Returns the original request that initiated this call.
Invokes the request immediately, and blocks until the response can be processed or is in error.
Schedules the request to be executed at some point in the future.
Cancels the request, if possible. Requests that are already complete cannot be canceled.
Returns true if this call has been either {@linkplain #execute() executed} or {@linkplain #enqueue(Callback) enqueued}. It is an error to execute a call more than once.