Magento. Rewrite save for admin custom field - magento

Question

I created new field and included it into system.xml (Pin_Init_Block_Adminhtml_System_Robots)
Show i controled by _getElementHtml or render.
How i can catch save?
<?php
class Pin_Init_Block_Adminhtml_System_Robots extends Mage_Adminhtml_Block_System_Config_Form_Field
{
protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element)
{
//self::__writeRobots($element->getEscapedValue());
$html = '<textarea id="'.$element->getHtmlId().'" name="'.$element->getName()
.'" '.$element->serialize($element->getHtmlAttributes()).'>'.self::__readRobots().'</textarea>'."\n";
$html.= $element->getAfterElementHtml();
return $html;
}
protected function __readRobots( )
{
$file = fopen( Mage::getBaseDir().'/robots.txt', "r");
if(!$file) return $this->content;
$content = '';
while (!feof($file)):
$content .= fgets($file);
endwhile;
fclose($file);
return $content;
}
protected function __writeRobots( $content )
{
$file = #fopen( Mage::getBaseDir().'/robots.txt', 'w');
#fwrite($file, "$content");
}
protected function _beforeSave()
{
self::__writeRobots('text1');
}
protected function _afterSave ()
{
self::__writeRobots('text1');
}
}
?>
File: /app/code/community/Pin/Init/Block/Adminhtml/System
XML:
<robots translate="label">
<label>Robots.txt</label>
<comment></comment>
<frontend_type>text</frontend_type>
<frontend_model>init/adminhtml_system_robots</frontend_model>
<sort_order>3</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</robots>

Answer 1

You can specify to your element a backend_model then implement in that model the methods _afterSave and _beforeSave.
You can use as an example the web/unsecure/base_url element. It is defined in app/code/core/Mage/Core/etc/system.xml and has the backend model adminhtml/system_config_backend_baseurl.
That is equivalent to the class Mage_Adminhtml_Model_System_Config_Backend_Baseurl.
See what happens in there.
UPDATE
Make your config look like this:
<robots translate="label">
<label>Robots.txt</label>
<comment></comment>
<frontend_type>text</frontend_type>
<frontend_model>init/adminhtml_system_robots</frontend_model>
<backend_model>init/adminhtml_system_backend_robots</backend_model><!-- add this line -->
<sort_order>3</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</robots>
Now create the following class (put it in the proper file):
<?php
class Pin_Init_Block_Adminhtml_System_Backend_Robots extends Mage_Core_Model_Config_Data{
protected function _beforeSave()
{
//this is called before the value is saved
$value = $this->getValue();//this is how you can get the value
//do your magic with $value
}
protected function _afterSave()
{
//this is called after the value is saved
$value = $this->getValue();//this is how you can get the value
//do your magic with $value
}
}