This CodeLet demonstrates how to hook into node’s view page in Drupal 7.x. It usese Drupal 7.x hook — hook_node_view($node, $view_mode, $langcode). Thise particular CodeLet adds a form with button. This CodeLet also demonstrate use of Drupal 7.x form API. This is one of the live projects code in work!
Read more: Custom Drupal form on Drupal node.
