Question

I am trying to extend shipping method with respect to cities.

senerio :

When i select country in flat rate, Automatically populate cities with respect to selected country.

How may i change or extend shipping method

Answer 1

To answer your own questions in the future always explore the methods in abstract classes and you find that every shipping method has a parent method isActive() defined in their object.

So your task is to write your restrictions to this method in your shipping method class that extends Mage_Shipping_Model_Carrier_Abstract

public function isActive(){
    parent::isActive();
    //your logic to follow and check the selected shipping or billing city and return boolean true/false 
}

or you can narrow your needs to specific methods in abstract( dont forget to call the parent method in your overwrite)

checkAvailableShipCountries()
isActive()
isCityRequired()
isStateProvinceRequired()
isZipCodeRequired()