Waiting state pattern

When modeling object lifecycles, there are often situations where the process cannot continue until some separate, but related, activity completes. This can be modeled by using a waiting pattern. In this pattern, a state is used to indicate that it is waiting for something to occur.

The following figure shows this pattern.

The example shows a situation where a payment instruction is received and forwarded to a payment gateway. The payment object is placed in the S_WaitingForAck state. At some later stage, an acknowledgment is returned from the gateway and some other component raises the E_PaymentAcknowledged event. This event causes the FSM to transition to completion.

By convention, the name of a state in this pattern should indicate that the object is waiting for something to happen.