Recap: State Pattern

One class per state, the context delegates, and one agreed place decides the next state.

State replaces a status field and its repeated checks with one class per state.

Intent. Let an object change its behaviour when its internal state changes, as if it changed its class.
Structure. The context holds the current state and passes each request to it; each concrete state handles requests for one state.
From a diagram. Each box is a class, and each arrow is a line in the class it leaves.
Who decides. Usually the state classes, sometimes the context, never the client, and always one of them for the whole design.
Against Strategy. The structure is the same; a strategy is chosen by the client, while states replace one another.
The trap. Transitions split between context and states, so a rule runs on an arrow the object never took.
When not. Two or three states with little behaviour, or transitions that are data.

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