###########
Controllers
###########

Controllers are the heart of your application, as they determine how
HTTP requests should be handled.

.. contents:: Page Contents

What is a Controller?
=====================

**A Controller is simply a class file that is named in a way that can be
associated with a URI.**

Consider this URI::

	example.com/index.php/blog/

In the above example, CodeIgniter would attempt to find a controller
named Blog.php and load it.

**When a controller's name matches the first segment of a URI, it will
be loaded.**

Let's try it: Hello World!
==========================

Let's create a simple controller so you can see it in action. Using your
text editor, create a file called Blog.php, and put the following code
in it::

	<?php
	class Blog extends CI_Controller {

		public function index()
		{
			echo 'Hello World!';
		}
	}

Then save the file to your *application/controllers/* directory.

.. important:: The file must be called 'Blog.php', with a capital 'B'.

Now visit the your site using a URL similar to this::

	example.com/index.php/blog/

If you did it right, you should see:

	Hello World!

.. important:: Class names must start with an uppercase letter.

This is valid::

	<?php
	class Blog extends CI_Controller {

	}
	
This is **not** valid::

	<?php
	class blog extends CI_Controller {

	}

Also, always make sure your controller extends the parent controller
class so that it can inherit all its methods.

Methods
=======

In the above example the method name is ``index()``. The "index" method
is always loaded by default if the **second segment** of the URI is
empty. Another way to show your "Hello World" message would be this::

	example.com/index.php/blog/index/

**The second segment of the URI determines which method in the
controller gets called.**

Let's try it. Add a new method to your controller::

	<?php
	class Blog extends CI_Controller {

		public function index()
		{
			echo 'Hello World!';
		}

		public function comments()
		{
			echo 'Look at this!';
		}
	}

Now load the following URL to see the comment method::

	example.com/index.php/blog/comments/

You should see your new message.

Passing URI Segments to your methods
====================================

If your URI contains more than two segments they will be passed to your
method as parameters.

For example, let's say you have a URI like this::

	example.com/index.php/products/shoes/sandals/123

Your method will be passed URI segments 3 and 4 ("sandals" and "123")::

	<?php
	class Products extends CI_Controller {

		public function shoes($sandals, $id)
		{
			echo $sandals;
			echo $id;
		}
	}

.. important:: If you are using the :doc:`URI Routing <routing>`
	feature, the segments passed to your method will be the re-routed
	ones.

Defining a Default Controller
=============================

CodeIgniter can be told to load a default controller when a URI is not
present, as will be the case when only your site root URL is requested.
To specify a default controller, open your **application/config/routes.php**
file and set this variable::

	$route['default_controller'] = 'blog';

Where 'blog' is the name of the controller class you want used. If you now
load your main index.php file without specifying any URI segments you'll
see your "Hello World" message by default.

For more information, please refer to the "Reserved Routes" section of the
:doc:`URI Routing <routing>` documentation.

Remapping Method Calls
======================

As noted above, the second segment of the URI typically determines which
method in the controller gets called. CodeIgniter permits you to override
this behavior through the use of the ``_remap()`` method::

	public function _remap()
	{
		// Some code here...
	}

.. important:: If your controller contains a method named _remap(),
	it will **always** get called regardless of what your URI contains. It
	overrides the normal behavior in which the URI determines which method
	is called, allowing you to define your own method routing rules.

The overridden method call (typically the second segment of the URI) will
be passed as a parameter to the ``_remap()`` method::

	public function _remap($method)
	{
		if ($method === 'some_method')
		{
			$this->$method();
		}
		else
		{
			$this->default_method();
		}
	}

Any extra segments after the method name are passed into ``_remap()`` as an
optional second parameter. This array can be used in combination with
PHP's `call_user_func_array() <http://php.net/call_user_func_array>`_
to emulate CodeIgniter's default behavior.

