wordpress-multisite-sidebar.php

<?php

add_action( 'template_redirect', 'wp_multisite_sidebar_save' );

function wp_multisite_sidebar_save() {
	if ( is_main_site() && isset( $_GET['get_sidebar'] ) ) {
		$sidebar = $_GET['get_sidebar'];
		ob_start();
		dynamic_sidebar( $sidebar );
		$markup = ob_get_clean();
		set_site_transient( 'sidebar_cache_' . $sidebar, $markup, HOUR_IN_SECONDS );
		die();
	}
}

function wp_multisite_sidebar( $sidebar ) {
	$markup = get_site_transient( 'sidebar_cache_' . $sidebar );
	if (!is_main_site() && $markup ) {
		echo $markup;
	} else {
		if (!is_main_site()) {
			wp_remote_get( add_query_arg( array('get_sidebar' => $sidebar), get_site_url( 1 ) ) );
			echo get_site_transient( 'sidebar_cache_' . $sidebar );
		} else {
			ob_start();
			dynamic_sidebar($sidebar);
			$markup = ob_get_clean();
			set_site_transient( 'sidebar_cache_' . $sidebar, $markup, HOUR_IN_SECONDS );
			echo $markup;
		}
	}
}

add_action( 'sidebar_admin_setup', 'wp_multisite_sidebar_clear_cache' );

function wp_multisite_sidebar_clear_cache() {
	global $wp_registered_sidebars;
	foreach($wp_registered_sidebars as $sidebar){
		delete_site_transient('sidebar_cache_' . $sidebar['id']);
	}
	
}