How to add fields to the widget options in wordpress?

Question

I wrote my first custom wordpress plugin. It is basically a copy of the default Recent Posts plugin (that comes out of the box), and then I am adding a filter to only get a certain post category. Originally, I just hardcoded this, but I figured I would just add a widget option that the user can change. This required adding an additional field in 'function form()'. I basically just copied and pasted the input field for the "title" text box (and then added the appropriate code for the new field - again copying and pastying from title). After I did this, the field showed up just fine, but I could the option would not save when clicking Save (the field just wipes out each time). Basically, the field is not posting properly (or something along those lines). My first question is, where are these fields stored? Also, am I supposed to be registering the field somewhere?

The code is below... The Category field is what I am trying to add. Please advise. Thanks.

<?php 
/**
 * Recent_Posts widget class
 *
 * @since 2.8.0
 */
class custom_RecentPostsByCategory extends WP_Widget {

    function __construct() {
        $widget_ops = array('classname' => 'widget_recent_entries', 'description' => __( "The most recent posts on your site (by Category)") );
        parent::__construct('recent-posts', __('Custom: Recent Posts'), $widget_ops);
        $this->alt_option_name = 'widget_recent_entries';

        add_action( 'save_post', array(&$this, 'flush_widget_cache') );
        add_action( 'deleted_post', array(&$this, 'flush_widget_cache') );
        add_action( 'switch_theme', array(&$this, 'flush_widget_cache') );
    }

    function widget($args, $instance) {
        $cache = wp_cache_get('widget_recent_posts', 'widget');

        if ( !is_array($cache) )
            $cache = array();

        if ( ! isset( $args['widget_id'] ) )
            $args['widget_id'] = $this->id;

        if ( isset( $cache[ $args['widget_id'] ] ) ) {
            echo $cache[ $args['widget_id'] ];
            return;
        }

        ob_start();
        extract($args);

        $title = apply_filters('widget_title', empty($instance['title']) ? __('Recent Posts') : $instance['title'], $instance, $this->id_base);
        if ( empty( $instance['number'] ) || ! $number = absint( $instance['number'] ) )
            $number = 10;

        $r = new WP_Query(array('posts_per_page' => $number, 'no_found_rows' => true, 'post_status' => 'publish', 'ignore_sticky_posts' => true, 'category_name' => $instance['cat']));
        if ($r->have_posts()) :
?>
        <?php echo $before_widget; ?>
        <?php if ( $title ) echo $before_title . $title . $after_title; ?>
        <ul class="twitter-list">
        <?php  while ($r->have_posts()) : $r->the_post(); ?>
            <li class="twitter-item">
                <a href="<?php the_permalink() ?>" title="<?php echo esc_attr(get_the_title() ? get_the_title() : get_the_ID()); ?>"><?php if ( get_the_title() ) the_title(); else the_ID(); ?></a><br/>
                <?php the_time("F j, Y"); ?>
            </li>
        <!--<li><a href="<?php the_permalink() ?>" title="<?php echo esc_attr(get_the_title() ? get_the_title() : get_the_ID()); ?>"><?php if ( get_the_title() ) the_title(); else the_ID(); ?></a></li>-->

        <?php endwhile; ?>
        </ul>
        <?php echo $after_widget; ?>

<?php
        // Reset the global $the_post as this query will have stomped on it
        wp_reset_postdata();

        endif;

        $cache[$args['widget_id']] = ob_get_flush();
        wp_cache_set('widget_recent_posts', $cache, 'widget');
    }

    function update( $new_instance, $old_instance ) {
        $instance = $old_instance;
        $instance['title'] = strip_tags($new_instance['title']);
        $instance['cat'] = strip_tags($new_instance['cat']);
        $instance['number'] = (int) $new_instance['number'];
        $this->flush_widget_cache();

        $alloptions = wp_cache_get( 'alloptions', 'options' );
        if ( isset($alloptions['widget_recent_entries']) )
            delete_option('widget_recent_entries');

        return $instance;
    }

    function flush_widget_cache() {
        wp_cache_delete('widget_recent_posts', 'widget');
    }

    function form( $instance ) {
        $title = isset($instance['title']) ? esc_attr($instance['title']) : '';
        $cat = isset($instance['cat']) ? esc_attr($instance['cat']) : '';
        $number = isset($instance['number']) ? absint($instance['number']) : 5;
?>
        <p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
        <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo $title; ?>" /></p>

        <p><label for="<?php echo $this->get_field_id('cat'); ?>"><?php _e('Category:'); ?></label>
                <input class="widefat" id="<?php echo $this->get_field_id('cat'); ?>" name="<?php echo $this->get_field_name('cat'); ?>" type="text" value="<?php echo $cat; ?>" /></p>

        <p><label for="<?php echo $this->get_field_id('number'); ?>"><?php _e('Number of posts to show:'); ?></label>
        <input id="<?php echo $this->get_field_id('number'); ?>" name="<?php echo $this->get_field_name('number'); ?>" type="text" value="<?php echo $number; ?>" size="3" /></p>
<?php
    }
}

function wpzoom_register_rpa_widget() {
    register_widget('custom_RecentPostsByCategory');
}
add_action('widgets_init', 'wpzoom_register_rpa_widget');
?>

Answer 1

You have to use unique id base for Widgets.

Change

parent::__construct('recent-posts', __('Custom: Recent Posts'), $widget_ops);

To

parent::__construct('recent-posts-custom', __('Custom: Recent Posts'), $widget_ops);