Using wp_category_checklist in a widget

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1. The problem is that to make the update method of widget class work, the name inputs on the form method should be set via $this->get_get_field_name('name_of_the_field'); but wp_category_checklist has no argument to setup the name of the inputs (checkboxes).

   However, wp_category_checklist uses a walker class to print the checkboxes and allow customizing it. By default the class used is Walker_Category_Checklist, and the method that prints the checkboxes is start_el.

   That method has no filter to allow editing of the input names, but we can create a custom walker, that accepts params to setup the name. If this walker extends Walker_Category_Checklist, we only need to override start_el method (mostly copying from original).

   The code:

   // This is required to be sure Walker_Category_Checklist class is available
   require_once ABSPATH . 'wp-admin/includes/template.php';
   /**
    * Custom walker to print category checkboxes for widget forms
    */
   class Walker_Category_Checklist_Widget extends Walker_Category_Checklist {
   
       private $name;
       private $id;
   
       function __construct( $name = '', $id = '' ) {
           $this->name = $name;
           $this->id = $id;
       }
   
       function start_el( &$output, $cat, $depth = 0, $args = array(), $id = 0 ) {
           extract( $args );
           if ( empty( $taxonomy ) ) $taxonomy = 'category';
           $class = in_array( $cat->term_id, $popular_cats ) ? ' class="popular-category"' : '';
           $id = $this->id . '-' . $cat->term_id;
           $checked = checked( in_array( $cat->term_id, $selected_cats ), true, false );
           $output .= "n<li id='{$taxonomy}-{$cat->term_id}'$class>" 
               . '<label class="selectit"><input value="' 
               . $cat->term_id . '" type="checkbox" name="' . $this->name 
               . '[]" id="in-'. $id . '"' . $checked 
               . disabled( empty( $args['disabled'] ), false, false ) . ' /> ' 
               . esc_html( apply_filters( 'the_category', $cat->name ) ) 
               . '</label>';
         }
   }
   

   Now, probably in same file we can write the widget class:

   /**
    * An example of widget using wp_category_checklist on form
    */
   class TestCategoryWidget extends WP_Widget {
   
       function __construct(){
           parent::__construct( false, 'TestWidget');
       }
   
       function widget( $args, $instance ) { 
           // Displays the widget on frontend 
       }
   
       function update( $new_instance, $old_instance ) {
           $instance = $old_instance;
           $instance['widget_categories'] = $new_instance['widget_categories'];
           return $instance;
       }
   
       function form( $instance ) {
           $defaults = array( 'widget_categories' => array() );
           $instance = wp_parse_args( (array) $instance, $defaults );    
           // Instantiate the walker passing name and id as arguments to constructor
           $walker = new Walker_Category_Checklist_Widget(
               $this->get_field_name( 'widget_categories' ), 
               $this->get_field_id( 'widget_categories' )
           );
           echo '<ul class="categorychecklist">';
           wp_category_checklist( 0, 0, $instance['widget_categories'], FALSE, $walker, FALSE );
           echo '</ul>';
       }
   
   }
   

   Finally, register the widget:

   function TestCategoryWidgetInit() {
       register_widget( 'TestCategoryWidget' );
   }
   
   add_action( 'widgets_init', 'TestCategoryWidgetInit' );
   

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