Custom wp_dropdown_pages walker class

class-custom-walker-page-dropdown.php

<?php

class Custom_Walker_PageDropdown extends Walker_PageDropdown {

	/**
	 * Starts the element output.
	 * @param array|string $args {
	 *
	 *    @type int|string|callable $selected     Value of the option that should be selected.
	 *                                            Default 0.
	 *                                            Or a callback that return true/false.
	 *    @type string|callable     $value_field  Post field or a callback used to populate
	 *                                            the 'value' attribute of the option elements.
	 *                                            Default 'ID'.
	 *
	 * @see Walker_PageDropdown
	 */
	public function start_el( &$output, $page, $depth = 0, $args = array(), $id = 0 ) {
		$pad = str_repeat('&nbsp;', $depth * 3);
		
		// Get value for <option>
		$value = $page->ID;
		if ( isset( $args['value_field'] ) ) {
			$value_field = $args['value_field'];
			if ( is_callable( $value_field ) ) {
				$value = call_user_func( $value_field, $page );
			} elseif ( isset( $page->{$value_field} ) ) {
				$value = $page->{$value_field};
			}
		}

		$output .= "\t<option class=\"level-$depth\" value=\"" . esc_attr( $value ) . "\"";
		
		// Check if <option> is selected
		if ( isset( $args['selected'] ) ) {
			$selected = $args['selected'];
			$is_selected = false;
			if ( is_callable( $selected ) ) {
				$is_selected = call_user_func( $selected, $page );
			} else {
				$is_selected = ( $value == $selected );
			}

			if ( $is_selected ) {
				$output .= ' selected="selected"';
			}
		}
		$output .= '>';

		$title = $page->post_title;
		if ( '' === $title ) {
			/* translators: %d: ID of a post */
			$title = sprintf( __( '#%d (no title)' ), $page->ID );
		}

		/**
		 * Filters the page title when creating an HTML drop-down list of pages.
		 *
		 * @since 3.1.0
		 *
		 * @param string $title Page title.
		 * @param object $page  Page data object.
		 */
		$title = apply_filters( 'list_pages', $title, $page );

		$output .= $pad . esc_html( $title );
		$output .= "</option>\n";
	}
}

usage.php

<?php
wp_dropdown_pages( [
	'value_field' => function ( $page ) {
		return $page->ID . ' - ' . $page->post_name;
	},
	'selected' => function ( $page ) {
		return $page->post_name == 'blog'; // get_data...()
	},
	'walker' => new Custom_Walker_PageDropdown,
] );