18. Using StateContext

18. Using StateContext
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StateContext is a one of a most important objects when working with a state machine as it is passed into various methods and callbacks to give status of a current state of a state machine and where it is possibly going. If simplifying things a little it can be considered to be a snapshot of a current state machine stage where it is at a time StateContext is passed on.

	Note
	In `Spring Statemachine 1.0.x` StateContext usage were relatively naive in terms of how it was used to just pass stuff around as a simple `POJO`. Starting from `Spring Statemachine 1.1.x` its role has been greatly improved by making it a first class citizen in a state machine.

In overall StateContext can be used as.

Access to current Message, Event or their MessageHeaders if known.
Access to state machine Extended State.
Access to StateMachine itself.
Access to possible state machine error.
Access to current Transition if applicable.
Access to source and target states where state machine is possibly getting from and going to.
Access to current Stage as described in Section 18.1, “Stages”.

StateContext is passed into various components interacting with user like Action and Guard.

18.1 Stages

Stage is representation of a stage on which a state machine is currently interacting with a user. Current stages are EVENT_NOT_ACCEPTED, EXTENDED_STATE_CHANGED, STATE_CHANGED, STATE_ENTRY, STATE_EXIT, STATEMACHINE_ERROR, STATEMACHINE_START, STATEMACHINE_STOP, TRANSITION, TRANSITION_START and TRANSITION_END which look very familiar as those match how user can interact with listeners as described in Chapter 20, Listening State Machine Events.

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