设计模式-命令模式（Command）

命令模式的目的

将服务请求封装成对象，从而可以不同的方式对请求进行操作。

它使client可以调用命令但不需要了解命令的执行细节。并且可以对命令进行修改而无需影响调用命令的client程序。

命令模式的结构

Command

声明执行操作的接口

ConcreteCommand

定义Receiver对象和动作的绑定

通过调用Receiver的相应操作实现Execute（）方法

Client

创建ConcreteCommand对象从而设置相应的receiver

Invoker

要求命令执行用户请求

Receiver

负责解析用户的请求并执行相应的操作

命令模式的类图

下面看一个例子（例子来源：https://github.com/iluwatar/java-design-patterns）

男巫(wizard)施加命令(command)给小怪。二具体的命令(invisibility spell,shrink spell)是让小怪（target）不可见或者变大或变小及取消施加命令和重新施加。

调用者

public class Wizard {

  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);

  private Deque<Command> undoStack = new LinkedList<>();
  private Deque<Command> redoStack = new LinkedList<>();

  public Wizard() {
    // comment to ignore sonar issue: LEVEL critical
  }

  /**
   * Cast spell
   */
  public void castSpell(Command command, Target target) {
    LOGGER.info("{} casts {} at {}", this, command, target);
    command.execute(target);
    undoStack.offerLast(command);
  }

  /**
   * Undo last spell
   */
  public void undoLastSpell() {
    if (!undoStack.isEmpty()) {
      Command previousSpell = undoStack.pollLast();
      redoStack.offerLast(previousSpell);
      LOGGER.info("{} undoes {}", this, previousSpell);
      previousSpell.undo();
    }
  }

  /**
   * Redo last spell
   */
  public void redoLastSpell() {
    if (!redoStack.isEmpty()) {
      Command previousSpell = redoStack.pollLast();
      undoStack.offerLast(previousSpell);
      LOGGER.info("{} redoes {}", this, previousSpell);
      previousSpell.redo();
    }
  }

  @Override
  public String toString() {
    return "Wizard";
  }

抽象的命令类

public abstract class Command {

  public abstract void execute(Target target);

  public abstract void undo();

  public abstract void redo();

  @Override
  public abstract String toString();

}

命令InvisibilitySpell 隐身

public class InvisibilitySpell extends Command {

  private Target target;

  @Override
  public void execute(Target target) {
    target.setVisibility(Visibility.INVISIBLE);
    this.target = target;
  }

  @Override
  public void undo() {
    if (target != null) {
      target.setVisibility(Visibility.VISIBLE);
    }
  }

  @Override
  public void redo() {
    if (target != null) {
      target.setVisibility(Visibility.INVISIBLE);
    }
  }

  @Override
  public String toString() {
    return "Invisibility spell";
  }
}

命令类ShrinkSpell 变小，收缩

public class ShrinkSpell extends Command {

  private Size oldSize;
  private Target target;

  @Override
  public void execute(Target target) {
    oldSize = target.getSize();
    target.setSize(Size.SMALL);
    this.target = target;
  }

  @Override
  public void undo() {
    if (oldSize != null && target != null) {
      Size temp = target.getSize();
      target.setSize(oldSize);
      oldSize = temp;
    }
  }

  @Override
  public void redo() {
    undo();
  }

  @Override
  public String toString() {
    return "Shrink spell";
  }
}

public abstract class Target {

  private static final Logger LOGGER = LoggerFactory.getLogger(Target.class);

  private Size size;

  private Visibility visibility;

  public Size getSize() {
    return size;
  }

  public void setSize(Size size) {
    this.size = size;
  }

  public Visibility getVisibility() {
    return visibility;
  }

  public void setVisibility(Visibility visibility) {
    this.visibility = visibility;
  }

  @Override
  public abstract String toString();

  /**
   * Print status
   */
  public void printStatus() {
    LOGGER.info("{}, [size={}] [visibility={}]", this, getSize(), getVisibility());
  }
}

小怪Goblin ，被施加命令的对象

public class Goblin extends Target {

  public Goblin() {
    setSize(Size.NORMAL);
    setVisibility(Visibility.VISIBLE);
  }

  @Override
  public String toString() {
    return "Goblin";
  }

}

客户端调用通过施加不同的命令，取消命令或重新施加命令，然后打印目标的状态查看命令是否施加成功。

public class App {

  /**
   * Program entry point
   * 
   * @param args command line args
   */
  public static void main(String[] args) {
    Wizard wizard = new Wizard();
    Goblin goblin = new Goblin();

    goblin.printStatus();

    wizard.castSpell(new ShrinkSpell(), goblin);
    goblin.printStatus();

    wizard.castSpell(new InvisibilitySpell(), goblin);
    goblin.printStatus();

    wizard.undoLastSpell();
    goblin.printStatus();

    wizard.undoLastSpell();
    goblin.printStatus();

    wizard.redoLastSpell();
    goblin.printStatus();

    wizard.redoLastSpell();
    goblin.printStatus();
  }
}

命令模式总结