getRegisterDefinitionsCallback example

    $mock_decorator = $this->getMockBuilder('Drupal\Component\Plugin\Discovery\StaticDiscoveryDecorator')
      ->disableOriginalConstructor()
      ->addMethods(['registeredDefinitionCallback'])
      ->getMock();

    // Set up the ::$registerDefinitions property.     $ref_register_definitions = new \ReflectionProperty($mock_decorator, 'registerDefinitions');
    if ($has_register_definitions) {
      // Set the callback object on the mocked decorator.       $ref_register_definitions->setValue(
        $mock_decorator,
        [$this->getRegisterDefinitionsCallback(), 'registerDefinitionsCallback']
      );
    }
    else {
      // There should be no registerDefinitions callback.       $ref_register_definitions->setValue($mock_decorator, NULL);
    }

    // Set up ::$definitions to an empty array.     $ref_definitions = new \ReflectionProperty($mock_decorator, 'definitions');
    $ref_definitions->setValue($mock_decorator, []);