设计模式第六集——命令模式（Command）

什么时候使用命令模式？当需要将发出请求的对象和执行请求的对象解耦的时候，使用命令模式。

命令模式：将请求封装成对象。

Client创建一个ConcreteCommend，并设置接受者。Command为所有命令生命了一个接口，调用命令对象的execute()方法，可以让接收者进行相关动作。Invoker是命令最终的调用者。

具体例子，现在设计一个遥控器，这个遥控器来控制房间内所有电器的开关。为这个遥控器编程。

以电风扇为例，RemoteLoader创建很多命令对象，然后将他们加载到遥控起的插槽中。每个命令对象都封装了某个家电自动化的一项要求。RemoteControl管理一组命令对象，每个按钮按下的时候就调用相应的onButtonWasPushed()或者offButtonWasPushed()，进而调用execute()方法被调用。

  1 package com;
  2 
  3 public interface Command {
  4     public void execute();
  5     public void undo();
  6 }
  7 
  8 package com;
  9 
 10 public class FanHighCommand implements Command {
 11     Fan fan;
 12     int prevSpeed;
 13 
 14     public FanHighCommand(Fan fan) {
 15         this.fan = fan;
 16     }
 17 
 18     public void execute() {
 19         prevSpeed = fan.getSpeed();
 20 
 21     }
 22 
 23     @Override
 24     public void undo() {
 25         if (prevSpeed == Fan.High) {
 26             fan.high();
 27         } else if (prevSpeed == Fan.Medium) {
 28             fan.medium();
 29         } else if (prevSpeed == Fan.Low) {
 30             fan.low();
 31         } else if (prevSpeed == Fan.Off) {
 32             fan.off();
 33         }
 34     }
 35 }
 36 
 37 package com;
 38 
 39 public class NoCommand implements Command{
 40 
 41     @Override
 42     public void execute() {
 43         // TODO Auto-generated method stub
 44         
 45     }
 46 
 47     @Override
 48     public void undo() {
 49         // TODO Auto-generated method stub
 50         
 51     }
 52 
 53 }
 54 
 55 package com;
 56 
 57 public class Fan {
 58     public static final int High=3;
 59     public static final int Medium=2;
 60     public static final int Low=1;
 61     public static final int Off=0;
 62     String location;
 63     int speed;
 64     
 65     public Fan(String location) {
 66         this.location = location;
 67         speed=Off;
 68     }
 69     //设置吊扇速度
 70     public void high(){
 71         speed=High;
 72     }
 73     public void medium(){
 74         speed=Medium;
 75     }
 76     public void low(){
 77         speed=Low;
 78     }
 79     public void off(){
 80         speed=Off;
 81     }
 82     //返回当前吊扇速度，为undo使用
 83     public int getSpeed(){
 84         return speed;
 85     }
 86 }
 87 
 88 
 89 package com;
 90 
 91 public class RemoteControl {
 92     Command[] onCommands;
 93     Command[] offCommands;
 94     Command undoCommand;
 95     
 96     public RemoteControl(){
 97         onCommands=new Command[3];//假设共控制三种电器
 98         offCommands=new Command[3];
 99         
100         Command noCommand=new NoCommand();
101         for(int i=0;i<3;i++){
102             onCommands[i]=noCommand;
103             offCommands[i]=noCommand;            
104         }
105         undoCommand=noCommand;
106     }
107     public void setCommand(int slot,Command onCommand,Command offCommand){
108         onCommands[slot]=onCommand;
109         offCommands[slot]=offCommand;
110     }
111     public void onButtonWasPushed(int slot){
112         onCommands[slot].execute();
113         undoCommand=onCommands[slot];
114     }
115     public void offButtonWasPushed(int slot){
116         offCommands[slot].execute();
117         undoCommand=offCommands[slot];
118     }
119     public void undoButtonWashPushed(){
120         undoCommand.undo();
121     }
122 }