{tech}.Ben Buckman.net

I'm using Views' "expose filters as a block" functionality in a custom search page. One of the filters is for content type, and users need to be able to select multiple options. Views can only generate multi-select lists as dropdowns, however, and the designs require checkboxes. Using a hook_form_alter to flip the #type from 'select' to 'checkboxes' fails because the form API structures those so differently. The views_filter_pack module claims to be able to convert from one to the other, but is extremely over-engineered for this purpose. Some searching around showed that other people had the same problem but no solutions seemed to work.

Then it dawned on me that I didn't need Drupal or Views to use checkboxes, I just needed the browser output to be rendered as checkboxes.

So I did a test in Firebug, rewriting the <select> as checkboxes, with the same name and value's, and it worked!

With that revelation, I made a hybrid of theme_select() and theme_checkboxes() called theme_select_as_checkboxes(), which takes a select element but renders checkboxes. Here's the code:

/**
 * hack to make exposed dropdowns appear as checkboxes
 * (can't do it w/ form_alter b/c whole structure is different)
 * just render the options directly as checkboxes, pass the values the same as the SELECT,
 * tricks the form api and views to think it's a dropdown
 * [registered w/ hook_theme, toggled w/ #theme in form_alter]
 */
function theme_select_as_checkboxes($element) {
  $output = '';
 
  $selected_options = (array) $element['#post'][$element['#name']];    // the selected keys from #options
 
  foreach($element['#options'] as $key=>$value) {
 
    $id = $element['#id'] . '-' . $key;      // custom
 
    // is this option selected?
    $selected = (array_search($key, $selected_options) !== false);    // (returns key or false)
 
    $checkbox = '<input type="checkbox" '
      . 'name="'. $element['#name'] . '[]' .'" '    // brackets are key -- just like select
        . 'id="'. $id .'" '
        . 'value="'. $key .'" '
        . ($selected ? ' checked="checked" ' : ' ')
        . drupal_attributes($element['#attributes']) .' />';
 
    $output .= '<label class="option" for="'. $id .'">' . $checkbox .' '. $value .'</label>' . "\n";
  }
  return theme_form_element($element, $output);    // wraps it neatly
}

To enable it, register the function in hook_theme() with 'select_as_checkboxes' = array( 'function' => 'theme_select_as_checkboxes'), and set the field you want, in my case 'type', to use the custom theme function: $form['type']['#theme'] = 'select_as_checkboxes';

And it works!
I find this a lot more sensible than converting the whole tree structure in a form_alter. The rendering itself is basically Drupal-standard, it just happens to mix up the types a little.