admin/settings/fields/select/select.php
<?php if ( ! defined( 'ABSPATH' ) ) { die; } // Cannot access directly.
/**
 *
 * Field: select
 *
 * @since 1.0.0
 * @version 1.0.0
 *
 */
if ( ! class_exists( 'ULF_Field_select' ) ) {
  class ULF_Field_select extends ULF_Fields {

    public function __construct( $field, $value = '', $unique = '', $where = '', $parent = '' ) {
      parent::__construct( $field, $value, $unique, $where, $parent );
    }

    public function render() {

      $args = wp_parse_args( $this->field, array(
        'placeholder' => '',
        'chosen'      => false,
        'multiple'    => false,
        'sortable'    => false,
        'ajax'        => false,
        'settings'    => array(),
        'query_args'  => array(),
      ) );

      $this->value = ( is_array( $this->value ) ) ? $this->value : array_filter( (array) $this->value );

      echo $this->field_before();

      if ( isset( $this->field['options'] ) ) {

        if ( ! empty( $args['ajax'] ) ) {
          $args['settings']['data']['type']  = $args['options'];
          $args['settings']['data']['nonce'] = wp_create_nonce( 'ulf_chosen_ajax_nonce' );
          if ( ! empty( $args['query_args'] ) ) {
            $args['settings']['data']['query_args'] = $args['query_args'];
          }
        }

        $chosen_rtl       = ( is_rtl() ) ? ' chosen-rtl' : '';
        $multiple_name    = ( $args['multiple'] ) ? '[]' : '';
        $multiple_attr    = ( $args['multiple'] ) ? ' multiple="multiple"' : '';
        $chosen_sortable  = ( $args['chosen'] && $args['sortable'] ) ? ' ulf-chosen-sortable' : '';
        $chosen_ajax      = ( $args['chosen'] && $args['ajax'] ) ? ' ulf-chosen-ajax' : '';
        $placeholder_attr = ( $args['chosen'] && $args['placeholder'] ) ? ' data-placeholder="'. esc_attr( $args['placeholder'] ) .'"' : '';
        $field_class      = ( $args['chosen'] ) ? ' class="ulf-chosen'. esc_attr( $chosen_rtl . $chosen_sortable . $chosen_ajax ) .'"' : '';
        $field_name       = $this->field_name( $multiple_name );
        $field_attr       = $this->field_attributes();
        $maybe_options    = $this->field['options'];
        $chosen_data_attr = ( $args['chosen'] && ! empty( $args['settings'] ) ) ? ' data-chosen-settings="'. esc_attr( wp_json_encode( $args['settings'] ) ) .'"' : '';

        if ( is_string( $maybe_options ) && ! empty( $args['chosen'] ) && ! empty( $args['ajax'] ) ) {
          $options = $this->field_wp_query_data_title( $maybe_options, $this->value );
        } else if ( is_string( $maybe_options ) ) {
          $options = $this->field_data( $maybe_options, false, $args['query_args'] );
        } else {
          $options = $maybe_options;
        }

        if ( ( is_array( $options ) && ! empty( $options ) ) || ( ! empty( $args['chosen'] ) && ! empty( $args['ajax'] ) ) ) {

          if ( ! empty( $args['chosen'] ) && ! empty( $args['multiple'] ) ) {

            echo '<select name="'. $field_name .'" class="ulf-hide-select hidden"'. $multiple_attr . $field_attr .'>';
            foreach ( $this->value as $option_key ) {
              echo '<option value="'. esc_attr( $option_key ) .'" selected>'. esc_attr( $option_key ) .'</option>';
            }
            echo '</select>';

            $field_name = '_pseudo';
            $field_attr = '';

          }

          // These attributes has been serialized above.
          echo '<select name="'. esc_attr( $field_name ) .'"'. $field_class . $multiple_attr . $placeholder_attr . $field_attr . $chosen_data_attr .'>';

          if ( $args['placeholder'] && empty( $args['multiple'] ) ) {
            if ( ! empty( $args['chosen'] ) ) {
              echo '<option value=""></option>';
            } else {
              echo '<option value="">'. esc_attr( $args['placeholder'] ) .'</option>';
            }
          }

          foreach ( $options as $option_key => $option ) {

            if ( is_array( $option ) && ! empty( $option ) ) {

              echo '<optgroup label="'. esc_attr( $option_key ) .'">';

              foreach ( $option as $sub_key => $sub_value ) {
                $selected = ( in_array( $sub_key, $this->value ) ) ? ' selected' : '';
                echo '<option value="'. esc_attr( $sub_key ) .'" '. esc_attr( $selected ) .'>'. esc_attr( $sub_value ) .'</option>';
              }

              echo '</optgroup>';

            } else {
              $selected = ( in_array( $option_key, $this->value ) ) ? ' selected' : '';
              echo '<option value="'. esc_attr( $option_key ) .'" '. esc_attr( $selected ) .'>'. esc_attr( $option ) .'</option>';
            }

          }

          echo '</select>';

        } else {

          echo ( ! empty( $this->field['empty_message'] ) ) ? esc_attr( $this->field['empty_message'] ) : esc_html__( 'No data available.', 'ulf' );

        }

      }

      echo $this->field_after();

    }

    public function enqueue() {

      if ( ! wp_script_is( 'jquery-ui-sortable' ) ) {
        wp_enqueue_script( 'jquery-ui-sortable' );
      }

    }

  }
}
