设计模式之命令模式
让我们从一个简单的项目（遥控器）了解命令模式，然后再给出他的定义：
此项目地址：
实现命令接口：
首先，让所有的命令对象实现相同的包含一个方法的接口。我们暂且使用一贯的名称execute（）。
这就是命令接口（Command.java）：
1 public interface Command { 2 public void execute(); //简单，只须一个方法 execute() 3 }
实现一个打开电灯的命令：
现在，假设想实现一个打开电灯的命令。有已知我们的Light类有两个方法：on()和off()。下面是如
何将它实现成一个命令（LightOnCommand.java）：
1 //这是一个命令，所以要实现Command接口 2 public class LightOnCommand implements Command{ 3 Light light; 4 5 // 构造器被传入某个电灯（比如客厅的电灯），以便让这 6 // 个命令控制。一旦调用execute()，就有这个电灯对象 7 // 成为接收者。负责接收请求。 8 public LightOnCommand(Light light){ 9 this.light = light; 10 } 11 12 // 这个execute()方法调用接收对象（我们正在控制的电灯）的on()方法 13 public void execute(){ 14 light.on(); 15 } 16 }
现在有了LightOnCommand类，接下来让我们看一下如何使用它（SimpleRemoteController.java）：
1 public class SimpleRemoteControl { 2 //有一个插槽持有命令，而这个命令控制着一个装置 3 Command slot; 4 5 public SimpleRemoteControl(){}; 6 7 //这个方法用来设置插槽控制的命令。如果想改变遥 8 // 控器按钮的行为，可以多次调用这个方法 9 public void setCommand(Command command){ 10 slot = command; 11 } 12 13 //当按下按钮时，这个方法就会被调用，使得当前命 14 // 令衔接插槽，并调用方法execute() 15 public void buttonWasPressed(){ 16 slot.execute(); 17 } 18 }
现在让我们测试一下（Main.java）：
1 public class Main { 2 3 public static void main(String[] args) { 4 SimpleRemoteControl remote = new SimpleRemoteControl(); 5 Light light = new Light(); 6 LightOnCommand lightOn = new LightOnCommand(light); 7 8 remote.setCommand(lightOn); 9 remote.buttonWasPressed(); 10 } 11 }
结果：
好啦！通过如上例子，现在我们该给出命令模式的定义了：
命令模式：将“请求”封装成对象，以便使用不同的请求、队列或者日志来参数化其他对象。命令
模式也支持撤销的操作。
来看看他的类图：
上面完成了简单遥控，但是要知道，一个遥控器可不止有一个插槽（每个插槽都具备了“开”和“关”
按钮），所以我们现在就要用到数组，作如下工作：
1 onCommand[0] = onCommand; 2 offCommand[0] = offCommand;
实现遥控器(RemoteController.java）:
1 public class RemoteControl { 2 //这时候，遥控器要处理7个开宇关的命令，使用相应的数组记录这些命令 3 Command[] onCommands; 4 Command[] offCommands; 5 6 public RemoteControl() { 7 //在构造器中，只需实例化并初始化这两个开与关的数组 8 onCommands = new Command[7]; 9 offCommands = new Command[7]; 10 11 Command noCommand = new NoCommand(); 12 for (int i = 0; i < 7; i++) { 13 onCommands[i] = noCommand; 14 offCommands[i] = noCommand; 15 } 16 } 17 18 //这个方法需要有3个参数，分别是插槽的位置，开的命令、关的命令。这些命令将记录 19 // 在开关数组中对应的插槽位置、以供稍后使用 20 public void setCommand(int slot, Command onCommand, Command offCommand) { 21 onCommands[slot] = onCommand; 22 offCommands[slot] = offCommand; 23 } 24 25 //当按下开或关的按钮，硬件就会负责调用相应的方法，也就是onButtonWasPushed 26 // 或offButtonWasPushed 27 public void onButtonWasPushed(int slot) { 28 onCommands[slot].execute(); 29 } 30 31 public void offButtonWasPushed(int slot) { 32 offCommands[slot].execute(); 33 } 34 35 public String toString() { 36 StringBuffer stringBuff = new StringBuffer(); 37 stringBuff.append("\n------ Remote Control -------\n"); 38 for (int i = 0; i < onCommands.length; i++) { 39 stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName() 40 + " " + offCommands[i].getClass().getName() + "\n"); 41 } 42 return stringBuff.toString(); 43 } 44 }
实现命令：
LightOffCommand.java:
1 public class LightOffCommand implements Command { 2 Light light; 3 4 public LightOffCommand(Light light) { 5 this.light = light; 6 } 7 8 public void execute() { 9 light.off(); 10 } 11 }
还有很多命令就不一一列举了，原理都一样，相当于很多机械操作。
开始测试遥控器：