Example::

	public function _remap($method, $params = array())
	{
		$method = 'process_'.$method;
		if (method_exists($this, $method))
		{
			return call_user_func_array(array($this, $method), $params);
		}
		show_404();
	}

Processing Output
=================

CodeIgniter has an output class that takes care of sending your final
rendered data to the web browser automatically. More information on this
can be found in the :doc:`Views <views>` and :doc:`Output Class
<../libraries/output>` pages. In some cases, however, you might want to
post-process the finalized data in some way and send it to the browser
yourself. CodeIgniter permits you to add a method named ``_output()``
to your controller that will receive the finalized output data.

.. important:: If your controller contains a method named ``_output()``,
	it will **always** be called by the output class instead of
	echoing the finalized data directly. The first parameter of the
	method will contain the finalized output.

Here is an example::

	public function _output($output)
	{
		echo $output;
	}

.. note::

	Please note that your ``_output()`` method will receive the
	data in its finalized state. Benchmark and memory usage data
	will be rendered, cache files written (if you have caching
	enabled), and headers will be sent (if you use that
	:doc:`feature <../libraries/output>`) before it is handed off
	to the ``_output()`` method.
	To have your controller's output cached properly, its
	``_output()`` method can use::

		if ($this->output->cache_expiration > 0)
		{
			$this->output->_write_cache($output);
		}

	If you are using this feature the page execution timer and
	memory usage stats might not be perfectly accurate since they
	will not take into account any further processing you do.
	For an alternate way to control output *before* any of the
	final processing is done, please see the available methods
	in the :doc:`Output Library <../libraries/output>`.

Private methods
===============

In some cases you may want certain methods hidden from public access.
In order to achieve this, simply declare the method as being private
or protected and it will not be served via a URL request. For example,
if you were to have a method like this::

	private function _utility()
	{
		// some code
	}

Trying to access it via the URL, like this, will not work::

	example.com/index.php/blog/_utility/

.. note:: Prefixing method names with an underscore will also prevent
	them from being called. This is a legacy feature that is left
	for backwards-compatibility.

Organizing Your Controllers into Sub-directories
================================================

If you are building a large application you might want to hierarchically
organize or structure your controllers into sub-directories. CodeIgniter
permits you to do this.

Simply create sub-directories under the main *application/controllers/*
one and place your controller classes within them.

.. note:: When using this feature the first segment of your URI must
	specify the folder. For example, let's say you have a controller located
	here::

		application/controllers/products/Shoes.php

	To call the above controller your URI will look something like this::

		example.com/index.php/products/shoes/show/123

Each of your sub-directories may contain a default controller which will be
called if the URL contains *only* the sub-directory. Simply put a controller
in there that matches the name of your 'default_controller' as specified in
your *application/config/routes.php* file.

CodeIgniter also permits you to remap your URIs using its :doc:`URI
Routing <routing>` feature.

Class Constructors
==================

If you intend to use a constructor in any of your Controllers, you
**MUST** place the following line of code in it::

	parent::__construct();

The reason this line is necessary is because your local constructor will
be overriding the one in the parent controller class so we need to
manually call it.

Example::

	<?php
	class Blog extends CI_Controller {

		public function __construct()
		{
			parent::__construct();
			// Your own constructor code
		}
	}

Constructors are useful if you need to set some default values, or run a
default process when your class is instantiated. Constructors can't
return a value, but they can do some default work.

Reserved method names
=====================

Since your controller classes will extend the main application
controller you must be careful not to name your methods identically to
the ones used by that class, otherwise your local functions will
override them. See :doc:`Reserved Names <reserved_names>` for a full
list.

.. important:: You should also never have a method named identically
	to its class name. If you do, and there is no ``__construct()``
	method in the same class, then your e.g. ``Index::index()``
	method will be executed as a class constructor! This is a PHP4
	backwards-compatibility feature.

That's it!
==========

That, in a nutshell, is all there is to know about controllers.