*******************
Content Negotiation
*******************

Content negotiation is a way to determine what type of content to return to the client based on what the client
can handle, and what the server can handle. This can be used to determine whether the client is wanting HTML or JSON
returned, whether the image should be returned as a jpg or png, what type of compression is supported and more. This
is done by analyzing four different headers which can each support multiple value options, each with their own priority.
Trying to match this up manually can be pretty challenging. CodeIgniter provides the ``Negotiator`` class that
can handle this for you.

=================
Loading the Class
=================

You can load an instance of the class manually through the Service class::

	$negotiator = \Config\Services::negotiator();

This will grab the current request instance and automatically inject it into the Negotiator class.

This class does not need to be loaded on it's own. Instead, it can be accessed through this request's ``IncomingRequest``
instance. While you cannot access it directly this way, you can easily access all of methods through the ``negotiate()``
method::

	$request->negotiate('media', ['foo', 'bar']);

When accessed this way, the first parameter is the type of content you're trying to find a match for, while the
second is an array of supported values.

===========
Negotiating
===========

In this section we will discuss the 4 types of content that can be negotiated and show how that would look using
both of the methods described above to access the negotiator.

Media
=====

The first aspect to look at is handling 'media' negotiations. These are provided by the ``Accept`` header and
is one of the most complex headers available. A common example is the client telling the server what format it
wants the data in. This is especially common in API's. For example, a client might request JSON formatted data
from an API endpoint::

	GET /foo HTTP/1.1
	Accept: application/json

The server now needs to provide a list of what type of content it can provide. In this example, the API might
be able to return data as raw HTML, JSON, or XML. This list should be provided in order of preference::

	$supported = [
		'application/json',
		'text/html',
		'application/xml'
	];

	$format = $request->negotiate('media', $supported);
	// or
	$format = $negotiate->media($supported);

In this case, both the client and the server can agree on formatting the data as JSON so 'json' is returned from
the negotiate method. By default, if no match is found, the first element in the $supported array would be returned.
In some cases, though, you might need to enforce the format to be a strict match. If you pass ``true`` as the
final value, it will return an empty string if no match is found::

	$format = $request->negotiate('media', $supported, true);
	// or
	$format = $negotiate->media($supported, true);

Language
========

Another common usage is to determine the language the content should be served in. If you are running only a single
language site, this obviously isn't going to make much difference, but any site that can offer up multiple translations
of content will find this useful, since the browser will typically send the preferred language along in the ``Accept-Language``
header::

	GET /foo HTTP/1.1
	Accept-Language: fr; q=1.0, en; q=0.5

In this example, the browser would prefer French, with a second choice of English. If your website supports English
and German you would do something like::

	$supported = [
		'en',
		'de'
	];

	$lang = $request->negotiate('language', $supported);
	// or
	$lang = $negotiate->language($supported);

In this example, 'en' would be returned as the current language. If no match is found, it will return the first element
in the $supported array, so that should always be the preferred language.

Encoding
========

The ``Accept-Encoding`` header contains the character sets the client prefers to receive, and is used to
specify the type of compression the client supports::

	GET /foo HTTP/1.1
	Accept-Encoding: compress, gzip

Your web server will define what types of compression you can use. Some, like Apache, only support **gzip**::

	$type = $request->negotiate('encoding', ['gzip']);
	// or
	$type = $negotiate->encoding(['gzip']);

See more at `Wikipedia <https://en.wikipedia.org/wiki/HTTP_compression>`_.

Character Set
=============

The desired character set is passed through the ``Accept-Charset`` header::

	GET /foo HTTP/1.1
	Accept-Charset: utf-16, utf-8

By default, if no matches are found, **utf-8** will be returned::

	$charset = $request->negotiate('charset', ['utf-8']);
	// or
	$charset = $negotiate->charset(['utf-8']);