1 public class RemoteLoader { 2 3 public static void main(String[] args) { 4 RemoteControl remoteControl = new RemoteControl(); 5 6 Light livingRoomLight = new Light("Living Room"); 7 Light kitchenLight = new Light("Kitchen"); 8 CeilingFan ceilingFan= new CeilingFan("Living Room"); 9 GarageDoor garageDoor = new GarageDoor(""); 10 Stereo stereo = new Stereo("Living Room"); 11 12 LightOnCommand livingRoomLightOn = new LightOnCommand(livingRoomLight); 13 LightOffCommand livingRoomLightOff = new LightOffCommand(livingRoomLight); 14 LightOnCommand kitchenLightOn = new LightOnCommand(kitchenLight); 15 LightOffCommand kitchenLightOff = new LightOffCommand(kitchenLight); 16 17 CeilingFanOnCommand ceilingFanOn = new CeilingFanOnCommand(ceilingFan); 18 CeilingFanOffCommand ceilingFanOff = new CeilingFanOffCommand(ceilingFan); 19 20 GarageDoorUpCommand garageDoorUp = new GarageDoorUpCommand(garageDoor); 21 GarageDoorDownCommand garageDoorDown = new GarageDoorDownCommand(garageDoor); 22 23 StereoOnWithCDCommand stereoOnWithCD = new StereoOnWithCDCommand(stereo); 24 StereoOffCommand stereoOff = new StereoOffCommand(stereo); 25 26 remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff); 27 remoteControl.setCommand(1, kitchenLightOn, kitchenLightOff); 28 remoteControl.setCommand(2, ceilingFanOn, ceilingFanOff); 29 remoteControl.setCommand(3, stereoOnWithCD, stereoOff); 30 31 System.out.println(remoteControl); 32 33 remoteControl.onButtonWasPushed(0); 34 remoteControl.offButtonWasPushed(0); 35 remoteControl.onButtonWasPushed(1); 36 remoteControl.offButtonWasPushed(1); 37 remoteControl.onButtonWasPushed(2); 38 remoteControl.offButtonWasPushed(2); 39 remoteControl.onButtonWasPushed(3); 40 remoteControl.offButtonWasPushed(3); 41 } 42 }
注：在遥控器中，我们不想每次都检查是否某个插槽都加载了命令。比方说，在这个onButtonWasP-
ushed()方法中，我们可能需要这样的代码：
1 public void onButtonWasPushed(int slot){ 2 if(onCommands[slot] != null){ 3 onCommands[slot].execute(); 4 } 5 }
So，为了避免上述做法，我们实现一个不做事前的命令：
1 public class NoCommand implements Command { 2 public void execute() { } 3 }
这么一来，在RemoteControl构造器中，我们将个每个插槽都预先制定或NoCommand对象，以便
确定每个插槽永远都有名命令对象。
1 Command noCommand = new NoCommand(); 2 for (int i = 0; i < 7; i++) { 3 onCommands[i] = noCommand; 4 offCommands[i] = noCommand; 5 }
所以在测试的输出中，没有被明确指定命令的插槽，其命令将是默认的NoCommand对象。
第二个项目代码地址：
https://github.com/Stray-Kite/Design-Pattern/tree/master/src/headfirst/designpatterns/command/remote
还记得我们之前说的undo操作嘛？现在我们来实现它：
代码地址：https://github.com/Stray-Kite/Design-Pattern/tree/master/src/headfirst/designpatterns/command/undo
我们只需要在Command.java，我们只需加上第三行代码就好:
1 public interface Command { 2 public void execute(); 3 public void undo(); 4 }
然后再从LightOnCommand开始下手：
1 public class LightOnCommand implements Command { 2 Light light; 3 int level; 4 public LightOnCommand(Light light) { 5 this.light = light; 6 } 7 8 public void execute() { 9 level = light.getLevel(); 10 light.on(); 11 } 12 13 //加上 14 public void undo() { 15 light.dim(level); 16 } 17 }
最后是RemoteControllerWithUndo.java:
