Add hook_views_bulk_operations_form_alter().

Sure, why not.

Yeah, that is definitely a bad idea.

So, we either document that in order to modify a VBO form you need to do:

function hook_form_alter(&$form, &$form_state, $form_id) {
  if (strpos($form_id, 'views_form_') === 0) {
    $vbo = _views_bulk_operations_get_field($form_state['build_info']['args'][0]);
  }
  // Not a VBO-enabled views form.
  if (empty($vbo)) {
    return;
  }

  if ($form_state['step'] == 'views_form_views_form') {
    // This is the initial step of the VBO form. Do what you need to do.
  }
}

Or maybe add a hook_views_bulk_operations_form_alter().

Thoughts?

I can't seem to edit that page for some reason.

I cannot get it work. How can I edit vbo form in my hook_form_alter?
In particular, I want to edit form in that way, that I would have my custom submit function (with my custom submit button). But I cannot do it because selecting vbo operation in operations fieldset is required.
But I cannot edit the fieldset in form hooks.
Is it possible to achieve this without a patch?

You need the code in #3.
You also need to make sure your module runs after views_bulk_operations.
So if views_bulk_operations has a weight of "0", your module needs to have at least the weight of 1.
Weight is a column in the {system} table (though there's probably a contrib module that handles it as well).

thanks it works OK

Tomas

I think add a hook_views_bulk_operations_form_alter hook makes alot of sense. I have done something similar in textformatter now, it makes things alot easier for people altering, and saves a few support queries too :)

@bojanz - Do you want a patch for this?

Yes, please!

Here is an initial patch for this, I'm not sure if this covers everything we need right off the bat but should be a good start.

With a newline at the end of the api.php file

How about this version?

Committed a tweaked version.