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Category: Unit Tests

On unit testing private behaviours

TL;DR; every time I find myself in a situation where I need to unit test a private behaviour, I consider that a clear indication that I might need to rethink my design.

Let me tell you a story

Some time ago, in what now seems like a galaxy far far away, I was introduced to unit testing by means of someone adding the following requirement to the project my team was developing: “code must be unit tested”.

As you can probably guess by the hint of sarcasm in the previous paragraph, unit testing, like almost everything else in life, is something you learn, not something you are born with. Which was painfully obvious at that time, as I have already written about.

But the thing is, a couple days ago I finally had the opportunity, since I was trapped in a plane for 13 hours, to catch up with some reading. In particular I was finally able to devour Sandy Metz’s new book.

The book is still unfinished, but I highly recommend grabbing a copy, and reading it cover to cover, as soon as possible.

There are plenty of potential quotes in that book, but I would like to focus on one in particular:

… the first step in learning the art of testing is to understand how to write tests that confirm what your code does without any knowledge of how your code does it.

Tests always tell at least two stories.

Unit tests always tell at least two stories. One, specially if the tests are well written, is a cristal clear description of the expectations you have about the behaviour of the code under test.

The second story, though, is usually one that nobody likes to hear. If your tests need to manipulate internal behaviour of the code under test, in order to test your external expectation of that code, then you need to rethink your design. And allow me to illustrate this with an example, that links with the introduction to this very same post you are reading.

When testing goes wrong.

Back to my story: no one in our team had any experience testing, so at the time we did not recognise the code smell, but here is what we were trying to do.

We had this one class that was supposed to be a one to one mapping to a RESTful API, what in the classic three-tier architecture would correspond to the data tier, service layer, transport layer, whatever you call it. The point is, this was the class that performed networking operations against a RESTful API. Let’s call it MediaService.

Part of the expected behaviour of MediaService was that it should cache data, using a key-value cache. This data cache would expire after a given time.

So, the flow would be like this: with every request, we would first check if there was data cached and not expired. If there was data cached, MediaService would return it, if there was no previously cached data or the existing data was marked a expired, MediaService would attack the network, fetch data, and add it to the cache, associating a timestamp to each key-value pair.

So, the next time we wanted to request data to the same endpoint, we would check the cache again, providing a timestamp. The cache would calculate if data was expired or not… rinse and repeat.

In code, that looked similar to the following snippet (those still were the Obj-C times, so this is a rough translation to Swift of my recollection of the events):

Well, not very idiomatic Swift code, of course, but again, those were the dark days before generics.

Now, if you wanted to test that cache was ignored when cached object have expired… how would you do that?


I don’t recall who said it, but someone said that good software engineering is all about arranging the code in a way that complexity does not collapse on you.

That usually implies breaking down your codebase, organising it into separate subsets or structures. Since you are already breaking down your code and organising it in smaller chunks, you might as well make each of those chunks contain code that is related, that deals with one concern. So, instead of one big ball of code, you would end up having smaller balls, each one of them taking care of one and only one smaller part of the big problem your software is trying to solve. (That’s how I visualise cohesion, by the way)

But of course, the problem your software tries to solve is bigger than each and everyone of those small, cohesive structures that you have used to organise your code. So they need to collaborate with each other.

And here is where dependencies are born. One block of code needs to collaborate with another block of code. One part of your solution needs to collaborate with another part of your solution. Or, if you will, one part of your solution depends on another part of the solution.

The tricky part here is how to set up those dependencies in a way that do not defy the whole purpose of breaking down your code into smaller structures. The key is setting up those collaborations in a way that the collaborators are still independent from each other.

Depending on abstractions not concretions

Yep, that’s kind of the definition of the Dependency Inversion Principle.

In the previous code sample, MediaService needs TimeCache. The key here is that it actually depends on the existence of TimeCache, because MediaService creates TimeCache, therefore the behaviour provided by TimeCache is, in a way, embedded within MediaService. Or, the MediaService abstraction depends on the implementation details of TimeCache.

But, why does that matter?

Firstly, MediaService is tightly coupled to TimeCache. So coupled to it, that it actually needs to create it in order to make itself functional.

Secondly, and due to the previous point, it is not possible to modify the behaviour of TimeCache, without modifying MediaService.

Thirdly, there is a more subtle issue here. The fact that there is a cache, and the fact that said cache expires, is not part of the public API of the MediaService class. It is an implementation detail, buried into the MediaService class code, but an implementation detail that is so significant that leaks outside its container. Because we know MediaService handles a cache that marks items as expired, because we expect actually expect that, however, the expectation is not declared anywhere in MediaService’s public interface.