1 public class RemoteControlWithUndo { 2 Command[] onCommands; 3 Command[] offCommands; 4 Command undoCommand; 5 6 public RemoteControlWithUndo() { 7 onCommands = new Command[7]; 8 offCommands = new Command[7]; 9 10 Command noCommand = new NoCommand(); 11 for(int i=0;i<7;i++) { 12 onCommands[i] = noCommand; 13 offCommands[i] = noCommand; 14 } 15 undoCommand = noCommand; 16 } 17 18 public void setCommand(int slot, Command onCommand, Command offCommand) { 19 onCommands[slot] = onCommand; 20 offCommands[slot] = offCommand; 21 } 22 23 public void onButtonWasPushed(int slot) { 24 onCommands[slot].execute(); 25 undoCommand = onCommands[slot]; 26 } 27 28 public void offButtonWasPushed(int slot) { 29 offCommands[slot].execute(); 30 undoCommand = offCommands[slot]; 31 } 32 33 public void undoButtonWasPushed() { 34 undoCommand.undo(); 35 } 36 37 public String toString() { 38 StringBuffer stringBuff = new StringBuffer(); 39 stringBuff.append("\n------ Remote Control -------\n"); 40 for (int i = 0; i < onCommands.length; i++) { 41 stringBuff.append("[slot " + i + "] " + onCommands[i].getClass().getName() 42 + " " + offCommands[i].getClass().getName() + "\n"); 43 } 44 stringBuff.append("[undo] " + undoCommand.getClass().getName() + "\n"); 45 return stringBuff.toString(); 46 } 47 }
测试类（RemoteLoader.java）:
1 public class RemoteLoader { 2 3 public static void main(String[] args) { 4 RemoteControlWithUndo remoteControl = new RemoteControlWithUndo(); 5 6 Light livingRoomLight = new Light("Living Room"); 7 8 LightOnCommand livingRoomLightOn = new LightOnCommand(livingRoomLight); 9 LightOffCommand livingRoomLightOff = new LightOffCommand(livingRoomLight); 10 11 remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff); 12 13 remoteControl.onButtonWasPushed(0); 14 remoteControl.offButtonWasPushed(0); 15 System.out.println(remoteControl); 16 remoteControl.undoButtonWasPushed(); 17 remoteControl.offButtonWasPushed(0); 18 remoteControl.onButtonWasPushed(0); 19 System.out.println(remoteControl); 20 remoteControl.undoButtonWasPushed(); 21 22 } 23 }
好了，实现电灯的撤销很简单。但是，通常，想要实现撤销的功能，需要记录一些状态。让我们是一个更有
趣的例子，比方说厂商类中的天花板的吊扇。吊扇允许有多种转动速度，当然也允许被关闭。
吊扇源码如下：
1 public class CeilingFan { 2 public static final int HIGH = 3; 3 public static final int MEDIUM = 2; 4 public static final int LOW = 1; 5 public static final int OFF = 0; 6 String location; 7 int speed; 8 9 public CeilingFan(String location) { 10 this.location = location; 11 speed = OFF; 12 } 13 14 public void high() { 15 speed = HIGH; 16 System.out.println(location + " ceiling fan is on high"); 17 } 18 19 public void medium() { 20 speed = MEDIUM; 21 System.out.println(location + " ceiling fan is on medium"); 22 } 23 24 public void low() { 25 speed = LOW; 26 System.out.println(location + " ceiling fan is on low"); 27 } 28 29 public void off() { 30 speed = OFF; 31 System.out.println(location + " ceiling fan is off"); 32 } 33 34 public int getSpeed() { 35 return speed; 36 } 37 }
加入撤销到吊扇的命令类（CeilingFanHighCommand.java）：
