Design Pattern 简明教程

Design Patterns - Memento Pattern

备忘录模式用于将对象的某一状态还原到以前的某个状态。备忘录模式归属于行为模式类别。

Memento pattern is used to restore state of an object to a previous state. Memento pattern falls under behavioral pattern category.

Implementation

备忘录模式使用三个参与者类。备忘录包含要还原的对象的状态。发起者创建和存储状态在备忘录对象中，看护者对象负责从备忘录还原对象的状态。我们创建了 Memento、Originator 和 CareTaker 类。

Memento pattern uses three actor classes. Memento contains state of an object to be restored. Originator creates and stores states in Memento objects and Caretaker object is responsible to restore object state from Memento. We have created classes Memento, Originator and CareTaker.

MementoPatternDemo，我们的演示类，将使用 CareTaker 和 Originator 对象来展示对象状态的还原。

MementoPatternDemo, our demo class, will use CareTaker and Originator objects to show restoration of object states.

Step 1

创建 Memento 类。

Create Memento class.

Memento.java

public class Memento {
   private String state;

   public Memento(String state){
      this.state = state;
   }

   public String getState(){
      return state;
   }
}

Step 2

创建 Originator 类

Create Originator class

Originator.java

public class Originator {
   private String state;

   public void setState(String state){
      this.state = state;
   }

   public String getState(){
      return state;
   }

   public Memento saveStateToMemento(){
      return new Memento(state);
   }

   public void getStateFromMemento(Memento memento){
      state = memento.getState();
   }
}

Step 3

创建 CareTaker 类

Create CareTaker class

CareTaker.java

import java.util.ArrayList;
import java.util.List;

public class CareTaker {
   private List<Memento> mementoList = new ArrayList<Memento>();

   public void add(Memento state){
      mementoList.add(state);
   }

   public Memento get(int index){
      return mementoList.get(index);
   }
}

Step 4

使用 CareTaker 和 Originator 对象。

Use CareTaker and Originator objects.

MementoPatternDemo.java

public class MementoPatternDemo {
   public static void main(String[] args) {

      Originator originator = new Originator();
      CareTaker careTaker = new CareTaker();

      originator.setState("State #1");
      originator.setState("State #2");
      careTaker.add(originator.saveStateToMemento());

      originator.setState("State #3");
      careTaker.add(originator.saveStateToMemento());

      originator.setState("State #4");
      System.out.println("Current State: " + originator.getState());

      originator.getStateFromMemento(careTaker.get(0));
      System.out.println("First saved State: " + originator.getState());
      originator.getStateFromMemento(careTaker.get(1));
      System.out.println("Second saved State: " + originator.getState());
   }
}

Step 5

验证输出。

Verify the output.

Current State: State #4
First saved State: State #2
Second saved State: State #3