Share Sidebars Across a Multisite Network

admin-widgets.js

jQuery(document).ready(function($){

	function wds_reset_footer_transient() {

	// Run our AJAX call to delete our site transient
        $.ajax({
            type : 'post',
            dataType : 'json',
            url : ajaxurl,
            data : {
                'action' : 'wds-reset-transient',
                'ei-widget-nonce' : wds_AJAX.wds_widget_nonce
            },
            error: function ( xhr, ajaxOptions, thrownError ) {
                console.log( thrownError );
                console.log( ajaxOptions );
            }
        });
	}

	// If one of our update buttons is clicked on a single widget
	$( '.widgets-holder-wrap' ).on( 'click', '.widget-control-remove, .widget-control-close, .widget-control-save', function() {

		// Get our parent, or sidebar, ID
		var widget_parent_div = $(this).parents().eq(5).attr( 'id' );

		// And if our parent div ID, or our sidebar ID, is one of the following
		if ( widget_parent_div == 'sidebar-footer-1' || widget_parent_div == 'sidebar-footer-2' || widget_parent_div == 'sidebar-footer-3' ) {

			// Run our function
			wds_reset_footer_transient();
		}
	});
});

ajax-functions.php

<?php

/**
 * Enqueue and localize our scripts
 */
add_action( 'admin_enqueue_scripts', 'wds_enqueue_ajax_scripts' );
function wds_enqueue_ajax_scripts() {
	global $current_screen;

	// Only register these scripts if we're on the widgets page
	if ( $current_screen->id == 'widgets' ) {
		wp_enqueue_script( 'wds_ajax_scripts', get_stylesheet_directory_uri() . '/js/admin-widgets.js', array( 'jquery' ), 1, true );
		wp_localize_script( 'wds_ajax_scripts', 'wds_AJAX', array( 'wds_widget_nonce' => wp_create_nonce( 'widget-update-nonce' ) ) );
	}
}

/**
 * Register our AJAX call
 */
add_action( 'wp_ajax_wds-reset-transient', 'wds_ajax_wds_reset_transient', 1 );

/**
 * AJAX Helper to delete our transient when a widget is saved
 */
function wds_ajax_wds_reset_transient() {

	// Delete our footer transient.  This runs when a widget is saved or updated.  Only do this if our nonce is passed.
	if ( ! empty( $_REQUEST['ei-widget-nonce'] ) )
		delete_site_transient( 'wds_footer_widgets' );
}

footer.php

<?php

// Display our stored footer widgets
$show_footer_sidebar = get_site_transient( 'wds_footer_widgets' );
echo $show_footer_sidebar;

functions.php

<?php

/**
 * Generate sidebar for footer across all sites
 */
add_action( 'init', 'wds_footer_widgets_all_sites' );
function wds_footer_widgets_all_sites() {

	if ( ! is_main_site() )
		return;

	if ( ! ( $wds_footer_widgets = get_site_transient( 'wds_footer_widgets' ) ) ) {
		// start output buffer
		ob_start();

		// Display our footer sidebars ?>
		<aside class="footer-widgets-1 widget-area col span_4">
			<?php if ( ! dynamic_sidebar( 'sidebar-footer-1' ) ) : ?>
			<?php endif; // end sidebar widget area ?>
		</aside>

		<aside class="footer-widgets-2 widget-area col span_4">
			<?php if ( ! dynamic_sidebar( 'sidebar-footer-2' ) ) : ?>
			<?php endif; // end sidebar widget area ?>
		</aside>

		<aside class="footer-widgets-3 widget-area col span_4">
			<?php if ( ! dynamic_sidebar( 'sidebar-footer-3' ) ) : ?>
			<?php endif; // end sidebar widget area ?>
		</aside>

		<?php // grab the data from the output buffer and put it in our variable
		$wds_footer_widgets = ob_get_contents();
		ob_end_clean();

		// Put sidebar into a transient for 4 hours
		set_site_transient( 'wds_footer_widgets', $wds_footer_widgets, 4*60*60 );
	}
}