So now, when it comes to unit testing, we face a very interesting problem: we need to test a behaviour that is a private implementation detail, but since we need to test it, it is not private anymore, it is a well-known expectation, but an expectation that we can not test because it is a private implementation detail. You see the infinite loop here, right?

And notice how I have not even mentioned networking…

No worries, mocks, stubs and spies can help!

Well, not anymore. In the old Objective-C days, we could just declare a category in the testing bundle to expose and override any private property of a class with a mock (or stub, or spy, to be honest, I never really understood the difference).

In this case, we could override cache and network, provide mocks, set our tests, and go on our merry way.

The problem with that is that we would still be testing private behaviour. And why should not do that?

The reason, is again, subtle. When we test private behaviour we are basically coupling our tests to the production code very tightly, because we are not testing public APIs anymore, we are testing internal implementation details we should not even be aware of.

Have you ever heard anyone complaining about how unit testing is a waste of time and effort because if you change the production code there is always going to be a bunch of tests that need to be rewritten? I bet the reason is because of vey tight coupling between tests and the code under testing. Or, in other words, because the tests are not testing why they should (public expectations) but private details about how those expectations are fulfilled.

Also, in the age of Swift, doing that is not even possible anymore. If MediaService is final, we can not subclass and override, and if cache and network are declared as constants with let there is no way to override them.

So, what to do?

Invert the dependencies

This is not the first time I blog about dependency injection. Once even using the same example I am using today, another time in a different context. And I am afraid it won’t be the last time I blog about it.

But that’s for a reason. Inverting the dependencies, in my experience, only has upsides. It makes code more decoupled, easier to test, and therefore easier to maintain in the long term.

In this case, the best solution would be something along these lines:

Cache and networking behaviours can be provide now through the MediaService initialiser. Notice the shift in the way we refer to caching and networking now: as behaviours. Because for MediaService, that’s what they are.

The details on how caching and networking work are not known to MediaService anymore. MediaService is provided the behaviours it needs to rely on, the behaviours it needs to collaborate with.

And that’s the beauty of inverting the dependencies. By carefully setting the way collaborators know about each other, and by carefully hiding whatever is not necessary for collaborators to know, and by shifting from concretions to abstractions we make each of our building blocks rely on the expectations that the other building blocks publish about their own behaviours.

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I have been thinking about how to approach this post for almost a week, and in the end, I decided that I am going to do it with honesty. The following are my opinions, and I take full ownership of everything I am going to write. So, fair warning, long rant incoming.

I am sick and tired of hearing bullshit related to unit testing. I have a really hard time figuring out how any developer can be fine with delivering untested code, code that nobody knows if it really works until it is tested in production, with real users. I am sick and tired of hearing always the same arguments, the same excuses, justifying not writing unit tests. And I hate how the idea that unit testing is not indispensable has made its way into some management circles.

These are some of the excuses I have heard to not write unit tests.

We don’t have time to write unit tests.

Well, when you say that, what I hear is “I don’t know how to write unit tests”.

Why? Because of this:

Unit testing is a software development process in which the smallest testable parts of an application, called units, are individually and independently scrutinized for proper operation.

That’s the first result when googling “unit tests”. And I think it is a very good definition.

A unit test should test a small unit of behaviour. If you expect a class to return a specific value from an specific method when some particular conditions are met, all you have to do is test that you get the value you expect from the method you are interested in after recreating your particular preconditions. That is not that hard. That is not that time consuming.

And if it is, if testing the smallest unit of behaviour in your system is hard and time consuming, then your smallest unit of behaviour is not as small as you think. Back to my argument: you would be testing it wrong.

If I change the production code I will have to change my tests.

Well, obviously, if you know that the behaviour and the public API of a module are going to change tomorrow, first thing in the morning, it might not make too much sense to unit test that module today.

But… how many times will you find yourself in that situation? Not many.

Of course, a codebase is similar to a living and breathing organism, that evolves and might change its behaviour over time. Well, since you will have to refactor your production code, your will also have to refactor your testing code to reflect the new behaviour of the system.

But justifying not writing tests because you will refactor your code in the future does not make any sense. Will you also refuse writing code, today, because you might need to refactor that code in the future? I don’t think so.

Testing is difficult and requires too much work.

Well, sorry, but if testing your production code is very difficult, your production code is not properly designed.

You might be trying to test classes with too many responsibilities (violating the Single Responsibility Principle) you might be testing private behaviour, or you might be testing multiple layers at once (therefore, trying to write something that is not actually a unit test). Either way, when testing gets hard, that’s a clear sign that the design of the production code is wrong.

Or, to put it another way, well factored code, with small, highly cohesive and loosely coupled units, is easy to unit test.

