Block type breaks when rewriting a block in custom module

Question

Just installed Magento CE 1.9.3.1 into my local system.

Then I tried to override the default text input field of Varien class just to add "placeholder" attribute.

etc/config.xml of my module:

<?xml version="1.0"?>
<config>
    <modules>
        <Custom_PlaceholderAttribute>
            <version>1.0.0</version>
        </Custom_PlaceholderAttribute>
    </modules>
    <global>
        <blocks>
            <placeholderattr>
                <rewrite>
                    <text>Varien_Data_Form_Element_Text</text>
                </rewrite>
            </placeholderattr>
        </blocks>
    </global>
</config>

Block class file of module:

<?php
class Custom_PlaceholderAttribute_Block_Adminhtml_Text extends Varien_Data_Form_Element_Text {
    public function getHtmlAttributes() {
        $attributes = parent::getHtmlAttributes();
        $attributes[] = 'placeholder';
        return $attributes;
    }
}

Now when I try to change fieldset type, in any block-form file, by adding below line, it is giving me Fatal Error.

$fieldset->addType('text', Mage::getConfig()->getBlockClassName('placeholderattr/adminhtml_text'));

The Fatal error I am getting is:

Fatal error: Class 'Mage_Placeholderattr_Text_Block_Adminhtml_Text' not found in C:\UwAmp\www\testmage1931\lib\Varien\Data\Form\Abstract.php on line 146

But if I try this, rewrite works fine:

$fieldset->addType('text','Custom_PlaceholderAttribute_Block_Adminhtml_Text');

Why block type doesn't work in getBlockClassName(), can somebody clarify/explain the reason for this ?

Answer 1

Add following code inside global tag

<blocks>
    <placeholderattr>
        <class>Custom_PlaceholderAttribute_Block_Block</class>
    </placeholderattr>
</blocks>

So your code looks like:

<config>
    <modules>
        <Custom_PlaceholderAttribute>
            <version>1.0.0</version>
        </Custom_PlaceholderAttribute>
    </modules>
    <global>
        <blocks>
            <placeholderattr>
                <class>Custom_PlaceholderAttribute_Block_Block</class>
            </placeholderattr>
        </blocks>
    </global>
</config>

Clear cache.

Answer 2

It is not a regular block you are trying to rewrite. You try to rewrite a lib.

Therefore you have to copy this class into the local codepool to do a rewrite. Have a look at this answer from Alan Storm: https://stackoverflow.com/a/6077165

In general, this guide will help you with all the rewrite types you might need: http://inchoo.net/magento/overriding-magento-blocks-models-helpers-and-controllers/