I’m injecting some jQuery to make the Alt Text field required in WordPress upload thick box.
It intercepts the Insert into Post button click and checks if the field has been filled or not.
It works ok for the Gallery and Media Library tabs, but the From Computer tab needs a needs a “listener” when an upload finishes to alter the behavior of the Insert into Post button.

I was trying with setInterval, but don’t know how to kill or recreate it, but maybe someone is aware if a listener exists, or even how to make this code work because I suspect my logic here is fuzzy…
Here’s the code, commented.
add_action('admin_head-media-upload-popup','so_11149675_required_alt_text');
function so_11149675_required_alt_text()
{
// Detect current tab ("From Computer" == "type")
$tab = isset($_GET['tab']) ? $_GET['tab'] : "type";
// ( 'From Computer' or ( 'Gallery' and 'Library' ) )
$jquery = ('type' == $tab) ? 'var refreshUpload = setInterval(function(){$(".savesend input").each(checkAltTextPermanent);},500);' : '$(".savesend input").each(checkAltTextOnce);';
echo <<<HTML
<script language="javascript" type="text/javascript">
// var refreshUpload; /* testing */
// Function called by From Computer tab
// should run only once -> when the upload table and fields are created
function checkAltTextPermanent() {
// Create the required asterisk symbol
// setInterval creates a loop here
jQuery('.image_alt th label').each(function(i,e) {
jQuery('<span class="alignright"><abbr title="required" class="required">*</abbr></span>').prependTo(this);
});
// Alter button behavior
// Another loop in the alert box
jQuery(this).click(function(e) {
// clearInterval(refreshUpload);
var value = jQuery(this).parent().parent().parent().find('.image_alt input').val();
if('' != value)
return true;
alert ('Please fill the Alt text');
return false;
});
}
// Function called by Gallery and Library tabs
function checkAltTextOnce() {
jQuery(this).click(function(e) {
var value = jQuery(this).parent().parent().parent().find('.image_alt input').val();
if('' != value)
return true;
alert ('Please fill the Alt text');
return false;
});
}
jQuery(document).ready(function($) {
// Defined in PHP, calls checkAltTextOnce or checkAltTextPermanent
{$jquery}
// Used in Gallery and Libray tabs
$('.image_alt th label').each(function(i,e) {
$('<span class="alignright"><abbr title="required" class="required">*</abbr></span>').prependTo(this);
});
});
</script>
HTML;
}
Try this: