How to mock any network call

Hi 👋

Have you ever heard of a type called URLProtocol?

This type is not widely known and that's a shame because it's super useful!

It basically enables you to mock any network call, without the need to make any change to your existing code 👌

So let's see how it works!

But first, I have a big thank you to a developer tool who's the sponsor of this email: Stream

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Let's take a look at this code:

This is a typical example of how a network call is implemented in an iOS app.

As you would expect, the network call is made using a URLSession.

And then its result is decoded with the framework Decodable.

But in its current form, this code doesn't provide a way to replace the actual network call by a mocked version that would return some predefined data.

And this a quite a strong limitation, because without this possibility you can't write reliable unit tests for the class UserAPI.

However, you can notice that it is possible to inject a custom URLSession when we initialise a UserAPI:

And as it turns out, thanks to URLProtocol, this possibility will be enough to implement a mocked network call!

So let's move to our testing target and actually start using URLProtocol!

URLProtocol is an abstract class that we can subclass in order to implement a type that will be able to intercept network calls made through a URLSession.

And when such a call is intercepted, the subclass of URLProtocol will be able to manually inject a predefined response to that call.

So let's implement it!

First, we need to define two methods called canInit(with request:) and canonicalRequest(for request:):

I won't go into the details of how they work, but by returning true for the first one and the unmodified request for the second, we're saying that we want MockURLProtocol to intercept all the requests made by a URLSession.

Then, we define a requestHandler:

This closure will take as its input a request and will return an HTTPURLResponse and some Data.

As you can imagine, it's through this closure that we will define the mocked data that we want the network call to return.

Now, we also need to implement the methods startLoading() and stopLoading().

Inside startLoading() we assert that a requestHandler has indeed been provided.

And then we get the mocked response and data from the handler and we send them to the URLSession:

For the method stopLoading(), we can leave an empty implementation because we don't need to do anything special if the network call gets canceled:

And now we're ready to use MockURLProtocol to write a unit test for our class UserAPI!

All we need to do is to create a custom URLSession and add our MockURLProtocol to its protocolClasses property.

Then, we inject that custom URLSession into our instance of UserAPI:

And from that, implementing a test is rather straightforward.

First, we create the mockData and we set a closure for the requestHandler that returns this mockData and that also asserts that the URL called is the correct one:

And finally, we make the API call and we assert that the parsed result we get is indeed the one that we expect!

One last thing, we also implement the tearDown() method so that we set the requestHandler back to nil after each test.

And that's it, thanks to the class URLProtocol we've been able to inject a mock into a network call without needing to make any change to our existing code!

All that was needed was the ability to inject a custom URLSession and if that's not the case with your code, all you need is to add it as an argument to the initializer 😌

And that's it, we've covered everything you need to start using URLProtocol in your own projects!

If you want to give it a try, all the code that I've just showed you is available at the end of this page!

That's all for this email, thanks for reading it!

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to your friends and colleagues 🙌

I wish you an amazing week!

❤️