A reusable organization of responsibilities and interactions for a recurring software-design problem. The Memento pattern separates snapshot creation/restoration from history management, while the Decorator pattern composes independent responsibilities behind one interface. A pattern specifies an arrangement rather than one mandatory class naming scheme.
Represent a request and the information needed to execute it as a software object with a common command interface. Undoable commands additionally retain enough information to invert their change: an insertion retains its position and value, a deletion retains the removed value, and a positional move retains the source and final destination indices.
For sequential undo/redo, a history manager stores applied commands on one stack and undone commands on another. Undo transfers the most recent applied command to the redo stack; redo transfers its head back after reapplying it. A fresh edit clears the redo branch. Command inverses are valid under this chronological protocol, not generally after unrelated intervening edits. Saving complete immutable versions instead gives a Memento pattern implementation of the same undo/redo interface.
Register consumers through an observation interface so that a producer can notify interested consumers without depending on all of their concrete classes. Registration can change independently of the producer's core state changes. Notifications should expose coherent states, and unregistration or update discipline is still needed to avoid stale subscriptions or notification cycles.
Expose sequential consumption through availability and next-element operations while hiding storage and traversal details. The abstraction supports common clients for arrays, linked structures or generated sequences. A separate iterator can own separate cursor state; an alias of the same iterator does not create an independent traversal.
An adapter presents an object interface expected by clients while delegating to a component with a different object interface. Correct adaptation preserves the meaning and order of operations, not merely object method names. Adapting a combined update-and-result operation to separate update and query operations requires storing the last result and specifying its initial value. An adapter should not perform a hidden dummy update just to manufacture that initial observation.
