Q:

Custom Magento Extension. How can I add the option to enable or disable it?

I have finally got my first extension built thanks to everyone here! I managed to get the extension added as a tab in the admin area as well. I have a menu that says enable or disable. I would like to make this function work giving the customer more control with this extension. Before it is mentioned, I am aware that you can enable and disable extensions under the advanced menu. However, most Magento customers are not. I would like to add this directly to my extension.
If the module is enabled I would like to overide the shipping.phtml file located in the checkout/cart/shipping.phtml with my own custom shipping template.

How can I make my enable or disable dropdown box work? Below is my code:

etc/system.xml
<?xml version="1.0"?>
<config>
<tabs>
    <beckinconfig translate="label" module="dropdownshipping">
        <label>Beckin Extensions</label>
        <sort_order>100</sort_order>
    </beckinconfig>
</tabs> 
<sections>  
    <dropdownshipping translate="label" module="dropdownshipping">
        <label>Drop Down Shipping Options</label>
        <tab>beckinconfig</tab>
        <frontend_type>text</frontend_type>
        <sort_order>1000</sort_order>
        <show_in_default>1</show_in_default>
        <show_in_website>1</show_in_website>
        <show_in_store>1</show_in_store>

        <groups>            
                  <general>
            <label>General</label>
            <frontend_type>text</frontend_type>
            <sort_order>1</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>1</show_in_store>       
              <fields>
                            <enable translate="label">
                            <label>Enable</label>
                            <comment>
                            <![CDATA[Enable or Disable this extension.]]>
                            </comment>
                            <frontend_type>select</frontend_type>
                                 <source_model>adminhtml/system_config_source_yesno</source_model>
                            <sort_order>1</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>1</show_in_store>                    
                            </enable>           
             </fields>                 
        </general>
        </groups>
    </dropdownshipping>
</sections>     
</config>

etc/config.xml
<?xml version="1.0"?>
<config>    
<modules>
<Beckin_DropDownShipping><version>1.0.0</version></Beckin_DropDownShipping>
    </modules>

<global>

            <blocks>
                 <beckin_dropdownshipping>
                      <class>Beckin_DropDownShipping_Block</class>
                 </beckin_dropdownshipping>
            </blocks>

    <helpers>
         <beckin_dropdownshipping>
         <class>Beckin_DropDownShipping_Helper</class>
         </beckin_dropdownshipping>
    </helpers>      
</global>

<frontend>
    <layout>
        <updates>
            <beckin>
                <file><!-- beckin_dropdownshipping.xml --></file>
            </beckin>
        </updates>
    </layout>
    <routers>
        <dropdownshipping>
            <use>standard</use>
            <args>
                <module>Beckin_DropDownShipping</module>
                <frontName>dropdownshipping</frontName>
            </args>
        </dropdownshipping>
    </routers>  
</frontend>

<adminhtml>
    <acl>
        <resources>
            <admin>
                <children>
                    <system>
                        <children>
                            <config>
                                <children>
                                    <dropdownshipping_options>
                                        <title>Beckin Drop Down Shipping Extension</title>
                                    </dropdownshipping_options>
                                </children>
                            </config>
                        </children>
                    </system>
                </children>
            </admin>
        </resources>
    </acl>
</adminhtml>

</config>

Helper/Data.php
<?php
class Beckin_DropDownShipping_Helper_Data extends Mage_Core_Helper_Abstract
{   

}

Block/Cart/Shipping.php
<?php

class Beckin_DropDownShipping_Block_Cart_Shipping extends Mage_Checkout_Block_Cart_Shipping
{

protected function _construct()
{

      if(Mage::getStoreConfig('beckin/dropdownshipping_options/enable', Mage::app()->getStore()->getId()){
     {
     $this->setTemplate('Beckin/dropdownshipping/drop_down_shipping.phtml');
     }
        else
        {
        $this->setTemplate('checkout/cart/shipping.phtml');
        }
}
}

Thanks for any help provided!! I am getting a blank admin page now when I go to the system/config section where my extension is located. I am sure it is something simple that I am missing. I have included every file above that I am using except for the template file which I believe is irrelevant. Can you spot my error? I wished it would show me an error instead of the page being blank :(

A:

Without known exactly what your module does and where and how your loading it. Assuming that it is not a custom shipping extension. You could do 
Eg. 1
if((Mage::getStoreConfig('deckin/dropdownshipping_options/beckin_enable', Mage::app()->getStore()->getId()){
   module enable/display .phtml
   assuming that beckin_enable value are 0 or 1
}

I'm not 100% sure what you mean by '... the template xml file to set the template to override the shipping.phtml file ...'
`
You may need to find and rewrite the block construct that set the template file for the shipping
Eg. 2
protected function _construct()
{
    if(Mage::getStoreConfig('deckin/dropdownshipping_options/beckin_enable', Mage::app()->getStore()->getId()){
        $this->setTemplate('.../my_custom_shipping.phtml');
     }
     else{
        $this->setTemplate('.../regular_shipping.phtml');
     }
}

Also, You don't need to create you own 'source model' for yes/no, magento have it pre-built 
<beckin_enable>
    ....
    <source_model>dropdownshipping/enable</source_model>                           
    ...                    
</beckin_enable>

You could have done
<beckin_enable>
    ....
    <source_model>adminhtml/system_config_source_yesno</source_model>                          
    ...                    
</beckin_enable>

