Magento 2: Mocking method, return value not working

Question

I am trying to learn unit testing. I wrote simple module to get better understanding about mocking. The test should return "Cat" but it return "Dog".

1. I can not see what is wrong with the code?

2. In the example code I have two classes, but how should the test be written. When the getVariable method is inside learningMock class? I have read about partialMocking but...

Simplistic\Learning\Model\learningMock:

class learningMock {
    public function __construct(\Simplistic\Learning\Model\learningHelper $learningHelper) {
        $this->_learningHelper=$learningHelper;
    }

 public function learningMethodReplace(){
     $var="Dog";
     if($this->_learningHelper->getVariable()=="Cat"){
         $var="Cat";
     }
     return $var;
 }

Simplistic\Learning\Model\learningHelper.php:

class learningHelper {

 public function getVariable() {
     return "Cat";
 }

Simplistic\Learning\Test\Unit\Model\learningMockTest:

class mockTest extends \PHPUnit_Framework_TestCase {

    public function setUp() {
    $this->learningHelper = $this->getMockBuilder("\Simplistic\Learning\Model\learningHelper")
            ->setMethods(['getVariable'])
            ->disableOriginalConstructor()
            ->getMock();
    $objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
    $this->_learningMock = $objectManager->getObject('Simplistic\Learning\Model\learningMock');
}

public function testLearningClassReplace() {
    $this->learningHelper->expects($this->once())
            ->method('getVariable')
            ->will($this->returnValue('Cat'));
    $this->assertEquals('Cat', $this->_learningMock->learningMethodReplace());
  }
}

EDITED: I have manage to get it working, but I do not understand what is happening in the code. Can anyone please explain me, why it is working now (from Magento point of view)?

Simplistic\Learning\Test\Unit\Model\learningMockTest:

use Simplistic\Learning\Model\learningMock;
class mockTest extends \PHPUnit_Framework_TestCase {

    public function setUp() {
    $this->learningHelper = $this->getMockBuilder("\Simplistic\Learning\Model\learningHelper")
            ->setMethods(['getVariable'])
            ->disableOriginalConstructor()
            ->getMock();
    $this->learning = new learningMock($this->learningHelper);
}

public function testLearningClassReplace() {
    $this->learningHelper->expects($this->once())
            ->method('getVariable')
            ->willReturn('Cat');
    $this->assertEquals("Cat", $this->learning->learningMethodReplace());
  }
}

Answer 1

You created a mock ($learningHelper), but did not do anything with it.

The object manager contains lots of magic, but not enough to know what you do in your test setup and what's your intention ;-)

Now you passed the mock directly as constructor argument instead of using the object manager, and that's the correct approach in unit tests.