Example extension that allows you to modify the final ExpressionEngine CP output

gistfile1.php

<?php 

/**
 * Modifying the final CP Output
 *
 * This extension demonstrates how you can access and modify the final ExpressionEngine
 * CP output.  It is not a hack, but it is a new technique that to my knowledge has not
 * been used before in an EE addon.  
 * 
 * This has not been road tested and its side effects are unknown, so use this at your own risk.
 * 
 * NB: Lines 74 and 75 will need to be modified by you before this extension will work.
 *
 * - Christopher Imrie
 * 
 */
class Example_ext {
    var $EE;

    var $name           = 'Example Extension';
    var $version        = '0.1a';
    var $description    = 'Allows access to the final ExpressionEngine CP output';
    var $settings_exist = 'n';
    var $docs_url       = '';

    var $settings       = array();



    /**
     * Constructor
     *
     * @param   mixed   Settings array or empty string if none exist.
     */
    function __construct($settings = '')
    {
        $this->EE =& get_instance();

        $this->settings = $settings;
    }



    /**
     * Initialize CI Hooks
     * 
     * Using CI Hooks is the only way that we can ensure that our method is 
     * LAST, after the EE CP Controller has finished.
     * 
     * @param  object   $Session      EE Session instance
     * @return null          
     */
    public function initialize_ci_hook($Session)
    {
        global $EXT;


        //If this request is a JS or AJAX call, dont bother to load initilise hooks.  
        //We want legit  CP page loads only
        if($this->EE->input->get("C") == "javascript"){
            return;
        }

        //Enable CI Hooks
        $EXT->enabled = TRUE;

        //Create the post_controller hook array if needed
        if(!isset($EXT->hooks['post_controller'])){
            $EXT->hooks['post_controller'] = array();
        }

        //Add our hook
        $EXT->hooks['post_controller'][] = array(
            'class'    => __CLASS__,
            'function' => 'modify_cp_output',
            'filename' => THIS-FILENAME,            // eg: ext.example.php
            'filepath' => DIRECTORY-TO-THIS-FILE,   // Relative to the system/expressionengine folder
            'params'   => array()
        );
    }




    /**
     * Post EE Controller
     *
     * This method will be called after the EE Controller has finished.
     * 
     * @return null
     */
    public function modify_cp_output()
    {
        //Fetch the final CP Output
        $html = $this->EE->output->final_output;


        //
        // GO CRAZY CHANGING ALL THE THINGS !
        //
        

        //Reset the final output to your modified version
        $this->EE->output->final_output = $html;
    }






    /**
     * Activate, Update and Delete
     */

    function activate_extension()
    {
        $this->settings = array();


        $data = array(
            'class'     => __CLASS__,
            'method'    => 'initialize_ci_hook',
            'hook'      => 'sessions_start',
            'settings'  => serialize($this->settings),
            'priority'  => 1,
            'version'   => $this->version,
            'enabled'   => 'y'
        );

        $this->EE->db->insert('extensions', $data);
    }




    function update_extension($current = '')
    {
        if ($current == '' OR $current == $this->version)
        {
            return FALSE;
        }

  

        $this->EE->db->where('class', __CLASS__);
        $this->EE->db->update(
                    'extensions',
                    array('version' => $this->version)
        );
    }


    function disable_extension()
    {
        $this->EE->db->where('class', __CLASS__);
        $this->EE->db->delete('extensions');
    }
}
// END CLASS

You could check REQ == CP early in the sessions_start hook, to make sure this is not called on every page load on front end too.

I dig this so damn much! A couple suggestions though:

1. I think you could avoid the CP checks by using the "cp_menu_array" hook instead of session_start. Just be sure to return the menu on the method.

2. For the CI Hook itself, you can set the file name and path dynamically like the below:

   $parts = explode(DIRECTORY_SEPARATOR, dirname(__FILE__));
   //Add our hook
   $EXT->hooks['post_controller'][] = array(
           'class' => __CLASS__,
           'function' => 'modify_cp_output',
           'filename' => basename(__FILE__), // eg: ext.example.php
           'filepath' => 'third_party/'.end($parts), // Relative to the system/expressionengine folder
           'params' => array()
   );