How can I get custom module collection on form page magento 2

Question

I want to set categories in dropdown field of form of subcategory module in magento2 . How can I get category collection on subcategory form page and set values in dropdown field.

I am trying to do this.
Code:

Model class:

namespace Sparx\Subcategory\Model;

use Magento\Framework\Exception\SubcategoryException;

/**
 * Subcategorytab subcategory model
 */
class Subcategory extends \Magento\Framework\Model\AbstractModel
{

    /**
     * @param \Magento\Framework\Model\Context $context
     * @param \Magento\Framework\Registry $registry
     * @param \Magento\Framework\Model\ResourceModel\AbstractResource $resource
     * @param \Magento\Framework\Data\Collection\Db $resourceCollection
     * @param array $data
     */
    protected $categorymodelFactory;
    public function __construct(
        \Magento\Framework\Model\Context $context,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
        \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
        \Sparx\Category\Model\ResourceModel\Category\CollectionFactory $categorymodelFactory,
        array $data = []
    ) {
        parent::__construct($context, $registry, $resource, $resourceCollection, $data);
        $this->categorymodelFactory = $categorymodelFactory;
    }

    /**
     * @return void
     */
    public function _construct()
    {
        $this->_init('Sparx\Subcategory\Model\ResourceModel\Subcategory');
    }   
}

Form class:

namespace Sparx\Subcategory\Block\Adminhtml\Subcategory\Edit\Tab;

class Basicinformation extends \Magento\Backend\Block\Widget\Form\Generic implements \Magento\Backend\Block\Widget\Tab\TabInterface {

    /**
     * @var \Magento\Store\Model\System\Store
     */
    protected $_systemStore;

    /**
     * @param \Magento\Backend\Block\Template\Context $context
     * @param \Magento\Framework\Registry $registry
     * @param \Magento\Framework\Data\FormFactory $formFactory
     * @param \Magento\Store\Model\System\Store $systemStore
     * @param array $data
     */
    public function __construct(
    \Magento\Backend\Block\Template\Context $context, \Magento\Framework\Registry $registry, \Magento\Framework\Data\FormFactory $formFactory, \Magento\Store\Model\System\Store $systemStore, array $data = array()
    ) {
        $this->_systemStore = $systemStore;
        parent::__construct($context, $registry, $formFactory, $data);
    }

    /**
     * Prepare form
     *
     * @return $this
     */
    protected function _prepareForm() {
        /* @var $model \Magento\Cms\Model\Page */
        $model = $this->_coreRegistry->registry('subcategory_subcategory');
        $isElementDisabled = false;
        /** @var \Magento\Framework\Data\Form $form */
        $form = $this->_formFactory->create();

        $form->setHtmlIdPrefix('page_');

        $fieldset = $form->addFieldset('base_fieldset', array('legend' => __('Basic Information')));

        if ($model->getId()) {
            $fieldset->addField('id', 'hidden', array('name' => 'id'));
        }

        $coll=$this->_categoryFactory->create()->getCollection();
        echo '<pre>';        print_r($coll);

        $fieldset->addField(
                'category', 'select', array(
            'name' => 'category',
            'label' => __('Select Category'),
            'title' => __('Select Category'),
            //'values' => $this->_categoryFactory->create()->getCollection(),
            'required' => true,
                )
        );

        $fieldset->addField(
                'title', 'text', array(
            'name' => 'title',
            'label' => __('Subcategory Title'),
            'title' => __('Subcategory Title'),
            'required' => true,
                )
        );


        $fieldset->addField(
                'status', 'select', array(
            'name' => 'status',
            'label' => __('status'),
            'title' => __('status'),
            'values' => array(0 => array('label' => 'Enable', 'value' => '1'), 1 => array('label' => 'disable', 'value' => '2')),
            'required' => true,
                )
        );
        /* {{CedAddFormField}} */

        if (!$model->getId()) {
            $model->setData('status', $isElementDisabled ? '2' : '1');
        }

        $form->setValues($model->getData());
        $this->setForm($form);

        return parent::_prepareForm();
    }

    /**
     * Prepare label for tab
     *
     * @return string
     */
    public function getTabLabel() {
        return __('Basic Information');
    }

    /**
     * Prepare title for tab
     *
     * @return string
     */
    public function getTabTitle() {
        return __('Basic Information');
    }

    /**
     * {@inheritdoc}
     */
    public function canShowTab() {
        return true;
    }

    /**
     * {@inheritdoc}
     */
    public function isHidden() {
        return false;
    }

    /**
     * Check permission for passed action
     *
     * @param string $resourceId
     * @return bool
     */
    protected function _isAllowedAction($resourceId) {
        return $this->_authorization->isAllowed($resourceId);
    }

}

It gives a blank page and I am not able to identify the exact problem.

Answer 1

Have you debugged what $this->_categoryFactory->create()->getCollection()->getData() gives you? Just outputting the collection data like that into a select box values field wont get you anywhere.

You need to do something like this i think:

$itemsArray = [];    
foreach($this->_categoryFactory->create()->getCollection()->getItems() as $item) {
    $itemsArray[] = $item->getName(); //or title or whatever it is
}

  $fieldset->addField(
            'category', 'select', array(
        'name' => 'category',
        'label' => __('Select Category'),
        'title' => __('Select Category'),
        'values' => $itemsArray,
        'required' => true,
            )
    );

This is a rough example. Some things might not be correct but should get you going.

Answer 2

To see problem uncomment #ini_set('display_errors', 1); line in app/bootstrap.php.

You class do not have dependency on categoryFactory. Add it to block constructor to use.

private $_categoryFactory;

public function __construct(
    \Magento\Backend\Block\Template\Context $context, 
    \Magento\Framework\Registry $registry, 
    \Magento\Framework\Data\FormFactory $formFactory,
    \Magento\Store\Model\System\Store $systemStore, 
    \Sparx\Category\Model\ResourceModel\Category\CollectionFactory $categorymodelFactory,
    array $data = array()
    ) {
        $this->_systemStore = $systemStore;
        $this->_categoryFactory = categorymodelFactory;
        parent::__construct($context, $registry, $formFactory, $data);
    }