How to get attribute information of a custom field in registration form

Question

I have created a custom attribute for customer in Magento 2 site.

The HTML of the field is this:

        <input type="text"
               name="<?= $block->escapeHtmlAttr($block->getFieldName('custom')) ?>"
               id="custom" 
               value="<?= $block->escapeHtmlAttr($block->getObject()->getCustom()) ?>"
               title="<?= $block->escapeHtmlAttr($block->getAttributeData()->getFrontendLabel('custom')) ?>"
               class="input-text" 
               data-validate="{required:true, 'validateData':true}">

Look at all $block calls. None of them works.

$block->getFieldName('custom') returns an empty string

$block->getObject()->getCustom() throws an exception because getObject is null

$block->getAttributeData()->getFrontendLabel('custom') throws an exception because $block->getAttributeData() is null.

How can I do it?

The field value is something that should be retrieved dynamically, because if there is an error in the registration form, and the page is reloaded, all fields should be repopulated. This works for all standard fields but not for the custom ones.

EDIT:

This is how I am creating the attribute:

public function addCustomAttribute()
{
    $eavSetup = $this->eavSetupFactory->create();


    $eavSetup->addAttribute(
        \Magento\Customer\Model\Customer::ENTITY,
        'custom',
        [
            'type' => 'varchar',
            'label' => 'Custom label',
            'input' => 'text',
            'required' => 1,
            'visible' => 1,
            'user_defined' => 1,
            'sort_order' => 1,
            'position' => 1,
            'system' => 0,
            'is_used_in_grid' => true,
            'is_visible_in_grid' => true,
            'is_filterable_in_grid' => true,
            'is_searchable_in_grid' => true,
        ]
    );

    $attributeSetId = $eavSetup->getDefaultAttributeSetId(Customer::ENTITY);
    $attributeGroupId = $eavSetup->getDefaultAttributeGroupId(Customer::ENTITY);

    $attribute = $this->eavConfig->getAttribute(Customer::ENTITY, 'custom');
    $attribute->setData('attribute_set_id', $attributeSetId);
    $attribute->setData('attribute_group_id', $attributeGroupId);

    $attribute->setData('used_in_forms', [
        'adminhtml_customer',
    ]);

    $this->attributeResource->save($attribute);
}

Answer 1

You need to assign the files into this forms as well.

$attribute->setData('used_in_forms', [
        'adminhtml_customer','customer_account_create','customer_account_edit'
    ]);

Answer 2

Since there is no a simple way to do this, I have just hardcoded the input field attributes. It is better than complicating things creating a custom block class just for this.