1 public class CeilingFanHighCommand implements Command { 2 CeilingFan ceilingFan; 3 int prevSpeed; 4 5 public CeilingFanHighCommand(CeilingFan ceilingFan) { 6 this.ceilingFan = ceilingFan; 7 } 8 9 public void execute() { 10 prevSpeed = ceilingFan.getSpeed(); 11 ceilingFan.high(); 12 } 13 14 public void undo() { 15 if (prevSpeed == CeilingFan.HIGH) { 16 ceilingFan.high(); 17 } else if (prevSpeed == CeilingFan.MEDIUM) { 18 ceilingFan.medium(); 19 } else if (prevSpeed == CeilingFan.LOW) { 20 ceilingFan.low(); 21 } else if (prevSpeed == CeilingFan.OFF) { 22 ceilingFan.off(); 23 } 24 } 25 }
添加测试类（RemoteLoader.java）：
1 public class RemoteLoader { 2 3 public static void main(String[] args) { 4 CeilingFan ceilingFan = new CeilingFan("Living Room"); 5 6 CeilingFanMediumCommand ceilingFanMedium = new CeilingFanMediumCommand(ceilingFan); 7 CeilingFanHighCommand ceilingFanHigh = new CeilingFanHighCommand(ceilingFan); 8 CeilingFanOffCommand ceilingFanOff = new CeilingFanOffCommand(ceilingFan); 9 10 remoteControl.setCommand(0, ceilingFanMedium, ceilingFanOff); 11 remoteControl.setCommand(1, ceilingFanHigh, ceilingFanOff); 12 13 remoteControl.onButtonWasPushed(0); 14 remoteControl.offButtonWasPushed(0); 15 System.out.println(remoteControl); 16 remoteControl.undoButtonWasPushed(); 17 18 remoteControl.onButtonWasPushed(1); 19 System.out.println(remoteControl); 20 remoteControl.undoButtonWasPushed(); 21 } 22 }
最后，我们相同时弄暗灯光、打开音响和电视、设置好DVD，并让热水器开始加温：
代码地址： https://github.com/Stray-Kite/Design-Pattern/tree/master/src/headfirst/designpatterns/command/party
MacroCommand.java:
1 public class MacroCommand implements Command { 2 Command[] commands; 3 4 public MacroCommand(Command[] commands) { 5 this.commands = commands; 6 } 7 8 public void execute() { 9 for (int i = 0; i < commands.length; i++) { 10 commands[i].execute(); 11 } 12 } 13 14 public void undo() { 15 for (int i = commands.length -1; i >= 0; i--) { 16 commands[i].undo(); 17 } 18 } 19 }
使用宏命令（RemoteLoader.java）：
1 public class RemoteLoader { 2 3 public static void main(String[] args) { 4 5 RemoteControl remoteControl = new RemoteControl(); 6 7 //首先创建想要进入宏的命令集合 8 Light light = new Light("Living Room"); 9 TV tv = new TV("Living Room"); 10 Stereo stereo = new Stereo("Living Room"); 11 Hottub hottub = new Hottub(); 12 13 LightOnCommand lightOn = new LightOnCommand(light); 14 StereoOnCommand stereoOn = new StereoOnCommand(stereo); 15 TVOnCommand tvOn = new TVOnCommand(tv); 16 HottubOnCommand hottubOn = new HottubOnCommand(hottub); 17 LightOffCommand lightOff = new LightOffCommand(light); 18 StereoOffCommand stereoOff = new StereoOffCommand(stereo); 19 TVOffCommand tvOff = new TVOffCommand(tv); 20 HottubOffCommand hottubOff = new HottubOffCommand(hottub); 21 22 //接下来创建两个数组，其中一个用来记录开启命令，另一个用来记录关闭命令，并在数组内放入对应的命令 23 Command[] partyOn = { lightOn, stereoOn, tvOn, hottubOn}; 24 Command[] partyOff = { lightOff, stereoOff, tvOff, hottubOff}; 25 26 MacroCommand partyOnMacro = new MacroCommand(partyOn); 27 MacroCommand partyOffMacro = new MacroCommand(partyOff); 28 29 //然后将宏命令指定给我们所希望的按钮 30 remoteControl.setCommand(0, partyOnMacro, partyOffMacro); 31 32 //最后，按一按按钮，测试一下 33 System.out.println(remoteControl); 34 System.out.println("--- Pushing Macro On---"); 35 remoteControl.onButtonWasPushed(0); 36 System.out.println("--- Pushing Macro Off---"); 37 remoteControl.offButtonWasPushed(0); 38 } 39 }
