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# Facebook\Helpers\FacebookCanvasHelper
The `FacebookCanvasHelper` is used to obtain an access token or signed request when working within the context of an [app canvas](https://developers.facebook.com/docs/games/canvas).
~~~
Facebook\Helpers\FacebookCanvasHelper( Facebook\FacebookApp $facebookApp )
~~~
## Usage {#usage}
If your app is loaded through Canvas, Facebook sends a POST request to your app with a signed request. This helper will handle validating and decrypting the signed request.
~~~
$fb = new Facebook\Facebook([/* */]);
$canvasHelper = $fb->getCanvasHelper();
$signedRequest = $canvasHelper->getSignedRequest();
if ($signedRequest) {
$payload = $signedRequest->getPayload();
var_dump($payload);
}
~~~
If a user has already authenticated your app, you can also obtain an access token.
~~~
$fb = new Facebook\Facebook([/* */]);
$canvasHelper = $fb->getCanvasHelper();
try {
$accessToken = $canvasHelper->getAccessToken();
} catch(Facebook\Exceptions\FacebookResponseException $e) {
// When Graph returns an error
echo 'Graph returned an error: ' . $e->getMessage();
} catch(Facebook\Exceptions\FacebookSDKException $e) {
// When validation fails or other local issues
echo 'Facebook SDK returned an error: ' . $e->getMessage();
}
if (isset($accessToken)) {
// Logged in.
}
~~~
The `$accessToken` will be `null` if the signed request did not contain any OAuth 2.0 data to obtain the access token.
## Instance Methods {#instance-methods}
### __construct() {#construct}
~~~~
public FacebookCanvasHelper __construct(FacebookApp $app, FacebookClient $client, $graphVersion = null)
~~~~
Upon instantiation, `FacebookCanvasHelper` validates and decrypts the signed request that was sent via POST if present.
### getAccessToken() {#get-access-token}
~~~
public Facebook\AccessToken|null getAccessToken()
~~~
Checks the signed request for authentication data and tries to obtain an access token access token.
### getUserId() {#get-user-id}
~~~
public string|null getUserId()
~~~
A convenience method for obtaining a user's ID from the signed request if present. This will only return the user's ID if a valid signed request can be obtained and decrypted and the user has already authorized the app.
~~~
$userId = $canvasHelper->getUserId();
if ($userId) {
// User is logged in
}
~~~
This is equivalent to accessing the user ID from the signed request entity.
~~~
$signedRequest = $canvasHelper->getSignedRequest();
if ($signedRequest) {
$userId = $signedRequest->getUserId();
// OR
$userId = $signedRequest->get('user_id');
}
~~~
### getAppData() {#get-app-data}
~~~
public string|null getAppData()
~~~
Gets the value that is set in the `app_data` property if present.
### getSignedRequest() {#get-signed-request}
~~~
public Facebook\SignedRequest|null getSignedRequest()
~~~
Returns the signed request as an instance of [`Facebook\SignedRequest`](/docs/php/SignedRequest) if present.
### getRawSignedRequest() {#get-raw-signed-request}
~~~
public string|null getRawSignedRequest()
~~~
Returns the raw encoded signed request as a `string` if present in the POST variables or `null`.