The hidden cost of `@testable`
29 Mar 2021If a Swift module is compiled with “testing enabled” it allows us to import that module using the @testable annotation to alter the visibility of our code.
Classes and their methods that are marked as internal or public become open, which allows subclassing and overriding in tests.
Other API marked as internal becomes public so that it is now visible to tests.
This is certainly useful when it’s required but it can often be used too eagerly without taking into account some of the issues it can lead to.
I’m going to look at a few potential design issues that could come from using @testable.
This post is not saying if you use @testable bad things will happen but it’s worth keeping in mind some of the design trade offs you are making.
All the issues I’m going to discuss have a common thread that revolves around my understanding of public API so it’s worth clarifying what I’m encompassing when referring to public API.
Public API
When an API is marked as public in code that is going to be shared it represents a commitment from the author.
The commitment is to the consumers of the code that public APIs will be stable, supported and the behaviour will not change unless some change management process is followed.
It is therefore beneficial for code authors to keep the surface area of their public APIs as small as possible and hide as much implementation from end users.
This set up gives the author the freedom to rework the internals as much as they like and as long as the observable public API remains unchanged then downstream users won’t bat an eyelid.
With that explained let’s look at some of the issues:
Overly specified code
Adding tests around code makes the code harder to change because we are locking in the behaviour or at the very least our current understanding of the behaviour. This is great for public APIs because we already discussed that public APIs should be stable. This rigidity is not so good for our non public implementation details that we want to be easier to change.
I’m sure we’ve all had this internal dialog with ourselves at some point
I only wrote these tests last week, why is it hindering my refactoring rather than helping?
This is normally a sign that we got carried away and we are testing the implementation details rather than the overall behaviour.
@testable makes this problem much easier to come by.
There’s been plenty of times I’ve hit code visibility issues in my tests and instinctively reached for @testable import instead of opting to mark the API I want to test as public.
The issue is, once the big ol’ @testable switch has been flipped it’s much easier to overly specify your code and write tests at the wrong level.
There are of course exceptions but I’d try to selectively mark things as public and prefer to only test those APIs. This does not mean that the code is any less tested, it’s just that the code is being exercised indirectly. If there is code that is not exercised when going through the public API then it’s probably just dead code that needs removing.
Loosens documentation and forces extremes
Something that @testable takes away from us is the documentation that we get when we mark an API as public.
As the default visibility for code is internal it means that unless otherwise stated all code you write in a single module is visible everywhere within that module.
This makes it really hard to differentiate what code should be stable and what code should be flexible.
This documenting of stable API is forced upon us, in a good way, when using multiple modules or we won’t be able to see any code from the imported modules. Unfortunately this is probably not the common case as many people will come to Swift for app development where working within a single module is the norm.
To resolve this we can go to extremes and mark all implementation details as private but this then removes our ability to use the escape hatch of @testable.
As a reminder this post is not saying @testable is bad as there are many times where you genuinely might get value from testing implementation details that you don’t want to be public.
But I use TDD
Using a TDD approach is not a panacea and when teamed with @testable I think it’s a winning combination to make it easy to fall into these design traps.
I’ve seen people TDD some code and come up with good solutions but then fall at the last hurdle.
It’s easy to forget that that tests are not the artifact we care about producing, instead they just help create working software.
The last step that is often missed is to ask the question
Are these tests at the right level?
Keeping in mind that tests make things less flexible and the things we preferably want to be stable are the public API. We should therefore see if we can restate any tests that are aimed at implementation details as tests of the public API.
I’ve been bitten by this many times where I’ve TDD’d some code and then returned sometime later to find it requires a lot of rework of tests to get things moving. This can often cause so much friction that I’ll just opt to leave the code to rot and incur more debt.
Different compilation
For @testable import to work your Swift module needs to be compiled differently.
I’m assuming it’s entirely safe as much smarter people than me decided it would be a good addition but I can’t help but feel most uses are unnecessary.
By choosing to lightly sprinkling code with public you get the benefits of a smaller public API surface area, better documentation of intent and the compiler is not having to do any special work.
Conclusion
There are no hard rules and context is key when making decisions.
As a starting point I’d mark things as public rather than use @testable as this forces you to consider how stable you want this API to be.
Also I’d use the visibility modifiers on code even when working within a single module to signal intent to future readers about whether the code should remain stable or is free to change.
Basically - don’t be afraid to use @testable just don’t make it the default tool.