Provides implementations of callbacks, objects that encapsulate a server request that is deferred, typically to allow a user to login or otherwise authenticate before proceeding with some other activity.

In practice, an implementation of validate(IRequestCycle) or IActionListener will create a callback, and assign it as a persistent page property of an application-specific login page. After the login completes, it can use the callback to return the user to the functionality that was deferred.

Another example use would be to collect billing and shipping information as part of an e-commerce site's checkout wizard.