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Test Doubles, Dummies, Mocks & Spies

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Now that we've seen all the things we can do with test doubles... we need to talk a bit more about some words and language that you'll see with prophecy... otherwise... it all looks a bit crazy.

Google for "php prophecy"... because, remember, this whole test double system in phpspec actually comes from prophecy.

In their documentation, they talk about four different types of objects! Dummy objects, stub objects, mock objects and spy objects. Woh! For me... this was... confusing! So let's walk through and see what's actually going on - it's really nothing new for us. These four different words all different ways to describe test doubles... the "things" that we just finished learning all about.


First, look at dummies. A dummy object is both what I look like if you ask me to remember where I left the car keys... and also the object you get if you add an argument with a type-hint in phpspec to get a test double... then do absolutely nothing with it. So if we get a test double and add no behavior and make no assertions on its methods, it's called a "dummy object".

Oh, and inside of their documentation, you'll see things like $prophecy->reveal(). That's a detail that we don't need to worry about because phpspec takes care of that for us. Score!


As soon as you start controlling even one return value of even one method... boom! This object is suddenly known as a stub. From the docs: "a stub is an object double" - all of these things are known as test doubles, or object doubles - that when put in a specific environment, behaves in a specific way. That's a fancy way of saying: as soon as we add one of these willReturn() things, it becomes a stub.

And actually, most of the documentation is spent talking about stubs and the different ways to control exactly how it behaves, including the Argument wildcarding that we saw earlier.


If you keep reading down, the next thing you'll find are "mocks". An object becomes a mock when you call shouldBeCalled(). So, if you want to add an assertion that a method is called a certain number of times and you want to put that assertion before the actual code - using shouldBeCalledTimes() or shouldBeCalled() - congratulations! Your object is now known as a mock.


And finally, at the bottom, we have spies. A spy is the exact same thing as a mock, except it's when you add the expectation after the code - like with shouldHaveBeenCalledTimes().

Putting it All Together

So there's a lot of language here... but these are all different words to describe the different things you can do with a test double. Doing nothing? It's a dummy. Controlling the return value? It's a stub. Adding an assertion to make sure a method was called? It's a mock or a spy, depending on your style. And yes, you can be a stub and a mock or spy at the same time - that's exactly what we're doing in our example.

I find these terms fun, but a little confusing and I don't think about them when I'm actually coding: I just think about what I need to get done. But now that you're familiar with them, reading the docs should be a breeze. And also, these words are used everywhere in the testing world - not just in phpspec - so it's kinda cool to know them. You're now part of the cool-kids testing club.

Next, let's do some more mocking... cause it's awesome! And we'll say hello to a helper method that we can use inside our spec class to run some code before every example.