Widget Example using dynamic params based off settings

widget-example.php

<?php
/**
 * Example Widget
 *
*/
class ck_example_widget extends WP_Widget {
	/** Constructor */
	function __construct() {
		parent::__construct( 'ck_example_widget', Example Widget, array( 'description' => 'My Example Widget' ) );
		add_filter( 'dynamic_sidebar_params', array( $this, 'widget_class' ), 10, 1 );
	}

	/** @see WP_Widget::widget */
	function widget( $args, $instance ) {

		$args['id']        = ( isset( $args['id'] ) )        ? $args['id']        : 'ck_example_widget';
		$instance['title'] = ( isset( $instance['title'] ) ) ? $instance['title'] : '';

		$title = apply_filters( 'widget_title', $instance['title'], $instance, $args['id'] );

		echo $args['before_widget'];

		if ( $title ) {
			echo $args['before_title'] . $title . $args['after_title'];
		}

		// Your widget content

		echo $args['after_widget'];
	}

	/** @see WP_Widget::update */
	function update( $new_instance, $old_instance ) {
		$instance = $old_instance;

		$instance['title']             = strip_tags( $new_instance['title'] );
		$instance['add_wrapper_class'] = isset( $new_instance['add_wrapper_class'] );

		return $instance;
	}

	/** @see WP_Widget::form */
	function form( $instance ) {

		$defaults = array(
			'title'             => '',
			'add_wrapper_class' => false,
		);

		$instance = wp_parse_args( (array) $instance, $defaults ); ?>
		<p>
			<label for="<?php echo esc_attr( $this->get_field_id( 'title' ) ); ?>">
				Widget Title
			</label>
			<input class="widefat" id="<?php echo esc_attr( $this->get_field_id( 'title' ) ); ?>" name="<?php echo esc_attr( $this->get_field_name( 'title' ) ); ?>" type="text" value="<?php echo $instance['title']; ?>"/>
		</p>

		<p>
			<input <?php checked( $instance['add_wrapper_class'], true ); ?> id="<?php echo esc_attr( $this->get_field_id( 'add_wrapper_class' ) ); ?>" name="<?php echo esc_attr( $this->get_field_name( 'add_wrapper_class' ) ); ?>" type="checkbox" />
			<label for="<?php echo esc_attr( $this->get_field_id( 'add_wrapper_class' ) ); ?>">
				Apply CSS Class to Wrapper?
			</label>
		</p>

		<?php
	}

	/**
	 * Check the widget options before it's wrapper loads to add classes
	 * @param $params
	 *
	 * @return array
	 */
	public function widget_class( $params ) {
		if ( strpos( $params[0]['widget_id'], 'ck_example_widget' ) !== false ) {
			$instance_id       = $params[1]['number'];
			$all_settings      = $this->get_settings();
			$instance_settings = $all_settings[ $instance_id ];

			if ( ! empty( $instance_settings['add_wrapper_class'] ) ) {
				
				/**
				 * Here, you can do some logic to determine what $class should be equal to.
				 *
				 * For example, on Easy Digital Downloads we look for the option and check if the cart
				 * is empty or not using something like:
				 * $class = ! empty( edd_get_cart_contents() ) ? 'cart-not-empty' : 'cart-empty';
				 */
				$class = 'my-awesome-css-class';

				$params[0]['before_widget'] = preg_replace( '/class="(.*?)"/', 'class="$1 ' . $class . '"', $params[0]['before_widget'] );
			}
		}

		return $params;
	}

}