cannot add handler to a view (error : Error: handler for <fieldname> doesn't exist!

Hi , thanks for replying.

I did have such a file, in views/modules/category
and it indeed extends from the correct parent.

I'm not changing the view core module code, just enhancing the view functionality of the category module which is very basic.
I don't care about the location of the file that holds the view hooks implementation.
Actually, I tried also to put the code in the category module itself without success.
Btw, views/modules contains such extensions for many modules.

Can you state the required and precise steps to add an handler ?

I'm not changing the view core module code, just enhancing the view functionality of the category module which is very basic.
I don't care about the location of the file that holds the view hooks implementation.
Actually, I tried also to put the code in the category module itself without success.
Btw, views/modules contains such extensions for many modules.

Can you state the required and precise steps to add an handler ?

Ok, thanks I solved the issue.

I have now a handler for category views that includes depth (based on taxonomy depth handler)
and it works !!

<?php
// $Id: views_handler_argument_numeric.inc,v 1.1 2008/09/03 19:21:28 merlinofchaos Exp $
/**
 * @file
 * Contains the numeric argument handler.
 */

/**
 * Basic argument handler for arguments that are numeric. Incorporates
 * break_phrase.
 *
 * @ingroup views_argument_handlers
 */
class views_handler_argument_category_depth extends views_handler_argument {
	
  function option_definition() {
    $options = parent::option_definition();

    $options['depth'] = array('default' => 0);
    $options['break_phrase'] = array('default' => FALSE);
    $options['set_breadcrumb'] = array('default' => FALSE);

    return $options;
  }

  function options_form(&$form, &$form_state) {
    parent::options_form($form, $form_state);
    $form['depth'] = array(
      '#type' => 'weight',
      '#title' => t('Depth'),
      '#default_value' => $this->options['depth'],
      '#description' => t('The depth will match nodes tagged with terms in the hierarchy. For example, if you have the term "fruit" and a child term "apple", with a depth of 1 (or higher) then filtering for the term "fruit" will get nodes that are tagged with "apple" as well as "fruit". If negative, the reverse is true; searching for "apple" will also pick up nodes tagged with "fruit" if depth is -1 (or lower).'),
    );

    $form['break_phrase'] = array(
      '#type' => 'checkbox',
      '#title' => t('Allow multiple terms per argument'),
      '#description' => t('If selected, users can enter multiple arguments in the form of 1+2+3. Due to the number of JOINs it would require, AND will be treated as OR with this argument.'),
      '#default_value' => !empty($this->options['break_phrase']),
    );

    $form['set_breadcrumb'] = array(
      '#type' => 'checkbox',
      '#title' => t("Set the breadcrumb for the term parents"),
      '#description' => t('If selected, the breadcrumb trail will include all parent terms, each one linking to this view. Note that this only works if just one term was received.'),
      '#default_value' => !empty($this->options['set_breadcrumb']),
    );
  }
/*
  function set_breadcrumb(&$breadcrumb) {
    if (empty($this->options['set_breadcrumb']) || !is_numeric($this->argument)) {
      return;
    }

    return views_taxonomy_set_breadcrumb($breadcrumb, $this);
  }

*/
  /**
   * Override default_actions() to remove summary actions.
   */
  function default_actions($which = NULL) {
    if ($which) {
      if (in_array($which, array('ignore', 'not found', 'empty', 'default'))) {
        return parent::default_actions($which);
      }
      return;
    }
    $actions = parent::default_actions();
    unset($actions['summary asc']);
    unset($actions['summary desc']);
    return $actions;
  }

  function query() {
    $this->ensure_my_table();

    if (!empty($this->options['break_phrase'])) {
      $cids = new stdClass();
      $cids->value = $this->argument;
      $cids = views_break_phrase($this->argument, $cids);
      if ($cids->value == -1) {
        return FALSE;
      }

      if (count($cids->value) > 1) {
        $placeholder = " IN (" . implode(', ', array_fill(0, sizeof($cids->value), '%d')) . ")";
      }
      else {
        $placeholder = " = %d";
      }

      $cids = $cids->value;
    }
    else {
      $placeholder = "= %d";
      $cids = array($this->argument);
    }
    $subquery = "\n  SELECT tn.vid FROM {category_node} tn\n";
    $where = "  WHERE tn.cid $placeholder\n";
    $args = $cids;
    $last = "tn";

    if ($this->options['depth'] > 0) {
      $subquery .= "    LEFT JOIN {category_hierarchy} th ON th.cid = tn.cid\n";
      $last = "th";
      foreach (range(1, abs($this->options['depth'])) as $count) {
        $subquery .= "    LEFT JOIN {category_hierarchy} th$count ON $last.parent = th$count.cid\n";
        $where .= "    OR th$count.cid $placeholder\n";
        $args = array_merge($args, $cids);
        $last = "th$count";
      }
    }
    else if ($this->options['depth'] < 0) {
      foreach (range(1, abs($this->options['depth'])) as $count) {
        $subquery .= "    LEFT JOIN {category_hierarchy} th$count ON $last.cid = th$count.parent\n";
        $where .= "    OR th$count.cid $placeholder\n";
        $args = array_merge($args, $cids);
        $last = "th$count";
      }
    }

    $this->query->add_where(0, "$this->table_alias.vid IN ($subquery$where  )", $args);
  }

  function title() {
    $term = category_get_category($this->argument);
    if (!empty($term)) {
      return check_plain($term->name);
    }
    // TODO review text
    return t('No name');
  }
	}

Do not worry, It's in the category module now, not in the views module code.