Multiline admin field is broken

[wintermute-84]

Preconditions

1. Magento version 2.1.3
2. PHP 7

Steps to reproduce

1. Add field type="multiline" in any form in admin.
2. Save values in multiple fields

Expected result

1. Values are displayed as saved
2. Ability to add new lines

Actual result

1. The plus button (or any other way to add additional lines) is missing Saved values are not exploded correctly when populating the fields.

[WaPoNe]

Any news on this issue?
I verified it on 2.1.7

[wintermute-84]

No, the functionality doesn't work.

BUT.

As a work around I've used other method:
See code bellow for a general idea.

system.xml

<field id="field_mapping" translate="label" sortOrder="410" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Field Mapping</label>
                    <frontend_model>...\...\Block\Adminhtml\Form\Field\Mapping</frontend_model>
                    <backend_model>...\...\Model\Config\Source\Backend\Mapping</backend_model>
                </field>

Backend\Mapping

class Mapping extends \Magento\Framework\App\Config\Value
{
    protected $_customtabs = null;

    /**
     * @param \Magento\Framework\Model\Context $context
     * @param \Magento\Framework\Registry $registry
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $config
     * @param \Magento\Framework\App\Cache\TypeListInterface $cacheTypeList
     * @param \Magento\Framework\Model\ResourceModel\AbstractResource $resource
     * @param \Magento\Framework\Data\Collection\AbstractDb $resourceCollection
     * @param array $data
     */
    public function __construct(
        \Magento\Framework\Model\Context $context,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\App\Config\ScopeConfigInterface $config,
        \Magento\Framework\App\Cache\TypeListInterface $cacheTypeList,
        \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
        \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
        array $data = []
    ) {
        parent::__construct($context, $registry, $config, $cacheTypeList, $resource, $resourceCollection, $data);
    }
    
    /**
     * Process data after load
     *
     * @return void
     */
    protected function _afterLoad()
    {
        $value = $this->getValue();
        $arr = unserialize($value);

        $this->setValue($arr);
    }

    /**
     * Prepare data before save
     *
     * @return void
     */
    public function beforeSave()
    {
        $value = $this->getValue();
        unset($value['__empty']);
        $arr = serialize($value);
        
        $this->setValue($arr);
    }
}

Field\Mapping

class Mapping extends \Magento\Config\Block\System\Config\Form\Field\FieldArray\AbstractFieldArray
{

    private $yesnoRenderer;

    /**
     * Prepare to render
     *
     * @return void
     */
    protected function _prepareToRender()
    {
        $this->addColumn('attribute_code', ['label' => __('Attribute Code')]);
        $this->addColumn('mapped_code', ['label' => __('Mapped Value')]);
        $this->addColumn('overwrite_status', ['label' => __('Overwrite?'), 'renderer'  => $this->getYesnoRenderer()]);
        $this->_addAfter = false;
        $this->_addButtonLabel = __('Add');
    }

    protected function getYesnoRenderer()
    {
        if (!$this->yesnoRenderer) {
            $this->yesnoRenderer = $this->getLayout()->createBlock(
                Yesno::class,
                '',
                ['data' => ['is_render_to_js_template' => true]]
            );
            $this->yesnoRenderer->setClass('overwrite_status');
        }
        return $this->yesnoRenderer;
    }

    /**
     * Prepare existing row data object
     *
     * @param \Magento\Framework\DataObject $row
     * @return void
     */
    protected function _prepareArrayRow(\Magento\Framework\DataObject $row)
    {
        $optionExtraAttr = [];

        $optionExtraAttr['option_' . $this->getYesnoRenderer()->calcOptionHash($row->getData('overwrite_status'))] =
            'selected="selected"';

        $row->setData('option_extra_attrs',$optionExtraAttr);
    }
}

[IlnitskiyArtem]

@Dukeofvampires, Thanks for reporting this issue.
We've created internal ticket MAGETWO-69696 to address this issue.