# Responses ¶

When an application finishes handling a request, it generates a response object and sends it to the end user. The response object contains information such as the HTTP status code, HTTP headers and body. The ultimate goal of Web application development is essentially to build such response objects upon various requests.

In most cases you should mainly deal with the `response` application component which is an instance of yii\web\Response, by default. However, Yii also allows you to create your own response objects and send them to end users as we will explain in the following.

In this section, we will describe how to compose and send responses to end users.

## Status Code ¶

One of the first things you would do when building a response is to state whether the request is successfully handled. This is done by setting the yii\web\Response::$statusCode property which can take one of the valid HTTP status codes. For example, to indicate the request is successfully handled, you may set the status code to be 200, like the following:

    Yii::$app->response->statusCode = 200;

However, in most cases you do not need to explicitly set the status code. This is because the default value of yii\web\Response::$statusCode is 200. And if you want to indicate the request is unsuccessful, you may throw an appropriate HTTP exception like the following:

    throw new \yii\web\NotFoundHttpException;

When the error handler catches an exception, it will extract the status code from the exception and assign it to the response. For the yii\web\NotFoundHttpException above, it is associated with the HTTP status 404. The following HTTP exceptions are predefined in Yii:

- yii\web\BadRequestHttpException: status code 400.
- yii\web\ConflictHttpException: status code 409.
- yii\web\ForbiddenHttpException: status code 403.
- yii\web\GoneHttpException: status code 410.
- yii\web\MethodNotAllowedHttpException: status code 405.
- yii\web\NotAcceptableHttpException: status code 406.
- yii\web\NotFoundHttpException: status code 404.
- yii\web\ServerErrorHttpException: status code 500.
- yii\web\TooManyRequestsHttpException: status code 429.
- yii\web\UnauthorizedHttpException: status code 401.
- yii\web\UnsupportedMediaTypeHttpException: status code 415.

If the exception that you want to throw is not among the above list, you may create one by extending from yii\web\HttpException, or directly throw it with a status code, for example,

    throw new \yii\web\HttpException(402);

## HTTP Headers ¶

You can send HTTP headers by manipulating the header collection in the `response` component. For example,

    $headers = Yii::$app->response->headers;

    // add a Pragma header. Existing Pragma headers will NOT be overwritten.
    $headers->add('Pragma', 'no-cache');

    // set a Pragma header. Any existing Pragma headers will be discarded.
    $headers->set('Pragma', 'no-cache');

    // remove Pragma header(s) and return the removed Pragma header values in an array
    $values = $headers->remove('Pragma');

> Info: Header names are case insensitive. And the newly registered headers are not sent to the user until the yii\web\Response::send() method is called.

## Response Body ¶

Most responses should have a body which gives the content that you want to show to end users.

If you already have a formatted body string, you may assign it to the yii\web\Response::$content property of the response. For example,

    Yii::$app->response->content = 'hello world!';

If your data needs to be formatted before sending it to end users, you should set both of the format and data properties. The format property specifies in which format the data should be formatted. For example,

    $response = Yii::$app->response;
    $response->format = \yii\web\Response::FORMAT_JSON;
    $response->data = ['message' => 'hello world'];

Yii supports the following formats out of the box, each implemented by a formatter class. You can customize these formatters or add new ones by configuring the yii\web\Response::$formatters property.

- HTML: implemented by yii\web\HtmlResponseFormatter.
- XML: implemented by yii\web\XmlResponseFormatter.
- JSON: implemented by yii\web\JsonResponseFormatter.
- JSONP: implemented by yii\web\JsonResponseFormatter.
- RAW: use this format if you want to send the response directly without applying any formatting.

While the response body can be set explicitly as shown above, in most cases you may set it implicitly by the return value of action methods. A common use case is like the following:

    public function actionIndex()
    {
        return $this->render('index');
    }

The `index` action above returns the rendering result of the `index` view. The return value will be taken by the `response` component, formatted and then sent to end users.

Because by default the response format is HTML, you should only return a string in an action method. If you want to use a different response format, you should set it first before returning the data. For example,

    public function actionInfo()
    {
        \Yii::$app->response->format = \yii\web\Response::FORMAT_JSON;
        return [
            'message' => 'hello world',
            'code' => 100,
        ];
    }

As aforementioned, besides using the default `response` application component, you can also create your own response objects and send them to end users. You can do so by returning such object in an action method, like the following,

    public function actionInfo()
    {
        return \Yii::createObject([
            'class' => 'yii\web\Response',
            'format' => \yii\web\Response::FORMAT_JSON,
            'data' => [
                'message' => 'hello world',
                'code' => 100,
            ],
        ]);
    }

> Note: If you are creating your own response objects, you will not be able to take advantage of the configurations that you set for the `response` component in the application configuration. You can, however, use dependency injection to apply a common configuration to your new response objects.

