisce of a particular episode in that era. Fair warning, some details have been changed to protect the innocent and or make my example a little more relevant.

We were building a video player. The requirements were quite simple: the UI should be easily customisable, and playback events should be trackable (i.e. play, pause, progress), and should be handled in a way that it should be easy to integrate with a second screen solution. Mind you, the company’s business domain was online video delivery, so as you could imagine, this was kind of a big deal.

So, off we went to build the thing, machete in hand and bug spray in pocket.

The key piece of the design was a “manager”, a block of logic that coordinated the playback status with the player’s UI state, while offering a default integration with a second screen solution (also developed in-house).

Now, and this is important, we needed to provide that integration with a second screen solution due to internal business reasons, but the design should be open enough so that other teams could provide their own integrations (i.e. AirPlay or Chromecast)

After some lively discussions we decided to provide what we believed complied with all the requirements.

So, this was, more or less, our manager, translated to Swift 3. Because those were still the Objective-C days.

The idea was simple. We provided the bulk of the business logic in our manager, and subclasses of our manager should be provided for different integrations with second screen solutions.

Basically, we designed an system modelled after the Template Method Pattern.

If a team wanted to integrate this video player with Chromecast, they would just need to subclass our manager, override the extension points we had provided, and go on their merry way. If another team wanted to integrate with AirPlay, they could do the same.

But, what would happen if a team wanted to integrate with AirPlay and Chromecast?

Well, they could subclass our manager with an integration with Chromecast, and then subclass their Chromecast integration with an AirPlay integration that could call super as part of each method that it overrides.

And that’s my issue with this pattern: it is not exactly what I would consider a very open design.

So, what would be the alternative? As usual, it depends.
To begin with, this pattern can be extremely useful sometimes, and provide very elegant solutions to some problems.

But, in this case, and I guess in any case where an open design is the main goal, it might be better to define an interface for each behaviour that we want to be customisable, and inject them.

For example, we could change the previous design to something like this:

The original design was not bad. This new design, however, has a couple advantages over the original design.

First, the manager and the different implementations of the second screens are more decoupled. The manager can work with multiple different implementation of the second screen, and those implementations of the second screen can collaborate with something other than the manager.

Second, these implementations of second screens are more cohesive, and therefore are going to be easier to test and maintain in the long term. And this part, in my experience, is extremely important. When classes do one thing and one thing only, and all the logic related to that thing is encapsulated in the same class, it is way easier to go back to it months after the code was written.

And third, and more important, now it is way easier to compose behaviours. For example (and I know this is a bad example, but it helps me illustrate my point), if we were asked to send play/stop events to both our InHouse solution and Chromecast and playback progress events only to Chromecast, we could reuse the implementations we already have, like this.

So, I guess, the whole point of this post is “favour composition over inheritance”.

Except, of course, when inheritance provides a better solution to the problem at hand.

So, on second thought, I guess the whole point of this post is that there are trade offs to every approach. A good engineer should try to choose the approach the solves the problem better, with the information she has at hand at that particular moment.