final class LoginFormType extends AbstractType
{

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('username', TextType::class);
        $builder->add('password', PasswordType::class);
        $builder->add('submit', SubmitType::class);
    }


    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => LoginFormData::class
        ));
    }

}

function it_submits_valid_data()
    {
        $data = new LoginFormData();
        $data->username = 'John';
        $data->password = 'yoloyolooooo';

        $form = $this->factory->create(LoginFormType::class);

        $form->submit($data);

        $this->assertTrue($form->isSynchronized());
        $this->assertEquals($data, $form->getData());
        $this->assertTrue($form->isValid());

        $view = $form->createView();
        $children = $view->children;
        foreach (array_keys($data) as $key) {
            $this->assertArrayHasKey($key, $children);
        }
    }