命令模式

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。命令模式也支持可撤销的操作。

一个命令对象通过在特定接受者上绑定一组动作来封装一个请求，要达到这一点，命令对象将动作和接受者包装进对象中。这个对象只暴露一个execute()，当此方法被调用的时候，接受者就会进行这些动作。

[]类图

[]命令模式的实现

命令接口

public interface Command {

void execute();

void undo();

}

命令实现类

public class NoCommand implements Command {

@Override
public void execute() {

}

@Override
public void undo() {

}

}

命令实现类

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();
}

}

命令实现类

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();
}

}

命令实现类

public class StereoOnWithCDCommand implements Command {

private Stereo stereo;

public StereoOnWithCDCommand(Stereo stereo) {
    this.stereo = stereo;
}

@Override
public void execute() {
    stereo.on();
    stereo.setCD();
    stereo.setVolume();
}

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

命令实现类

public class StereoOffWithCDCommand implements Command {

private Stereo stereo;

public StereoOffWithCDCommand(Stereo stereo) {
    this.stereo = stereo;
}

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

@Override
public void undo() {
    stereo.on();
    stereo.setCD();
    stereo.setVolume();
}
}

命令实现类

public class MacroCommand implements Command {

private Command[] commands;

public MacroCommand(Command[] commands) {
    this.commands = commands;
}

@Override
public void execute() {
    for (int i = 0; i < commands.length; i++) {
        commands[i].execute();
    }

}

@Override
public void undo() {

}
}

接受者类

public class Light {

public void on() {
    System.out.println("light on");
}

public void off() {
    System.out.println("light off");
}

}

接受者类

public class Stereo {

public void on() {
    System.out.println("stereo on");
}

public void setCD() {
    System.out.println("stereo setCD");
}

public void setVolume() {
    System.out.println("stereo setVolume");
}

public void off() {
    System.out.println("stereo off");
}

}

调用者类

public class RemoteControl {

private Command[] onCommands;
private Command[] offCommands;
private Command undoCommand;

public RemoteControl() {
    onCommands = new Command[7];
    offCommands = new Command[7];

    NoCommand noCommand = new NoCommand();
    for (int i = 0; i < 7; i++) {
        onCommands[i] = noCommand;
        offCommands[i] = noCommand;
    }

    undoCommand = noCommand;
}

public void setCommand(int slot, Command onCommand, Command offCommand) {
    onCommands[slot] = onCommand;
    offCommands[slot] = offCommand;
}

public void onButtonWasPressed(int slot) {
    onCommands[slot].execute();
    undoCommand = onCommands[slot];
}

public void offButtonWasPressed(int slot) {
    offCommands[slot].execute();
    undoCommand = offCommands[slot];
}

public void undoButtonWasPressed() {
    undoCommand.undo();
}

}

测试

public static void main(String args[]) {

    RemoteControl control = new RemoteControl();

    Light light = new Light();
    Stereo stereo = new Stereo();

    Command lightOnCommand = new LightOnCommand(light);
    Command lightOffCommand = new LightOffCommand(light);
    Command stereoOnCommand = new StereoOnWithCDCommand(stereo);
    Command stereoOffCommand = new StereoOffWithCDCommand(stereo);

    control.setCommand(0, lightOnCommand, lightOffCommand);
    control.setCommand(1, stereoOnCommand, stereoOffCommand);

    control.onButtonWasPressed(0);
    System.out.println("****************");
    control.offButtonWasPressed(1);
    System.out.println("****************");
    control.undoButtonWasPressed();
    System.out.println("****************");

    Command[] partyOn = {lightOnCommand, stereoOnCommand};
    Command[] partyOff = {lightOffCommand, stereoOffCommand};

    MacroCommand partyOnMacro = new MacroCommand(partyOn);
    MacroCommand partyOffMacro = new MacroCommand(partyOff);

    control.setCommand(2, partyOnMacro, partyOffMacro);

    control.onButtonWasPressed(2);
    System.out.println("****************");
    control.offButtonWasPressed(2);
}

[]
()命令模式的用途：队列请求，日程安排，线程池，工作队列等。
()命令模式将发送请求的对象和执行请求的对象解耦。
()在被解耦的两者之间是通过命令对象进行沟通的，命令对象封装了接收者和一个或者一组动作。
()调用者通过调用命令对象的execute()发出请求，这会使得接收者的动作被调用。
()命令可以支持撤销，做法是实现一个undo()来回到execute()被执行前的状态。
()宏命令是命令的一种简单延伸，允许调用多个命令，宏方法也可以支持撤销。