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Writing an intelligent hook_nodeapi function in Drupal

For Drupal module developers, the hook_nodeapi function affords a lot of flexibility for interacting with nodes at various operation.  If you just start pasting or writing code, you’ll quickly end up with a giant, messy switch statement.  But there is a simpel way to keep your code nicely organized.  First, lets take a look at what operations we can affect:

  • “alter”: the $node->content array has been rendered, so the node body or teaser is filtered and now contains HTML. This op should only be used when text substitution, filtering, or other raw text operations are necessary.
  • “delete”: The node is being deleted.
  • “delete revision”: The revision of the node is deleted. You can delete data associated with that revision.
  • “insert”: The node is being created (inserted in the database).
  • “load”: The node is about to be loaded from the database. This hook can be used to load additional data at this time.
  • “prepare”: The node is about to be shown on the add/edit form.
  • “prepare translation”: The node is being cloned for translation. Load additional data or copy values from $node->translation_source.
  • “print”: Prepare a node view for printing. Used for printer-friendly view in book_module
  • “rss item”: An RSS feed is generated. The module can return properties to be added to the RSS item generated for this node. See comment_nodeapi() and upload_nodeapi() for examples. The $node passed can also be modified to add or remove contents to the feed item.
  • “search result”: The node is displayed as a search result. If you want to display extra information with the result, return it.
  • “presave”: The node passed validation and is about to be saved. Modules may use this to make changes to the node before it is saved to the database.
  • “update”: The node is being updated.
  • “update index”: The node is being indexed. If you want additional information to be indexed which is not already visible through nodeapi “view”, then you should return it here.
  • “validate”: The user has just finished editing the node and is trying to preview or submit it. This hook can be used to check the node data. Errors should be set with form_set_error().
  • “view”: The node content is being assembled before rendering. The module may add elements $node->content prior to rendering. This hook will be called after hook_view(). The format of $node->content is the same as used by Forms API.

That’s 15 operations, multiplied by the number of content types on your site, that’s potentially 15N cases we’ll have to account for.  If you want your code to run regardless of the content type, thats another 15 cases.  How much do you like spaghetti?

Our own function naming convention to the rescue

By using a simple naming convention, we can make sure that a) we can drop in code to run for any operation and/or content type b) encapsulate the code within its own function.  To do this, your actual hook_nodeapi implementation will simply delagate execution to these other functions.  First, if a function named module_nodeapi_operation exists, we’ll call it.  Then if the function module_nodeapi_operation_content-type exists, execute it.

Let’s assume you have a module foo, the hook_nodeapi function looks like:

 

  1. <?php
  2. /**
  3. * Implementation of hook_nodeapi that delegates operations to other functions
  4. * @see http://api.drupal.org/api/function/hook_nodeapi/6
  5. * @author Oscar Merida
  6. */
  7. function foo_nodeapi(&$node, $op,$a3 = NULL, $a4 = NULL)
  8. {
  9. // our own all too clever function api
  10. // first call foo_$op if it exists
  11. // then call foo_$op_$node->type if it exists
  12. $f_base = ‘foo_nodeapi_’ . $op;
  13. if (function_exists($f_base))
  14. {
  15. $f_base(&$node, $a3, $a4);
  16. }
  17. $f_content = $f_base . ‘_’ . $node->type;
  18. if (function_exists($f_content))
  19. {
  20. $f_content(&$node, $a3, $a4);
  21. }
  22. }

If you need to alter something about all nodes before they are saved, you would create a function named foo_nodeapi_presave.

  1. <?php
  2. function foo_nodeapi_presave(&$node, $op, $a3, $a4)
  3. {
  4. // do something to all nodes before they are saved
  5. }

Likewise, to affect what is loaded with a story node, we need a function named foo_nodeapi_load_story:

  1. <?php
  2. function foo_nodeapi_load_story(&$node, $op, $a3, $a4)
  3. {
  4. // do something to story nodes when they are loaded
  5. }

Caveats

Keep in mind that both functions are called, but the content type specific one is called after the more general one so you can undo/override the latter. As a side note, I was trying to use the presave operation to set the value of a CCK Text field if it was empty. Since the order of hook calls depend on the weight of the module in the system table, I had to make sure the content module had a higher weight than my own. If you’re trying to set the value of a CCK field but it’s not being saved, you’ll have to do the same.