Customizer: Introduce WP_Customize_Control::active() method to determine whether the control is relevant to the current context (i.e. to the current URL being previewed).

Control can indicate its active state by a subclass overriding the 'active_callback' method, by supplying a callable 'active_callback' argument into the control's constructor, or by filtering 'customize_control_active'.

props westonruter.
see #27993.

git-svn-id: https://develop.svn.wordpress.org/trunk@29051 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-widgets.php

@@ -1068,7 +1068,33 @@ final class WP_Customize_Widgets {
 	 * @param array $widget Rendered widget to tally.
 	 */
 	public function tally_rendered_widgets( $widget ) {
-		$this->rendered_widgets[$widget['id']] = true;
+		$this->rendered_widgets[ $widget['id'] ] = true;
+	}
+
+	/**
+	 * Determine if a widget is rendered on the page.
+	 *
+	 * @since 4.0.0
+	 * @access public
+	 *
+	 * @param string $widget_id
+	 * @return bool
+	 */
+	public function is_widget_rendered( $widget_id ) {
+		return in_array( $widget_id, $this->rendered_widgets );
+	}
+
+	/**
+	 * Determine if a sidebar is rendered on the page.
+	 *
+	 * @since 4.0.0
+	 * @access public
+	 *
+	 * @param string $sidebar_id
+	 * @return bool
+	 */
+	public function is_sidebar_rendered( $sidebar_id ) {
+		return in_array( $sidebar_id, $this->rendered_sidebars );
 	}
 
 	/**