My solution works in two ways: it disables selection of two identical revisions by modifying the list-revisions Javascript file, and if by some means two identical revisions do get selected, it offers a real error message.
This screenshot gives you a quick overview of the error message, and the effect of the new Javascript. As you can see, it is no longer possible to select the same revision twice. If you want to see the effect you either have to disable the Javascript (comment out the add_action(('wp_default_scripts', ... line) or manually change the URL.
It is written as a plugin. For a multisite installation you could maybe drop it in mu-plugins, and let the initialization depend on a database configuration option. That way you can enable or disable it for different clients.
Comments on my plugin writing style are welcome, I tried to separate the code into three logical blocks.
Main plugin file. This file switches the list-revisions Javascript file with a "safer" version that prevents the selection of the same revision twice. It also checks the admin_action_diff action and shortcuts to an own page with an error message if the two revisions are equal. The new page is written in ui-identical.php to separate code and layout.
<?php
/*
Plugin Name: Myxomatosis
Plugin URI: http://www.monkeyman.be/
Description: Kill the Easter Bunny and his eggs!
Version: 1.0
Author: Jan Fabry
*/
class Myxomatosis
{
function __construct()
{
add_action('admin_action_diff', array(&$this, 'checkEqualDiff'));
add_action('wp_default_scripts', array(&$this, 'loadSafeListRevisions'), 20);
}
function checkEqualDiff()
{
global $left, $right;
if (!array_key_exists('left', $_REQUEST) || !array_key_exists('right', $_REQUEST)) {
return;
}
$left = absint($_REQUEST['left']);
$right = absint($_REQUEST['right']);
if (!$left || !$right || $left != $right) {
// Nothing wrong, they are not equal
return;
}
// They are equal. Set up globals so UI can be included
// Which globals do I need here?
global $title, $hook_suffix, $pagenow, $is_iphone, $current_screen, $user_identity, $post, $wp_locale;
$post = get_post($left);
if ($post->post_parent) {
$post = get_post($post->post_parent);
}
require_once(plugin_dir_path(__FILE__) . 'ui-identical.php');
exit();
}
function loadSafeListRevisions(&$scripts)
{
$scripts->remove('list-revisions');
$scripts->add('list-revisions', plugin_dir_url(__FILE__) . 'safe-list-revisions.dev.js', null, '20101026' );
}
}
$myxomatosis_instance = new Myxomatosis();
File ui-identical.php, in the plugin directory. This creates the page to show if the two revisions are the same. The revision list is shown again.
<?php
wp_enqueue_script('list-revisions');
$post_title = '<a href="' . get_edit_post_link() . '">' . get_the_title() . '</a>';
$h2 = sprintf( __( 'Compare Revisions of “%1$s”' ), $post_title );
$title = __( 'Revisions' );
require_once(ABSPATH . '/wp-admin/admin-header.php');
?>
<div class="wrap">
<h2 class="long-header"><?php echo $h2; ?></h2>
<div class="error"><p><?php _e( 'You selected two identical revisions' ); ?></p></div>
<h2><?php echo $title; ?></h2>
<?php
$args = array( 'format' => 'form-table', 'parent' => true, 'right' => $right, 'left' => $left );
if ( ! WP_POST_REVISIONS || !post_type_supports($post->post_type, 'revisions') )
$args['type'] = 'autosave';
wp_list_post_revisions( $post, $args );
?>
</div>
<?php
require_once(ABSPATH . '/wp-admin/admin-footer.php');
File safe-list-revisions.dev.js, in the plugin directory. The logic is split up in two loops: one to read the current checked buttons, and one to hide and show the others. This is might simpler than the original version, and allows some tweaking in the middle.
(function(w) {
var init = function() {
var pr = document.getElementById('post-revisions'),
var inputs = pr ? pr.getElementsByTagName('input') : [];
if (inputs.length <= 2) {
// Sanity check: if there is only one revision, hide nothing
// Not that we should get here...
return;
}
// For simplicity, only one click handler is registered on the whole #post-revisions table
pr.onclick = function() {
var i, checkCount = 0, side;
var checkedIdx = {};
for (i = 0; i < inputs.length; i++) {
if (inputs[i].checked) {
side = inputs[i].getAttribute('name');
checkedIdx[side] = i;
}
}
if (checkedIdx['left'] + 1 == checkedIdx['right']) {
// The same revisions are checked
// Move left down (or right up, if left is at the bottom)
// There should always be at least two revisions (see sanity check above)
if (checkedIdx['left'] + 2 < inputs.length) {
checkedIdx['left'] += 2;
inputs[checkedIdx['left']].checked = true;
} else {
checkedIdx['right'] -= 2;
inputs[checkedIdx['right']].checked = true;
}
}
if (checkedIdx['left'] < checkedIdx['right']) {
// 'Old' is newer than 'New', so switch them
// We sometimes get this - don't know why
var origLeft = checkedIdx['left'];
checkedIdx['left'] = checkedIdx['right'] - 1;
checkedIdx['right'] = origLeft + 1;
inputs[checkedIdx['left']].checked = true;
inputs[checkedIdx['right']].checked = true;
}
// Loop again, hide what we should not see
// 'left' buttons should be hidden until we see a checked 'right' button
// 'right' buttons should be visible until we see a checked 'left' button
for (i = 0; i < inputs.length; i++) {
side = inputs[i].getAttribute('name');
if ('left' == side) {
inputs[i].style.visibility = (i < checkedIdx['right'] ? 'hidden' : 'visible');
} else if ('right' == side) {
inputs[i].style.visibility = (checkedIdx['left'] < i ? 'hidden' : 'visible');
}
}
}
pr.onclick();
}
if ( w && w.addEventListener )
w.addEventListener('load', init, false);
else if ( w && w.attachEvent )
w.attachEvent('onload', init);
})(window);
