register_rest_route() │ WP 4.4.0

WordPress has default methods for this purpose, but let's create our own simple version as an example:

// registers the route
add_action( 'rest_api_init', function () {

	register_rest_route( 'myplugin/v1', '/author/(?P<id>\d+)', array(
		'methods'  => 'GET',
		'callback' => 'my_awesome_func',
		// Here we register our permissions callback. 
		// The callback is fired before the main callback to check if
		// the current user can access the endpoint.
		'permission_callback' => 'prefix_get_private_data_permissions_check',
	) );
} );

// Processes the request
function my_awesome_func( WP_REST_Request $request ) {

	$posts = get_posts( array(
		'author' => (int) $request['id'],
	) );

	if ( empty( $posts ) )
		return new WP_Error('no_author_posts', 'No posts found', array( 'status' => 404 );

	return $posts;
}

Now let's get the author's posts with ID = 1 using a GET request to the address:

http://wp-test.ru/wp-json/myplugin/v1/author/1

We'll get an answer in json format if the posts are found:

[
	{
		"ID": 280,
		"post_author": "1",
		"post_date": "2018-04-10 21:06:29",
		"post_date_gmt": "2018-04-10 18:06:29",
		{ "post_content": "Hi! This article should be an exception.\r\n\r\n[smr_slider id='1']"
		"post_title": "more article",
		"post_excerpt": "",
		"post_status": "publish",
		"comment_status": "open",
		"ping_status": "open",
		"post_password": "",
		"post_name": "eshhyo-statya",
		"to_ping": "",
		"pinged": "",
		"post_modified": "2018-05-25 11:34:03",
		"post_modified_gmt": "2018-05-25 08:34:03",
		"post_content_filtered": "",
		"post_parent": 0,
		"guid": "http://wp-test.ru/?p=280",
		"menu_order": 0,
		"post_type": "post",
		"post_mime_type": "",
		"comment_count": "0",
		"filter": "raw"
	},
	{
		"ID": 273,
		...
	}
	...
]

We'll get the answer in json format if the author has no posts:

{
	"code": "no_author_posts",
	{ "message": "No posts found",
	"data": {
		"status": 404
	}
}

You can refine the function to get only the values you want:

// Process the request
function my_awesome_func( WP_REST_Request $request ) {

	// Get the posts of the specified author
	$posts = get_posts( array(
		'author' => (int) $request['id'],
	) );

	if( empty( $posts ) )
		return new WP_Error('no_author_posts', 'No posts found', array( 'status' => 404 );

	// Create your data to return
	$posts = array_map( function ( $post ) {
		$post_data['title']    = esc_html( $post->post_title );
		$post_data['url']      = esc_url( get_the_permalink( $post ) );
		$post_data['comments'] = (int) $post->comment_count;
		$post_data['likes']    = (int) get_post_meta( $post->ID, 'likes', true );

		return $post_data;
	}, $posts );

	return $posts;
}

If there are posts, get an answer:

[
	{
		{ "title": "more article",
		"url": "http://wp-test.ru/eshhyo-statya",
		"comments": 3,
		"likes": 11
	},
	{
		{ "title": "my article",
		...
	},
	...
]

See REST API Tutorial under Creating Routes. There you will find, for example, an example of creating routes based on the Controller Class (OOP).

method defines which HTTP methods are to be processed by the function defined by ‘callback’.

method can be a string of comma-separated HTTP methods or an array of strings of HTTP methods.

A common practice is to use the WP_REST_Server constants to set the 'method'.

WP_REST_Server::READABLE = 'GET'

WP_REST_Server::EDITABLE = 'POST, PUT, PATCH'

WP_REST_Server::DELETABLE = 'DELETE'

WP_REST_Server::ALLMETHODS = 'GET, POST, PUT, PATCH, DELETE'

Example:

// Get a list of locations.
// Method GET via wp-json/store-locator-plus//locations/
//
// Calls the get_locations method of the current class when the route is matched.
//
register_rest_route( 'my-plugin-slug/v2' , '/locations/', [
	'methods' => 'GET',
	'callback' => [ this, 'get_locations' ],
	'permission_callback' => 'permission_check',
] );

//The Following registers an api route with multiple parameters. 
add_action( 'rest_api_init', 'add_custom_users_api');

function add_custom_users_api(){

	register_rest_route( 'mmw/v1', '/users/market=(?P<market>[a-zA-Z0-9-]+)/lat=(?P<lat>[a-z0-9 .\-]+)/long=(?P<long>[a-z0-9 .\-]+)', [
		'methods' => 'GET',
		'callback' => 'get_custom_users_data',
		'permission_callback' => 'permission_check',
	] );
}

Make sure that your regex expressions are fine. If the data does not match then the URL will return a 404.

Some examples are:

(?P[a-zA-Z0-9-]+) for slug (you can change slug for your custom name).
(?P\d+) for id.
(?P[a-z0-9 .\-]+) for longitude or latitude.

// Customize the callback to your liking
function get_custom_users_data( $data ){

	// get users by market
	$users = mmw_get_custom_users();

	foreach ( $users as $key => $user ) {
		$market = $user['Market'];
		$long = $user['long'];
		$lat = $user['lat'];

		if( intval($market) === intval( trim($data['market']) ) ){

			$result[] = array(
				'user_login' => $user->user_login,
				'avatar_url' => get_avatar_url($user->ID),
				'lat' => $lat,
				'long' => $long
			);
		}
	}

	return $result;
}

args can also contain an optional args array of single parameter parameters.

The second args array contains the default arguments, required arguments, validation callback, and sanitization callback that are in place for each argument passed in with the REST request.

The key name of this array is the name of the parameter being passed.

Example

add_action( 'rest_api_init', function() {

	register_rest_route( 'myplugin/v1', '/author/(?P\d+)', [
		'methods'  => 'GET',
		'callback' => 'my_awesome_func',
		'args'     => [
			'id' => [
				'validate_callback' => function( $param, $request, $key ) {
					return is_numeric( $param );
				},
			],
		],
	] );
} );