List of checkboxes not saving in WordPress widget - php

Question

I try to get the names of the pages in a loop.
In the widget, the page titles are displayed correctly, but the values are not saved.
How I can save the checked inputs?
Below Full Widget Code:
class Foo_Widget extends WP_Widget {
/** Register widget with WordPress.*/
function __construct() {
parent::__construct(
'foo_widget', // Base ID
esc_html__( 'Widget Title', 'text_domain' ), // Name
array( 'description' => esc_html__( 'A Foo Widget', 'text_domain' ), ) // Args
);
}
/*** Front-end display of widget.*/
public function widget( $args, $instance ) {
echo $args['before_widget'];
if ( ! empty( $instance['title'] ) ) {
echo $args['before_title'] . apply_filters( 'widget_title', $instance['title'] ) . $args['after_title'];
}
echo esc_html__( 'Hello, World!', 'text_domain' );
echo $args['after_widget'];
}
/*** Back-end widget form.*/
public function form( $instance ) {
$mypages = get_pages( array(
'sort_column' => 'post_date',
'sort_order' => 'desc'
) );
foreach( $mypages as $page ) {
$title = $page->post_title;
$slug = $page->post_name;
?>
<p>
<label>
<input type='checkbox' name='the_pages' id='<?php echo $slug ?>' value='<?php echo esc_attr( $title ); ?>'><?php echo esc_attr( $title ); ?>
</label>
</p>
<? } ?>
<?php }
/*** Sanitize widget form values as they are saved.*/
public function update( $new_instance, $old_instance ) {
$instance = array();
$instance['title'] = $new_instance[ 'title' ];
return $instance;
}
}
// register Foo_Widget widget
function register_foo_widget() {
register_widget( 'Foo_Widget' );
}
add_action( 'widgets_init', 'register_foo_widget' );
?>
How should update() function looks like ?