Testing private behaviour is very hard.

This point is not too different from the previous one.

If a behaviour is private, it should not be tested. If you really really want to test it, it means it is not private. So, we are back to swore one: the code is badly designed.

Now, allow me to digress a little bit, and tell you a story. A couple years ago, my team spent an insane amount of time trying to test a class that was performing some network requests, and the cached the behaviour. Cache was supposed to expire after five minutes, if I recall correctly. While caches were valid, the class performing the networking would return cached values, when caches were invalid, it would trigger a new network request. The production code looked like this:

The team complained a lot about how difficult it was to test that behaviour, how difficult it was to mock the cache, and spent an insane amount of time researching mocking frameworks.

All we needed was turn to the Dependency Inversion Principle. Inject the dependency like this:

And now the behaviour of the cache could be tested in isolation. And the behaviour of the class doing the networking could be tested in isolation. And the collaboration between the both of them could be tested in isolation as well.

Testing the UI is impossible. How are you going to unit test a view controller?

What do you mean by “unit testing a view controller”? Or “unit testing an activity”? Because what I have found people usually mean by that is something along the lines of “how am I going to test that when I tap a button the text in a textfield is updated with new content dowloaded from a server”?

The UI layer should be thin. Very thin. It should only render data, and propagate down user interaction. Period. If it does something else, the design is wrong. Period.

Some people call it The Humble Object Pattern, but I don’t think we really need a name for it. The UI layer should not do any “business” logic. If the UI layer needs to coordinate state between UI elements, that logic should be extracted to different units, because of modularity, and encapsulation, and good separation of concerns, but also because as an added benefit, it will be easier to test.

By now, I am sure you see the pattern: well designed code is easy to test. But the opposite is even more accurate: poorly designed code is difficult to test. And the more difficult it is to test a particular unit of code, the poorer its design.

I can’t waste time! I need to ship!

Do you want to ship code that works? Or do you want to just toss a dice?

Did this post offended you? If that’s the case, to be honest, I don’t care.

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Unit testing collection and table view cell outlets

This is the scenario (I hope a picture is really worth a thousand words):

Screen Shot 2015-12-29 at 11.17.07 AM

This is a very simple cell, with just two outlets, a UILabel and a UIImageView. I would usually build cells this simple programatically, but in this case I was going after something very quick, so I decided to take the opportunity to improve my IB kung fu.

Warning! Read this!

There is an important precondition to this post: the project I’m working on does not use Storyboards, and does not use xibs to declare the layout of view controllers. So, the cell xib is completely isolated!

Moving on

This is the cell in Interface Builder:

Screen Shot 2015-12-29 at 11.17.22 AM

Nothing fancy: a vertical UIStackView that covers the whole cell.

Incoming context switch!

Pardon the deviation, but this is how I like to populate cells lately:

Why on earth I don’t populate the outlets in the collection/tableview datasource? Well, I feel like letting the actual cell (in this case a subclass of UICollectionViewCell) take care of populating its own UI is the way to go. It provides better encapsulation, and better separation of concerns. The cell takes care of the UI, and the datasource only takes care of dequeuing a cell and providing it with a data object. Single responsibility principle FTW!

Back to unit tests

So, now I want to unit test that cell. Which means that I want to test that my expectation about the behaviour of that cell are fulfilled.

What is my expectation? I expect that, if I pass it an Item with a given title, the cell label will contain that Item’s title.

It might sound like something obvious and not worth testing, but that’s the whole point of unit testing: being able to break expectations down into small chunks, almost trivial chunks, that don’t require too much context to be tested. That way, tests are easy to understand, and make great documentation about the behaviour of the whole system with a level of details very difficult to achieve by other means.

Load all the things!

The issue here is that I need to load a xib within the testing bundle. So, the first thing I need to do is add the xib to the testing bundle:

Screen Shot 2015-12-29 at 11.29.28 AM

That’s something I don’t really like, but I am afraid it is impossible to avoid.

Now, there is one more thing to do. Make sure to specify, in Interface Builder, the Module that the cell’s custom class belongs to:

Screen Shot 2015-12-29 at 11.30.23 AM

This step is important. If you leave the default value, Xcode will try to find FeaturedCell in the testing bundle.

So, now, in your unit test class, you need to load the xib (the one in the testing bundle), and extract its first element, casting it to your custom class, qualified with the module name. Qualifying the module name is important as well, it will tell the test to look for your class in your module, not in the tests bundle:

And, finally, test your expectation

Now, it is just a matter of testing your actual expectation, which in my case was that setting an instance of Item as the cell’s data property, will assign the Item’s title to the cell’s titleLabel.


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