Choice state pattern

When modeling object lifecycles, there are often situations where there is a choice to transition to one of several states depending on the outcome of some logic. This can be modeled by using a choice pattern. In this pattern, a state is used to indicate that some evaluation logic is being performed. Typically, an action has just been called to execute the evaluation logic. The action raises an event appropriate to the outcome, which causes the transition to the desired state.

The following graphic shows this pattern.

In this example, E_SomeEvent causes the object to transition into the S_DoingSomething state, and then the A_DoSomething action is called. This action performs its function and, depending on its outcome, raises either an E_Outcome1 or an E_Outcome2 event. These events cause the object to transition to either the S_Outcome1 or S_Outcome2 state.

By convention, the name of a state in this pattern should indicate what is happening to the object while it is in this state.