Lesson 6 - Dependency Injection and Test doubles
In the previous lesson, PHP Testing - Xdebug and coverage reports, we learned to work with Xdebug and generate code-coverage reports. In today's PHP tutorial, we're going to start with the last topic associated with unit tests and that's mocking.
Our unit tests worked only with simple classes that had no
dependencies so far. But how to test classes that need some
other class instances to work and even these instances may need another
instances of other classes? We often talk about the System Under
Test (SUT), which refers to just a small part of a
larger system we're currently testing. As for our calculator, we can imagine it
needs some logger to work, which stores computed problems into a file. That
still wouldn't be such a problem, we could test the logger as well, and the
calculator test would normally log the problems and we could just live with
that. But how would we test classes that work with a web API? For example, the
calculator would share the results on Facebook. We certainly don't want to run
such a "madness" in tests and share something on our Facebook at the end of the
tests. Tests shouldn't even require an Internet connection unless it's our
intention to test the communication between our own APIs with the integration
tests. Here we use mocking that allows us to create "fake
objects", for example, an object that behaves as an instance of the
FacebookApi class, but it actually provides prepared "fake" data
only and calling its methods won't cause a real requests to
To be able to use mocking, which means to provide fake objects in our tests, we need to use Dependency injection. It's a design pattern that is so important that we can
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In the tutorial, we'll explain the Dependency Injection design pattern and introduce the test doubles - Dummy, Fake, Stub, Spy and, of course, Mock.
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