Add Custom Fieldset Without making a Core module Rewrite

Question

I am wondering, is there any way to add a custom fieldset to an admin form, without overwriting Form container class of that admin-form.

I will explain it little bit. Suppose we need to add a new fieldset in CMS_Block. We can achieve this by rewrite _preapreForm() method of class Mage_Adminhtml_Block_Cms_Block_Edit_Form. Here we need to do somewhat this in order to achieve our goal

class Namespace_Modulename_Block_Adminhtml_Cms_Block_Edit_Form extends Mage_Adminhtml_Block_Cms_Block_Edit_Form{
    
    protected function _prepareForm()
    {
        parent::_prepareForm();

        /*
           add new fieldset and fields here
        */
    }
}

However I don't like to do rewrite of a core class (Since Its not a good practice). Is there any other approach exist to achieve the same result. I just want to add a new field sets with a custom module on an admin form with two conditions

1. Without touching core files

2. Without doing rewrite

Answer 1

You can try to observe the event adminhtml_block_html_before.
but that gets dispatched for every block that is a child of Mage_Adminhtml_Block_Template.
So you will need to to a check first to see if the block matches your need.
something like this:

public function addFields($observer) {
    $block = $observer->getEvent()->getBlock();
    if ($block instanceof Mage_Adminhtml_Block_Cms_Block_Edit_Form) {
         //do your magic here
    }
}