设计模式（18）——Memento(备忘录）

待我称王封你为后i 2022-08-08 07:01 182阅读 0赞

### 备忘录是什么？ ###

备忘录模式是在不破坏封装性的前提下捕获一个对象的内部状态，并在对象之外保存这个状态。这样以后就可以将对象回复到原先保存的状态。

![memento.gif][]

### 备忘录模式示例 ###

下面的示例中，实现了undo功能，把状态暂时保存到Memento，然后再取回来。

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));   
        }
    }

[memento.gif]: