备忘录模式-撤销功能的实现

在idea写代码的过程中，会经常用到一个快捷键——“crtl + z”,即撤销功能。“备忘录模式”则为撤销功能提供了一个设计方案。

1 备忘录模式

备忘录模式提供一种状态恢复机制。在不破坏封装的前提下，捕获对象内部状态并在该对象之外保存这个状态。可以在以后将对象恢复到原先保存的状态。

图备忘录UML

Originator：原发器，一个需要保存内部状态的普通类，可以创建一个备忘录，也可通过备忘录来恢复其在某个时候的状态。

Memento：备忘录，存储原发器的内部状态。除了原发器本身及管理者类外，备忘录不能直接供其他类使用。

CareTaker：管理者，负责保存备忘录，但不能对备忘录内容进行操作或检查，无须知道备忘录的细节。

@Data
public class IdeaCodeOriginator {private String code;private Integer position;public IdeaCodeOriginator(String code, Integer position) {this.code = code;this.position = position;}public void restoreMemento(IdeaCodeMemento memento) {this.code = memento.getCode();this.position = memento.getPosition();}public IdeaCodeMemento createMemento() {return new IdeaCodeMemento(code,position);}}@Data
public class IdeaCodeMemento {private String code;private Integer position;public IdeaCodeMemento(String code, Integer position) {this.code = code;this.position = position;}
}public class CareTaker {private final Stack<IdeaCodeMemento> mementoStack = new Stack<>();public IdeaCodeMemento getMemento(int num) {IdeaCodeMemento memento = null;while (num-- > 0 && !mementoStack.isEmpty()) {memento = mementoStack.pop();}return mementoStack.isEmpty() ? memento : mementoStack.pop();}public void saveMemento(IdeaCodeMemento memento) {mementoStack.push(memento);}}public class IdeaCodeEditor {public static void main(String[] args) {CareTaker careTaker = new CareTaker();IdeaCodeOriginator originator1 = new IdeaCodeOriginator("hello", 1);careTaker.saveMemento(originator1.createMemento());IdeaCodeOriginator originator2 = new IdeaCodeOriginator("word", 2);careTaker.saveMemento(originator2.createMemento());IdeaCodeOriginator originator3 = new IdeaCodeOriginator("and", 3);careTaker.saveMemento(originator3.createMemento());IdeaCodeOriginator originator4 = new IdeaCodeOriginator("java", 4);careTaker.saveMemento(originator4.createMemento());//撤回到2步前originator4.restoreMemento(careTaker.getMemento(2));System.out.println(originator4); // IdeaCodeOriginator(code=word, position=2)}}

1.1 优化

1）要避免其他类操作或者检查备忘录的内容。

2）备忘录管理者应该具有通用性。

public interface Memento {
}@Data
public class IdeaCodeOriginator {private String code;private Integer position;public IdeaCodeOriginator(String code, Integer position) {this.code = code;this.position = position;}@Dataprivate static class InnerMemento implements Memento {private String code;private Integer position;public InnerMemento(String code, Integer position) {this.code = code;this.position = position;}}public Memento createMemento() {return new InnerMemento(code,position);}public void restoreMemento(Memento memento) throws Exception {if (memento instanceof InnerMemento) {InnerMemento innerMemento = (InnerMemento) memento;this.code = innerMemento.getCode();this.position = innerMemento.position;} else {throw new Exception("类型错误");}}}public class CareTaker<T> {Stack<T> stack = new Stack<>();public T getMemento(int num) {T memento = null;while (num-- > 0 && !stack.isEmpty()) {memento = stack.pop();}return stack.isEmpty() ? memento : stack.pop();}public void saveMemento(T memento) {stack.push(memento);}}public class IdeaCodeEditor2 {public static void main(String[] args) throws Exception {CareTaker<Memento> objectCareTaker = new CareTaker<>();IdeaCodeOriginator originator1 = new IdeaCodeOriginator("hello", 1);objectCareTaker.saveMemento(originator1.createMemento());IdeaCodeOriginator originator2 = new IdeaCodeOriginator("word", 2);objectCareTaker.saveMemento(originator2.createMemento());IdeaCodeOriginator originator3 = new IdeaCodeOriginator("and", 3);objectCareTaker.saveMemento(originator3.createMemento());IdeaCodeOriginator originator4 = new IdeaCodeOriginator("java", 4);objectCareTaker.saveMemento(originator4.createMemento());originator4.restoreMemento(objectCareTaker.getMemento(3)); // IdeaCodeOriginator(code=hello, position=1)System.out.println(originator4);}}