Example derivative

test.php

<?php

/**
 * Implements hook_islandora_derivative().
 */
function test_islandora_derivative(AbstractObject $object) {
  return array(
    'source_dsid' => 'TEST',
    'destination_dsid' => 'TEST',
    'function' => 'test_test_derivative',
  );
}

 /**
  * Example callback for derivative recursion.
  */
 function test_test_derivative(AbstractObject $object, $force = FALSE) {
   if (isset($object['TEST'])) {
     $content = $object['TEST']->content;
     if ($content != 'om') {
       $object['TEST']->content = 'om';
     }
   }
 }

test.txt

nom