enable Selected option for a custom field extending JFormFieldList

Question

I am creating a custom field extending JFormFieldList as below

Form field is defined as below

    <field name="reltype_rev_id" 
           multiple="true"
           type="customfield"
           default="0"
           label="Custom Field"
           />

Options (data) for the field is formed as below

public function getOptions($id='') {
    $options = [(object)['key' => '', 'value' => 'Custom Field']];

    $val_1 = new stdClass();
    $val_1->key='1';
    $val_1->value='One';

    $val_2 = new stdClass();
    $val_2->key='2';
    $val_2->value='Two';

    $key_values = array($val_1, $val_2);

    $options = array_merge($options, $key_values);

    $default_values = array(1);

    return JHtml::_('select.options', $options, 'key', 'value', $default_values);
}

How can we mark some of the options as selected while forming the field.

The output comes as below

Update:

Optimized the code based on @zollie and the result is the same as shown in the above screenshot

    $data = array('1' => 'One', '2' => 'Two');
    return JHtmlSelect::options($data, 'value' , 'text', array('1'), false);

Answer 1

If you're extending JFormFieldList then you don't need to use JHtml::_('select...) as well. Instead you just return an array of text/value combinations for the options of the HTML select. You can specify the default values by setting the protected variable $value.

class JFormFieldCustomfield extends JFormFieldList

protected $type = 'Customfield';

public function getOptions() {

    $key_values = array(
        array('value' => 1, 'text' => 'One'),
        array('value' => 2, 'text' => 'Two'),
        array('value' => 3, 'text' => 'Three'),
    );

    // merge with any options from the XML file
    $options = array_merge(parent::getOptions(), $key_values);

    // pre-select values 2 and 3
    $this->value = array(2, 3);

    return $options; 
}

(To allow you to select multiple values, you have to have multiple="true" in the XML file, as you already have). See https://docs.joomla.org/Creating_a_custom_form_field_type