PHPSpec with Mock example test always returns null but implementation works as expected

Question

I want to start with PHPSpec, so I'm working with two simple classes. The first one is responsible for applying a percentage reduce or enlarge to number, and the second one is responsible for calculate a product price with use of percentage applyer (like a Mock).

PercentageToNumberApplyerSpec (spec):

namespace spec\My;

use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

class PercentageToNumberApplyerSpec extends ObjectBehavior
{
    function it_is_initializable()
    {
        $this->shouldHaveType('My\PercentageToNumberApplyer');
    }

    function it_enlarges_a_number_with_a_given_percentage()
    {
        $this->enlarge(100, 20)->shouldReturn(120);
        $this->enlarge(80, 25)->shouldReturn(100);
        $this->enlarge(20, 50)->shouldReturn(30);
    }

    function it_reduces_a_number_with_a_given_percentage()
    {
        $this->reduce(100, 20)->shouldReturn(80);
        $this->reduce(80, 10)->shouldReturn(72);
        $this->reduce(250, 20)->shouldReturn(200);
    }
}

PercentageToNumberApplyer (implementation):

<?php

namespace My;

class PercentageToNumberApplyer
{
    /**
     * Enlarge given number with a given percentage
     *
     * @param $number
     * @param $percentage
     * @return float
     */
    public function enlarge($number, $percentage)
    {
        return $this->calculate($number, $percentage) + $number;
    }

    /**
     * Reduce given number with a given percentage
     *
     * @param $number
     * @param $percentage
     * @return mixed
     */
    public function reduce($number, $percentage)
    {
        return $number - $this->calculate($number, $percentage);
    }

    /**
     * @param $number
     * @param $percentage
     * @return float
     */
    private function calculate($number, $percentage)
    {
        return $number * $percentage / 100;
    }
}

PriceCalculatorSpec (spec):

<?php

namespace spec\My;

use My\PercentageToNumberApplyer;
use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

class PriceCalculatorSpec extends ObjectBehavior
{
    function let(PercentageToNumberApplyer $percentageToNumberApplyer)
    {
        $this->beConstructedWith($percentageToNumberApplyer);
    }

    function it_calculates_price_discount($percentageToNumberApplyer)
    {
        $number = 100;
        $discount = 20;

        $percentageToNumberApplyer->reduce($number, $discount)->shouldBeCalled();

        $this->applyDiscountTo($number, $discount)->shouldReturn(80);
    }
}

The Problem

The problem is that in the above example after running phpspec run the result is:

- it calculates price discount
expected [integer:80], but got null

PriceCalculator (implementation):

<?php

namespace My;

class PriceCalculator
{
    /**
     * @var PercentageToNumberApplyer
     */
    private $percentageToNumberApplyer;

    /**
     * @param PercentageToNumberApplyer $percentageToNumberApplyer
     */
    public function __construct(PercentageToNumberApplyer $percentageToNumberApplyer)
    {
        $this->percentageToNumberApplyer = $percentageToNumberApplyer;
    }

    /**
     * @param $basePrice
     * @param $discount
     * @return mixed
     */
    public function applyDiscountTo($basePrice, $discount)
    {
        return $this->percentageToNumberApplyer->reduce($basePrice, $discount);
    }
}

Why the following use case is working, even if the test fails:

$priceCalculator = new \My\PriceCalculator(new \My\PercentageToNumberApplyer());

$price = $priceCalculator->applyDiscountTo(100, 20);

$price have a 80 value...

Answer 1

You don't need a mock in your case. Stub will do. Read more on test doubles in PHP Test doubles patterns with prophecy.

Instead of mocking a call:

$percentageToNumberApplyer->reduce($number, $discount)->shouldBeCalled();

Stub it:

$percentageToNumberApplyer->reduce($number, $discount)->willReturn(80);

Next, you only need to expect that what's calculated is actually returned:

$percentageToNumberApplyer->reduce($number, $discount)->willReturn(80);

$this->applyDiscountTo($number, $discount)->shouldReturn(80);

This is because you shouldn't care if a call was made. You're only interested in the result.

As a rule of thumb, you'll usually:

• mock collaborators that perform an action (command methods)
• stub collaborators that return something (query methods)

Most of the time it's better to have a separation between the two (Command/Query Separation), and we won't need to mock and stub in the same time.