Shallow History   A shallow history is a kind of pseudostate that acts as marker or placeholder in a composite state. It represents the state or condition of the modeled element at the time it was last exited. It is not a state itself but a diagrammatic representation or marker for the "condition" of the composite state down to a single level, at the time it was last exited.

Explanation

Events cause a state machine to move from one state to another. If a composite state is exited and is then returned to, it is often useful to be able to start the machine at the point or state at, which it was left. If one is in the middle of doing something, and is called away, when you return, you often want to resume your task at the point that you left it - the history state defines (or remembers) this point.

Important Points

A shallow history state can only act as a placeholder within the enclosing state.

A shallow history has got a shallow "memory". The marker will only act as a placeholder for the active substate of the enclosing state at the time it was last exited.

Related Entries

Deep History  More General A deep history is related to a shallow history in the sense that it is has the same fundamental meaning except that it is more general. While a shallow history represents the condition of a composite state when it was last exited, a deep history "remembers" the state of the composite state, and the state of this state (if it too is also a composite state), down to any level of nesting.