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<title>Memento 模式</title>
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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復原機制，如果您直接在物件中建立復原機制，這會使得物件本身的職責加重，并且使得物件的重用性降低。<br>
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與其在物件內建立復原機制，不如將復原機制從物件中脫離出來，這個時候您可以使用Memento模式來達成這個功能。<br>
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Memento模式在Originator中保留一個Memento成員，這個Memento可以包括Originator的成員資訊，在外部的話，
Memento可以由一個Caretaker維護，每對Originator作一個動作，Caretaker就保留Originator動作前的成員狀
態，如果以后要復原物件狀態，只要從Caretaker取回Memento物件，對Originator進行狀態復原。<br>
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Memento模式的 UML 類別結構圖如下所示： <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;">圖中的Caretaker是用來保留原發者所創造的備忘錄物件，以供日后復原時取回，state表示一個內部狀態，內部狀態多時，也可以將之組織為一個類別，Caretaker維護的Memento可以是多個，用來實現Redo與Undo多次的功能。<br>
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下面提供一個簡單的實作，看看如何實現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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可以結合 <a class="wikilink" href="CommandPattern.htm">Command 模式</a>
來實作Redo/Undo的功能，將操作前后的物件狀態記錄下來，并記錄所使用的命令，當要實現Undo/Redo時，只要取回Memento物件以復原
物件狀態即可。<br>
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