Magento 2 Unit Tests – Unit Test with Mock Data Not Working

I'm trying to write unit tests for my code, but for some reason I can't get it to work. To simplify it, I have the following:

namespace Custom\Shipping\Model\Config\Source;

use Custom\Foundation\Model\Config\Source\AbstractSource;
use Custom\Shipping\Helper\Config;


/**
 * Class ProductTypes
 * @package Custom\Shipping\Model\Config\Source
 */
class ProductTypes extends AbstractSource
{
    /**
     * @var Config
     */
    protected $configHelper;

    /**
     * ProductTypes constructor.
     * @param Config $configHelper
     */
    public function __construct(
        Config $configHelper
    )
    {
        $this->configHelper = $configHelper;
    }

    /**
     * @return array
     */
    public function toArray()
    {
        $returnValues = [];

        $productTypes = $this->configHelper->getProductTypes();
        foreach ($productTypes as $key => $value) {
            $returnValues[$key] = $key;
        }

        return $returnValues;
    }
}

Now, I want to mock the result of $this->configHelper->getProductTypes(), so I set up my unit test like this:

namespace Custom\Shipping\Test\Unit\Model\Config\Source;

use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;

/**
 * Class ProductTypesTest
 */
class ProductTypesTest extends \PHPUnit_Framework_TestCase
{
    /**
     * @var \Custom\Shipping\Model\Config\Source\ProductTypes
     */
    protected $sourceModel;

    /**
     * @var \Custom\Shipping\Helper\Config|\PHPUnit_Framework_MockObject_MockObject
     */
    protected $configHelperMock;

    /**
     * Set up before every test
     */
    protected function setUp()
    {
        // Mock our config helper for this test:
        $this->configHelperMock = $this->getMockBuilder('Custom\Shipping\Helper\Config')
            ->disableOriginalConstructor()
            ->getMock();

        // Create our class to test:
        $this->sourceModel = (new ObjectManager($this))
            ->getObject(
                'Custom\Shipping\Model\Config\Source\ProductTypes',
                [
                    'Config' => $this->configHelperMock
                ]
            );
    }

    /**
     * @param $productTypes
     * @param $expectedResult
     * @dataProvider toArrayDataProvider
     */
    public function testToArray($productTypes, $expectedResult)
    {
        $this->configHelperMock
            ->method('getProductTypes')
            ->willReturn($productTypes);

        $this->sourceModel->toArray();

        // $this->assertEquals($expectedResult, $this->sourceModel->toArray());
    }

    /**
     * @return array
     */
    public function toArrayDataProvider()
    {
        return [
            [['Foo' => 2.95, 'Bar' => 4.95], ['Foo' => 'Foo', 'Bar' => 'Bar']]
        ];
    }
}

Now, as you can see in testToArray() a tell my mock object to return the data from the data provider when getProductTypes() is called.

However … the return value is always NULL.

The strange part is, that:

• When I do $this->configHelperMock->getProductTypes() in my test, it returns the correct result.
• When I check with get_class() in my actual class I get the mocked object.
• When I set $this->configHelper->method('getProductTypes')->willReturn('foo') in my actual class it returns foo not NULL (as expected).
• When I move my constructing / creating of my mock object from setUp to my actual test it still returns NULL.
• Even if I put everything in the test and don't use setUp() it still … returns … NULL:

This also doesn't work:

public function testToArray($productTypes, $expectedResult)
{
    // Mock our config helper for this test:
    $this->configHelperMock = $this->getMockBuilder('Custom\Shipping\Helper\Config')
        ->disableOriginalConstructor()
        ->getMock();

    $this->configHelperMock
        ->method('getProductTypes')
        ->willReturn($productTypes);

    // Create our class to test:
    $this->sourceModel = (new ObjectManager($this))
        ->getObject(
            'Custom\Shipping\Model\Config\Source\ProductTypes',
            [
                'Config' => $this->configHelperMock
            ]
        );

    $this->sourceModel->toArray();

    // $this->assertEquals($expectedResult, $this->sourceModel->toArray());
}

So why can't I get it to work? I've used a similar approach on another (helper) class and that works just fine! This has been bugging me for a couple of hours now!

Any help is greatly appreciated.

EDIT: I included the source of Config. No magic getters ….

namespace Custom\Shipping\Helper;

use Magento\Framework\App\Helper\AbstractHelper;

/**
 * Class Config
 * @package Custom\Shipping\Helper
 */
class Config extends AbstractHelper
{
    /**
     * @return array
     */
    public function getProductTypes()
    {
        $configValue = $this->scopeConfig->getValue('carriers/custom/product_types');

        $returnValue = [];

        $lines = explode("\n", $configValue);
        foreach ($lines as $line) {
            $line = trim($line);
            if (!empty($line)) {
                $arr = explode('::', $line);
                if (count($arr) === 2) {
                    $returnValue[trim($arr[0])] = (float) $arr[1];
                }
            }
        }

        return $returnValue;
    }
}