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Response Formatting

When handling a RESTful API request, an application usually takes the following steps that are related with response formatting:

  1. Determine various factors that may affect the response format, such as media type, language, version, etc. This process is also known as content negotiation.
  2. Convert resource objects into arrays, as described in the Resources section. This is done by [[yii\rest\Serializer]].
  3. Convert arrays into a string in the format as determined by the content negotiation step. This is done by [[yii\web\ResponseFormatterInterface|response formatters]] registered with the [[yii\web\Response::formatters|response]] application component.

Content Negotiation

Yii supports content negotiation via the [[yii\filters\ContentNegotiator]] filter. The RESTful API base controller class [[yii\rest\Controller]] is equipped with this filter under the name of contentNegotiator. The filter provides response format negotiation as well as language negotiation. For example, if a RESTful API request contains the following header,

Accept: application/json; q=1.0, */*; q=0.1

it will get a response in JSON format, like the following:

$ curl -i -H "Accept: application/json; q=1.0, */*; q=0.1" "http://localhost/users"

HTTP/1.1 200 OK
Date: Sun, 02 Mar 2014 05:31:43 GMT
Server: Apache/2.2.26 (Unix) DAV/2 PHP/5.4.20 mod_ssl/2.2.26 OpenSSL/0.9.8y
X-Powered-By: PHP/5.4.20
X-Pagination-Total-Count: 1000
X-Pagination-Page-Count: 50
X-Pagination-Current-Page: 1
X-Pagination-Per-Page: 20
Link: <http://localhost/users?page=1>; rel=self,
      <http://localhost/users?page=2>; rel=next,
      <http://localhost/users?page=50>; rel=last
Transfer-Encoding: chunked
Content-Type: application/json; charset=UTF-8

[
    {
        "id": 1,
        ...
    },
    {
        "id": 2,
        ...
    },
    ...
]

Behind the scene, before a RESTful API controller action is executed, the [[yii\filters\ContentNegotiator]] filter will check the Accept HTTP header in the request and set the [[yii\web\Response::format|response format]] to be 'json'. After the action is executed and returns the resulting resource object or collection, [[yii\rest\Serializer]] will convert the result into an array. And finally, [[yii\web\JsonResponseFormatter]] will serialize the array into a JSON string and include it in the response body.

By default, RESTful APIs support both JSON and XML formats. To support a new format, you should configure the [[yii\filters\ContentNegotiator::formats|formats]] property of the contentNegotiator filter like the following in your API controller classes:

use yii\web\Response;

public function behaviors()
{
    $behaviors = parent::behaviors();
    $behaviors['contentNegotiator']['formats']['text/html'] = Response::FORMAT_HTML;
    return $behaviors;
}

The keys of the formats property are the supported MIME types, while the values are the corresponding response format names which must be supported in [[yii\web\Response::formatters]].

Data Serializing

As we have described above, [[yii\rest\Serializer]] is the central piece responsible for converting resource objects or collections into arrays. It recognizes objects implementing [[yii\base\ArrayableInterface]] as well as [[yii\data\DataProviderInterface]]. The former is mainly implemented by resource objects, while the latter resource collections.

You may configure the serializer by setting the [[yii\rest\Controller::serializer]] property with a configuration array. For example, sometimes you may want to help simplify the client development work by including pagination information directly in the response body. To do so, configure the [[yii\rest\Serializer::collectionEnvelope]] property as follows:

use yii\rest\ActiveController;

class UserController extends ActiveController
{
    public $modelClass = 'app\models\User';
    public $serializer = [
        'class' => 'yii\rest\Serializer',
        'collectionEnvelope' => 'items',
    ];
}

You may then get the following response for request http://localhost/users:

HTTP/1.1 200 OK
Date: Sun, 02 Mar 2014 05:31:43 GMT
Server: Apache/2.2.26 (Unix) DAV/2 PHP/5.4.20 mod_ssl/2.2.26 OpenSSL/0.9.8y
X-Powered-By: PHP/5.4.20
X-Pagination-Total-Count: 1000
X-Pagination-Page-Count: 50
X-Pagination-Current-Page: 1
X-Pagination-Per-Page: 20
Link: <http://localhost/users?page=1>; rel=self,
      <http://localhost/users?page=2>; rel=next,
      <http://localhost/users?page=50>; rel=last
Transfer-Encoding: chunked
Content-Type: application/json; charset=UTF-8

{
    "items": [
        {
            "id": 1,
            ...
        },
        {
            "id": 2,
            ...
        },
        ...
    ],
    "_links": {
        "self": {
            "href": "http://localhost/users?page=1"
        },
        "next": {
            "href": "http://localhost/users?page=2"
        },
        "last": {
            "href": "http://localhost/users?page=50"
        }
    },
    "_meta": {
        "totalCount": 1000,
        "pageCount": 50,
        "currentPage": 1,
        "perPage": 20
    }
}