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<h3><a href="http://caterpillar.onlyfun.net/GossipCN/index.html">From
Gossip@caterpillar</a></h3>
<h1><a href="CppGossip.html">Design Pattern: Memento 模式</a></h1>
您希望您的程式中具有复原机制，如果您直接在物件中建立复原机制，这会使得物件本身的职责加重，并且使得物件的重用性降低。<br>
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与其在物件内建立复原机制，不如将复原机制从物件中脱离出来，这个时候您可以使用Memento模式来达成这个功能。<br>
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Memento模式在Originator中保留一个Memento成员，这个Memento可以包括Originator的成员资讯，在外部的话，
Memento可以由一个Caretaker维护，每对Originator作一个动作，Caretaker就保留Originator动作前的成员状
态，如果以后要复原物件状态，只要从Caretaker取回Memento物件，对Originator进行状态复原。<br>
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Memento模式的 UML 类别结构图如下所示： <br>
<div style="text-align: center;"><img style="width: 526px; height: 150px;" alt="Memento" title="Memento" src="images/memento-1.jpg"><br>
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<div style="text-align: left;">图中的Caretaker是用来保留原发者所创造的备忘录物件，以供日后复原时取回，state表示一个内部状态，内部状态多时，也可以将之组织为一个类别，Caretaker维护的Memento可以是多个，用来实现Redo与Undo多次的功能。<br>
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下面提供一个简单的实作，看看如何实现Memento模式：<br>
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<li> Originator.java
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<pre>public class Originator { <br> private String name; <br> private String phone;<br><br> public Originator(String name, String phone) { <br> this.name = name; <br> this.phone = phone; <br> }<br><br> // Some operations make state changed <br> public void someOperation() { <br> name = "noboby"; <br> phone = "911-911"; <br> } <br><br> // recover object's state <br> public void setMemento(Memento m) { <br> this.name = m.getName(); <br> this.phone = m.getPhone(); <br> }<br><br> public Memento createMemento() { <br> return new Memento(name, phone); <br> }<br><br> public void showInfo() { <br> System.out.println("Name: " + name + <br> "\nPhone: " + phone + "\n"); <br> } <br>} <br></pre>
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<ul>
<li> Memento.java
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</ul>
<pre>public class Memento { <br> private String name; <br> private String phone;<br><br> public Memento(String name, String phone) { <br> this.name = name; <br> this.phone = phone; <br> }<br><br> public String getName() { <br> return name; <br> }<br><br> public String getPhone() { <br> return phone; <br> }<br><br> public void setName(String name) { <br> this.name = name; <br> }<br><br> public void setPhone(String phone) { <br> this.phone = phone; <br> } <br>} <br></pre>
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<ul>
<li> Caretaker.java
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</ul>
<pre>public class Caretaker { <br> private Memento memento;<br><br> public void setMemento(Memento memento) { <br> this.memento = memento; <br> }<br><br> public Memento getMemento() { <br> return memento; <br> }<br>} <br></pre>
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<ul>
<li> Main.java
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</ul>
<pre>public class Main { <br> public static void main(String[] args) { <br> Originator originator = <br> new Originator("Justin", "888-8888"); <br> Caretaker caretaker = new Caretaker(); <br><br> // save object's memento <br> caretaker.setMemento(originator.createMemento()); <br><br> originator.showInfo(); <br> // some operations make the object's state changed <br> originator.someOperation(); <br> originator.showInfo(); <br><br> // use memento to recover object's state <br> originator.setMemento(caretaker.getMemento()); <br> originator.showInfo(); <br> } <br>} <br></pre>
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可以结合 <a class="wikilink" href="CommandPattern.htm">Command 模式</a>
来实作Redo/Undo的功能，将操作前后的物件状态记录下来，并记录所使用的命令，当要实现Undo/Redo时，只要取回Memento物件以复原
物件状态即可。<br>
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