設計模式（18）——Memento(備忘錄）

備忘錄是什麼？

備忘錄模式是在不破壞封裝性的前提下捕獲一個對象的內部狀態，並在對象之外保存這個狀態。這樣以後就可以將對象回覆到原先保存的狀態。

備忘錄模式示例

下面的示例中，實現了undo功能，把狀態暫時保存到Memento，然後再取回來。

import java.util.List;
import java.util.ArrayList;
class Originator {
    private String state;
    // The class could also contain additional data that is not part of the
    // state saved in the memento..
 
    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(this.state);
    }
 
    public void restoreFromMemento(Memento memento) {
        this.state = memento.getSavedState();
        System.out.println("Originator: State after restoring from Memento: " + state);
    }
 
    public static class Memento {
        private final String state;
 
        public Memento(String stateToSave) {
            state = stateToSave;
        }
 
        public String getSavedState() {
            return state;
        }
    }
}
 
class Caretaker {
    public static void main(String[] args) {
        List<Originator.Memento> savedStates = new ArrayList<Originator.Memento>();
 
        Originator originator = new Originator();
        originator.set("State1");
        originator.set("State2");
        savedStates.add(originator.saveToMemento());
        originator.set("State3");
        // We can request multiple mementos, and choose which one to roll back to.
        savedStates.add(originator.saveToMemento());
        originator.set("State4");
 
        originator.restoreFromMemento(savedStates.get(1));   
    }
}