How to detect changed fields only in admin section?

Question

I need to insert some values to custom database table based on the values of changed custom field, if the specific custom field value (in a custom shipping method) had changed.I need to check this in my Observer.php event that I'm firing is admin_system_config_changed_section_carriers

is there any possible way to do this ?

Answer 1

If this custom field comes from your own module, you can use a backend model to do additional things before save:

Add this to the field definition in system.xml

<backend_model>your_module/system_config_backend_custom</backend_model>

And write the model like this:

class Your_Module_Model_System_Config_Backend_Custom extends Mage_Core_Model_Config_Data
{
    protected function _afterSave()
    {
        if ($this->isValueChanged()) {
            // do your thing here
        }
    }
}

Actually this also works for existing modules without changing their code: you can override the backend model in your own system.xml and make your module depend on the original one to make sure, yours is loaded afterwards.