The Magic of the Subject

We now know that the ObjectBehavior class forwards all method calls to the underlying Dinosaur object thanks to some magic methods. But, there is still one more big piece of magic. When we call $this->getLength(), phpspec will ultimately call getLength() on the Dinosaur object and that will return the integer 15. So then... what absolute madness is allowing us to call a method on that?! Let's find out!

This time var_dump($this->getLength()).

Will this be the integer 15? An object? An emoji? Let's find out!

./vendor/bin/phpspec run

Ah! It's an instance of a Subject object. Stop! We know that object! That is the exact same class type that is stored on the $object property of our base ObjectBehavior class! Just like before, Subject is a wrapper object that gives us some magic. Let's find out how it works: type Shift+Shift and look for the Subject.php file.

Woh! See all these @method things on top? This tells PhpStorm that we can call any of these methods on this object and they will work. We need this because, if you look, these methods don't actually exist in this class! They work by magic - we'll see that in a minute.

This class works a lot like ObjectBehavior. Scroll down until you find the all-important __call() method. When we call getLength(), it gives us a Subject object. And then when we call shouldBeGreaterThan(), it's handled by __call(). The logic here is awesome: if the method name starts with should, it calls $this->callExpectation() - which finds and executes the correct "matcher". So, why do all matchers need to begin with should? Because of this line right here.

Next, if the method name starts with beConstructedThrough or beConstructedWith, it calls some code that allows us to control how the Dinosaur object is instantiated. We'll use this really soon.

And ultimately, if it is not one of those special cases, it executes code that forwards the call onto the underlying object and returns that value. Well, it returns the value wrapped in, yet another, Subject class. This is exactly what happens when we call $this->getLength(): this last line calls getLength() on the Dinosaur object and then wraps it in a Subject object. Thanks to that, we can then call shouldBeGreaterThan to call our matcher.

So, yes, it is all magic - super impressive magic! But it's magic that's done via a couple of wrapper object and the __call() method.

Let's remove our debug code, and make sure the tests are still passing:

./vendor/bin/phpspec run

Cool! Now, back to testing! Next: let's learn how to "describe" a special part of our object's behavior: how it's instantiated.