Process for Using Asynchronous Callouts

Invoking an Asynchronous Callout in an Action Method

To invoke an asynchronous callout, call the external service by using a Continuation instance in your Visualforce action method. When you create a continuation, you can specify a timeout value and the name of the callback method. For example, the following creates a continuation with a 60-second timeout and a callback method name of processResponse.

Continuation cont = new Continuation(60);
cont.continuationMethod = 'processResponse';

Next, associate the Continuation object to an external callout. To do so, create the HTTP request, and then add this request to the continuation as follows:

String requestLabel = cont.addHttpRequest(request);

The method that invokes the callout (the action method) must return the Continuation object to instruct Visualforce to suspend the current request after the system sends the callout and waits for the callout response. The Continuation object holds the details of the callout to be executed.

This is the signature of the method that invokes the callout. The Object return type represents a Continuation.

public Object calloutActionMethodName()

Defining a Callback Method

The response is returned after the external service finishes processing the callout. You can specify a callback method for asynchronous execution after the callout returns. This callback method must be defined in the controller class where the callout invocation method is defined. You can define a callback method to process the returned response, such as retrieving the response for display on a Visualforce page.

The callback method doesn’t take any arguments and has this signature.

public Object callbackMethodName()

The Object return type represents a Continuation, a PageReference, or null. To render the original Visualforce page and finish the Visualforce request, return null in the callback method.

If the action method uses JavaScript remoting (is annotated with @RemoteAction), the callback method must be static and has the following supported signatures.

public static Object callbackMethodName(List< String> labels, Object state)

Or:

public static Object callbackMethodName(Object state)

The labels parameter is supplied by the system when it invokes the callback method and holds the labels associated with the callout requests made. The state parameter is supplied by setting the Continuation.state property in the controller.

This table lists the return values for the callback method. Each return value corresponds to a different behavior.