How to load a stylesheet into wp_head from a custom widget?

Question

The code below is part of a widget that goes with my theme. I'm trying to include a stylesheet that the widget needs by placing an add_action call to wp_head. However, the function never fires in this context.

What am I missing?

   function widget( $args, $instance ) {
        if(!get_option('my_slider')) return;
        add_action('wp_head', 'my_sliderCSS');
        extract($args);
        $title = apply_filters( 'widget_title', empty($instance['title']) ? '' : $instance['title'], $instance );
        $text = apply_filters( 'widget_text', $instance['text'], $instance );
        $hide_title = isset( $instance['hide_title'] ) ? $instance['hide_title'] : false;
        echo $before_widget;
        if ( $title && !$hide_title )echo $before_title . $title . $after_title;
        if ( $text )?><div id="slider"><dl><?php echo $instance['filter'] ? wpautop($text) : $text; ?></dl></div>
        <?php echo $after_widget;
    }

Answer 1

There are various approaches (including this one), but you could consider, since you're talking about a stylesheet and not a script, simply enqueueing it from the Plugin, regardless of whether the Widget is active. (Which is worse: the few bytes of Widget-specific CSS, or the server overhead to determine if the Widget is active?)

Just add the following to your Plugin file (outside of the Widget class definition):

<?php
function plugin-name_print_stylesheet() {
    ?>
    <style type="text/css">
    /* CSS definitions go here */
    </style>
    <?php
}
add_action( 'wp_print_styles', 'plugin-name_print_stylesheet' );
?>

Or, if you've got a .css file with your Widget's stylesheet:

<?php
function plugin-name_enqueue_stylesheet() {
    wp_enqueue_style( 'plugin-name-stylesheet', plugins_dir( 'stylesheet.css', __FILE__ ) );
}
add_action( 'wp_enqueue_scripts', 'plugin-name_enqueue_stylesheet' );
?>

p.s. don't use a generic term such as my_slider as a custom-function prefix. You should use your plugin-slug instead, as a prefix for all of your Plugin's custom functions.