Here's what I'm doing to accomplish this. I'm posting this as a possible solution, and also to get feedback from anyone with more WP experience than me. If you see any problems with my implementation or anything I'm not considering, please let me know.
**Known limitations:**
- It only works with scripts that are properly enqueued via `wp_enqueue_script`
- It outputs all scripts in the footer even if they were registered to only be output in the `<head>` section of the page. This might be easy to work around, but for my implementation I prefer it this way.
The problem I saw with the [head.js plugin][1] was that it only seemed to work on scripts in the `<head>` section, and the fact that it didn't allow for any easy CDN fallback functionality which is a really nice benefit of loading scripts this way in my opinion.
Ok, here's the code. This is part of a class that is in my general functionality plugin, but it could just as easily be in `functions.php` or organized in some other way. I'm using [yepnope][2] for now, but this could easily be modified to use another script loader.
class example {
function __construct() {
/*
Hook into the script printing functionality and use our own resource loader to load
scripts in a non-blocking, asynchronous, parallel fancy way
*/
if( !is_admin() ) {
add_action( 'wp_print_scripts', array( &$this, 'deploy_script_loader' ) );
}
}
/*
If we have any javascripts queued for the page, grab the handles of
all of the ones to be loaded in the header and dequeue them now.
Then, we will check again and reload any that weren't queued
yet in the footer.
*/
function deploy_script_loader( ) {
global $wp_scripts;
if ( !empty( $wp_scripts->queue ) && !is_admin() ) {
// Get the queue in our class property, and dequeue everything
foreach ( $wp_scripts->queue as $handle ) {
/*
Check if this script is supposed to be loaded in the header (group isn't 1).
If it is, we'll grab it now and dequeue it. We'll save the rest of the dequeuing
for the footer script or else we'll miss some scripts that are queued after
this hook is run.
*/
if ( 1 !== $wp_scripts->registered[$handle]->extra['group'] ) {
/*
Just dequeuing a script here isn't enough to prevent it from being
printed in the header if another script that we haven't dequeued (ie a footer
script) depends on it. So, we need to make sure that all of the
scripts we just dequeued will be ok loading in the footer (where they will
get dequeued for real before they are printed).
*/
$wp_scripts->registered[$handle]->extra['group'] = 1;
}
}
// Add our hook to load everything in the footer
add_action( 'wp_footer', array( &$this, 'output_script_loader' ) );
}
}
/*
Function to be ran in wp_footer to output the js script loader
html content.
*/
function output_script_loader() {
global $wp_scripts;
// Process the scripts dependency tree, but don't print anything
ob_start();
$script_queue = $wp_scripts->do_items();
ob_clean();
// Add our script loader
echo '<script type="text/javascript" src="' . plugins_url( '/scripts/yepnope.1.0.2-min.js', __FILE__ ) . '"></script><script type="text/javascript">';
// Loop through the queued scripts and get all of their localization output objects first
foreach( $script_queue as $handle ) {
echo $wp_scripts->registered[$handle]->extra['data'];
}
echo 'yepnope.errorTimeout = 4000; yepnope([';
$i = 0;
$count = count( $script_queue );
// Loop through the queued scripts again, this time output the script loader syntax
foreach( $script_queue as $handle ) {
if ( 'jquery' === $handle ) {
$jquery_cdn_url = ( is_ssl() ) ? 'https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js' : 'http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js';
echo ' { load: "'. $jquery_cdn_url .'", callback: function () { if (!window.jQuery) { yepnope("' . $wp_scripts->base_url . $wp_scripts->registered['jquery']->src . '"); } } },';
} else {
$src = $wp_scripts->registered[$handle]->src;
// Check if this is a relative path or if we have the whole thing
if( 'http' !== strtolower( substr( $src, 0, 4 ) ) ) {
$src = $wp_scripts->base_url . $src;
} else {
$src = ( is_ssl() ) ? 'https' . substr( $src, strpos( $src, ':' ), strlen( $src ) ) : 'http' . substr( $src, strpos( $src, ':' ), strlen( $src ) );
}
$comma = ( $i == ( $count - 1 ) ) ? '' : ',';
echo '{ load: "' . $src . '" }' . $comma;
}
$i++;
}
echo ']);</script>';
}
}
// Instantiate the class
new example;
[1]: http://wordpress.org/extend/plugins/headjs-loader/
[2]: http://yepnopejs.com/