Command Pattern 命令模式

Command Pattern: encapsulates a request as an object, thereby letting you parameterize other objects with different requests, queue or log requests, and support undoable operations.

命令模式把一个请求或者操作封装到一个对象中。命令模式允许系统使用不同的请求把客户端参数化，对请求排队或者记录请求日志，可以提供命令的撤销和恢复功能。

命令模式是对命令的封装。命令模式把发出命令的责任和执行命令的责任分割开，委派给不同的对象。每一个命令都是一个操作：请求的一方发出请求要求执行一个操作；接收的一方收到请求，并执行操作。

Client: 生成Command，指定Invoker执行Command。

Command: 含有Receiver的引用，将Receiver的方法（一些方法）封装成一个execute()方法。成为一个Command Object。在某个Client产生，在某个Invoker执行。

Receiver: 一个普通的类。

Invoker: 执行Command的类，调用execute()方法。

1. The client creates a command object.

2. The client does a setCommand() to store the command object in the invoker.

3. The client asks the invoker to execute the command.

以遥控器控制灯为例：

定义Receiver：

public class Light {
public void on() {
// light on
}
public void off() {
// light off
}
}

定义命令接口：

public interface Command {
public void execute();
public void undo();
}

定义具体命令（Command）：

public class LightOnCommand implements Command {
private Light light;
public LightOnCommand(Light light) {
this.light = light;
}

@Override
public void execute() {
light.on();
}

@Override
public void undo() {
light.off();
}
}

public class LightOffCommand implements Command {
private Light light;
public LightOffCommand(Light light) {
this.light = light;
}

@Override
public void execute() {
light.off();
}

@Override
public void undo() {
light.on();
}
}

定义Invoker：

public class RemoteControl {
private Command onCommand;
private Command offCommand;
private Command undoCommand;
public RemoteControl() {
Command noCommand = new NoCommand();
onCommand = noCommand;
offCommand = noCommand;
}
public void setCommmand(Command onCommand, Command offCommand) {
this.onCommand = onCommand;
this.offCommand = offCommand;
}
public void onButtonPushed() {
onCommand.execute();
undoCommand = onCommand;
}
public void offButtonPushed() {
offCommand.execute();
undoCommand = offCommand;
}
}

定义Client：

public class RemoteLoader {
public static void main(String[] args) {
Light light = new Light();
LightOnCommand lightON = new LightOnCommand(light);
LightOffCommand lightOFF = new LightOffCommand(light);

RemoteControl control = new RemoteControl();
control.setCommmand(lightON, lightOFF);
control.onButtonPushed();
control.offButtonPushed();
}
}

如果命令包括更多的实现细节，则可把Receiver省略，生成一个self-contain的命令，内含execute()方法，便于Invoker调用。如Computer-Space模型下的Job，Task，Result。

A null object is useful when you don't have a meaningful object to return, and yet you want to remove the responsibility for handling
null from the client. Instead of declaring a reference to a null, you can declare it to an object which does nothing.

应用：Remote Control