Option Group support for Select/Select Advanced Field Types

Looks like I missed the closing a tag on the link to optgroup. Sorry, wish I could edit, but that's not possible.

Update: there seems to be a special field type (not in the documentation, by the way) called a 'select-tree' that was brought up in this thread: https://support.metabox.io/topic/custom-select-tree/

Seems like a better starting point then the standard select field....

I was able to achieve something building off of the standard select field. I created a new field type called: 'selectomatic' which is defined as so:

array(
    'type'    => 'selectomatic',
    'id'      => 'animal_select',
    'name'    => 'Animals',
    'options' => array(
        array( 'value' => 'monkey', 'label' => 'Monkeys' ),
            array( 'value' => 'king_kong', 'label' => 'King Kong', 'parent' => 'monkey' ),
            array( 'value' => 'curious_george', 'label' => 'Curious George', 'parent' => 'monkey' ),
        array( 'value' => 'donkey', 'label' => 'Donkeys' ),
            array( 'value' => 'eeyore', 'label' => 'Eeyore', 'parent' => 'donkey' ),
            array( 'value' => 'guss', 'label' => 'Gus', 'parent' => 'donkey' ),
    ),
    'flatten' => false
),

Then I extended the Select Field Class and Select Walker Class like so:

class RWMB_Walker_Selectomatic extends RWMB_Walker_Select {

		public function start_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			$label  = $this->db_fields['label'];
			$id     = $this->db_fields['id'];
			$meta   = $this->meta;
			
			if($depth){
				$output .= sprintf(
					'<option value="%s" %s>%s</option>',
					esc_attr( $object->$id ),
					selected( in_array( $object->$id, $meta ), true, false ),
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
			else{
				$output .= sprintf(
					'<optgroup label="%s">',
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
		}

		public function rwmb_end_html_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			if(!$depth){
				$output .= '</optgroup>';
			}
		}
	}

It's not beautiful, but it's a work-around that... works!

Here are the full selectomatic custom field extended classes based on the Select field:

if ( class_exists( 'RWMB_Field' ) ) {
	class RWMB_Selectomatic_Field extends RWMB_Select_Field {
		public static function walk( $field, $options, $db_fields, $meta ) {
			$attributes = self::call( 'get_attributes', $field, $meta );
			$walker     = new RWMB_Walker_Selectomatic( $db_fields, $field, $meta );
			$output     = sprintf(
				'<select %s>',
				self::render_attributes( $attributes )
			);
			if ( false === $field['multiple'] ) {
				$output .= $field['placeholder'] ? '<option value="">' . esc_html( $field['placeholder'] ) . '</option>' : '';
			}
			$output .= $walker->walk( $options, $field['flatten'] ? - 1 : 0 );
			$output .= '</select>';
			$output .= self::get_select_all_html( $field );
			return $output;
		}
	}

	class RWMB_Walker_Selectomatic extends RWMB_Walker_Select {

		public function start_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			$label  = $this->db_fields['label'];
			$id     = $this->db_fields['id'];
			$meta   = $this->meta;

			if($depth){
				$output .= sprintf(
					'<option value="%s" %s>%s</option>',
					esc_attr( $object->$id ),
					selected( in_array( $object->$id, $meta ), true, false ),
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
			else{
				$output .= sprintf(
					'<optgroup label="%s">',
					esc_html( RWMB_Field::filter( 'choice_label', $object->$label, $this->field, $object )
					)
				);
			}
		}

		public function rwmb_end_html_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {
			if(!$depth){
				$output .= '</optgroup>';
			}
		}
	}
}