Execute rules when toggling Source only

I have a rule

      // Editor status: 0
      // Executes when in html source mode.
      if ( htmlFilter ) {          
        htmlFilter.addRules({
          // Loop through all elements
          elements: {
            // Perform this when it found image tag.
            img : function( element) {
              // Replace inline image with INLINE placeholder (comment).
              // Only run this when toggling "Source" button
              if (element.attributes.class == 'wysiwyg-inline-image') {
                // Return the image alt value.
                return new CKEDITOR.htmlParser.comment(element.attributes.alt);
              }
              // Return this when saving the node using "Save" button.
              else {
                return element;
              }
            }
          }
        });
      }

Anyone knows what is the API that I can use to wrap this code below so that it will execute only when I switch to html source using "Source" button?

// Replace inline image with INLINE placeholder (comment).
// Only run this when toggling "Source" button
if (element.attributes.class == 'wysiwyg-inline-image') {
   // Return the image alt value.
   return new CKEDITOR.htmlParser.comment(element.attributes.alt);
}

Thanks