## Browser Redirection ¶

Browser redirection relies on sending a `Location` HTTP header. Because this feature is commonly used, Yii provides some special support for it.

You can redirect the user browser to a URL by calling the yii\web\Response::redirect() method. The method sets the appropriate `Location` header with the given URL and returns the response object itself. In an action method, you can call its shortcut version yii\web\Controller::redirect(). For example,

    public function actionOld()
    {
        return $this->redirect('http://example.com/new', 301);
    }

In the above code, the action method returns the result of the `redirect()` method. As explained before, the response object returned by an action method will be used as the response sending to end users.

In places other than an action method, you should call yii\web\Response::redirect() directly followed by a chained call to the yii\web\Response::send() method to ensure no extra content will be appended to the response.

    \Yii::$app->response->redirect('http://example.com/new', 301)->send();

> Info: By default, the yii\web\Response::redirect() method sets the response status code to be 302 which instructs the browser that the resource being requested is temporarily located in a different URI. You can pass in a status code 301 to tell the browser that the resource has been permanently relocated.

When the current request is an AJAX request, sending a `Location` header will not automatically cause the browser to redirect. To solve this problem, the yii\web\Response::redirect() method sets an `X-Redirect` header with the redirection URL as its value. On the client-side, you may write JavaScript code to read this header value and redirect the browser accordingly.

> Info: Yii comes with a `yii.js` JavaScript file which provides a set of commonly used JavaScript utilities, including browser redirection based on the `X-Redirect` header. Therefore, if you are using this JavaScript file (by registering the yii\web\YiiAsset asset bundle), you do not need to write anything to support AJAX redirection. More information about `yii.js` can be found in the Client Scripts Section.

## Sending Files ¶

Like browser redirection, file sending is another feature that relies on specific HTTP headers. Yii provides a set of methods to support various file sending needs. They all have built-in support for the HTTP range header.

- yii\web\Response::sendFile(): sends an existing file to a client.
- yii\web\Response::sendContentAsFile(): sends a text string as a file to a client.
- yii\web\Response::sendStreamAsFile(): sends an existing file stream as a file to a client.

These methods have the same method signature with the response object as the return value. If the file to be sent is very big, you should consider using yii\web\Response::sendStreamAsFile() because it is more memory efficient. The following example shows how to send a file in a controller action:

    public function actionDownload()
    {
        return \Yii::$app->response->sendFile('path/to/file.txt');
    }

If you are calling the file sending method in places other than an action method, you should also call the yii\web\Response::send() method afterwards to ensure no extra content will be appended to the response.

    \Yii::$app->response->sendFile('path/to/file.txt')->send();

Some Web servers have a special file sending support called X-Sendfile. The idea is to redirect the request for a file to the Web server which will directly serve the file. As a result, the Web application can terminate earlier while the Web server is sending the file. To use this feature, you may call the yii\web\Response::xSendFile(). The following list summarizes how to enable the `X-Sendfile` feature for some popular Web servers:

- Apache: X-Sendfile
- Lighttpd v1.4: X-LIGHTTPD-send-file
- Lighttpd v1.5: X-Sendfile
- Nginx: X-Accel-Redirect
- Cherokee: X-Sendfile and X-Accel-Redirect

## Sending Response ¶

The content in a response is not sent to the user until the yii\web\Response::send() method is called. By default, this method will be called automatically at the end of yii\base\Application::run(). You can, however, explicitly call this method to force sending out the response immediately.

The yii\web\Response::send() method takes the following steps to send out a response:

1. Trigger the yii\web\Response::EVENT_BEFORE_SEND event.
2. Call yii\web\Response::prepare() to format response data into response content.
3. Trigger the yii\web\Response::EVENT_AFTER_PREPARE event.
4. Call yii\web\Response::sendHeaders() to send out the registered HTTP headers.
5. Call yii\web\Response::sendContent() to send out the response body content.
6. Trigger the yii\web\Response::EVENT_AFTER_SEND event.

After the yii\web\Response::send() method is called once, any further call to this method will be ignored. This means once the response is sent out, you will not be able to append more content to it.

As you can see, the yii\web\Response::send() method triggers several useful events. By responding to these events, it is possible to adjust or decorate the response.