PhpUnit dataProviders vs Coverage - phpunit

Question

I decided, that it will be fine if I use data providers but when i try to generate code coverage whole tested class has 0% coverage.. Can someone tell me why?
Test class:
class AuthorDbManagerTest extends AbstractDbManagerTest
{
public function setUp()
{
parent::setUp();
}
/**
* #dataProvider instanceOfProvider
* #param bool $isInstanceOf
*/
public function testInstances(bool $isInstanceOf)
{
$this->assertTrue($isInstanceOf);
}
public function instanceOfProvider()
{
$manager = new AuthorDbManager($this->getEntityManagerMock());
return [
"create()" => [$manager->create() instanceof Author],
"save()" => [$manager->save(new Author()) instanceof AuthorDbManager],
"getRepository" => [$manager->getRepository() instanceof EntityRepository],
];
}
}
Tested class:
class AuthorDbManager implements ManagerInterface
{
protected $entityManager;
protected $repository;
public function __construct(EntityManager $entityManager)
{
$this->entityManager = $entityManager;
$this->repository = $entityManager->getRepository(Author::class);
}
public function create(array $data = [])
{
return new Author();
}
public function getRepository(): EntityRepository
{
return $this->repository;
}
public function save($object): ManagerInterface
{
$this->entityManager->persist($object);
$this->entityManager->flush();
return $this;
}
}
Why my code coverage is 0% on AuthorDbManager?
Screen

Answer 1

The data in the DataProvider is collected before the actual tests start - and there is nothing useful being tested within the testInstances() method.
If you passed the classname and expected class into testInstances($methodName, $expectedClass):
public function testInstances(callable $method, $expectedClass)
{
$this->assertInstanceOf($expectedClass, $method());
}
The dataprovider could return a callable, and the expected result:
"create()" => [[$manager,'create'], Author::class],
then you'd at least be running the code with in the actual test. You may also be better to just pass back a string methodname - 'create', and run that with a locally created $manager instance - $manager->$method() in the test.
In general, it's best to test something as specific as you can - not just letting it convert to a true/false condition.