History States

Sometimes leaving a compound state and coming back should resume where you were, not restart. A history pseudo-state records a compound’s last active configuration so a transition targeting it re-enters that configuration instead of the compound’s initial child.

Two flavors:

• Shallow (HistoryShallow) restores the compound’s last active direct child.
• Deep (HistoryDeep) restores the full nested leaf configuration, all the way down.

Declare a history pseudo-state inside a SuperState block with History, and give it a DefaultTo fallback for the first entry, before any history exists:

b.SuperState(Editing).
    History(EditingHistory, state.HistoryDeep).
    DefaultTo(Composing).
    SubState(Composing).On(Format).GoTo(Formatting).
    SubState(Formatting).On(Done).GoTo(Composing).
    EndSuperState()

// A transition that resumes the compound where it left off:
b.Transition(Previewing).On(Resume).GoTo(EditingHistory)

A history pseudo-state is structure, not a real child: it never counts toward the compound’s substates and is never eligible as the initial state. The kernel resolves it at transition time: if the compound was last in Formatting, Resume lands you back in Formatting, not Composing.

stateDiagram-v2
    [*] --> Editing
    state Editing {
        [*] --> Composing
        Composing --> Formatting: Format
        Formatting --> Composing: Done
    }
    Editing --> Previewing: Preview
    Previewing --> Editing: Resume (via H*)
    note right of Editing
        H* resumes the last
        active configuration
    end note

History is captured automatically on exit, so it survives snapshots too: restore re-arms exactly the configuration a Resume would have rejoined.