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The memento pattern is used by two objects: the ''originator'' and a ''caretaker''. The originator is some object that has an internal state. The caretaker is going to do something to the originator, but wants to be able to undo the change. The caretaker first asks the originator for a memento object. Then it does whatever operation (or sequence of operations) it was going to do. To rollback to the state before the operations, it returns the memento object to the originator. The memento object itself is an Opaque Object (one which the caretaker can not, or should not, change). When using this pattern, care should be taken if the originator may change other objects or resources - the memento pattern operates on a single object. Classic examples of the memento pattern include the seed of a Pseudorandom Number Generator and the state in a Finite State Machine . EXAMPLE Java The following Java programm illustrates the "undo" usage of the Memento Pattern. The output is: Originator: Setting state to State1 Originator: Setting state to State2 Originator: Saving to Memento. Originator: Setting state to State3 Originator: Saving to Memento. Originator: Setting state to State4 Originator: State after restoring from Memento: State3
class Memento { private String state; public Memento(String stateToSave) { state = stateToSave; } public String getSavedState() { return state; } } class Originator { private String state;
public void set(String state) { System.out.println("Originator: Setting state to "+state); this.state = state; } public Memento saveToMemento() { System.out.println("Originator: Saving to Memento."); return new Memento(state); } public void restoreFromMemento(Memento m) { state = m.getSavedState(); System.out.println("Originator: State after restoring from Memento: "+state); } } class Caretaker { private ArrayList public void addMemento(Memento m) { savedStates.add(m); } public Memento getMemento(int index) { return savedStates.get(index); } } class MementoExample { public static void main(String {Link without Title} args) { Caretaker caretaker = new Caretaker(); Originator originator = new Originator(); originator.set("State1"); originator.set("State2"); caretaker.addMemento( originator.saveToMemento() ); originator.set("State3"); caretaker.addMemento( originator.saveToMemento() ); originator.set("State4"); originator.restoreFromMemento( caretaker.getMemento(1) ); } } SEE ALSO EXTERNAL LINKS |
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