设计模式系列

2023-06-10 18:08| 来源: 网络整理| 查看: 265

案例

假如我们接到一个研发做饭机器人的需求，需求比较简单，这个机器人只会做三件事：走路，做饭，关机。做每件事都有前提条件，只有在开机状态下才能走路，做饭和关机，类似的，在做饭状态下可以进行走路或者继续做饭。拿到需求分析后，一种简单的设计和实现是，设计一个类，此类有走路、做饭和关机的方法，在执行方法前，用 if 或者 switch 语句判断当前机器人的状态是否运行执行。这种把状态和方法耦合在一起再加上分支条件判断的方案，代码会随着需求的变化复杂度会增加而且难以维护。有没有有一种设计方案，符合开闭原则-面向新增开放，面向修改关闭的原则，当机器人增加状态时，原有代码设计不做改动，只新增类或相关代码。答案是有的，就是我们即将讲解的状态模式。

状态模式

状态模式的核心思想是把一个对象的状态和行为分开，用不同的接口或类去定义，用状态去限制该对象的行为，行为变化时相应的状态可能也发生变化。主要参与者如下图：在这里插入图片描述 Context

对外提供对象的主要行为方法维护一个具体的状态实例为当前状态

Sate

状态接口，主要来封装某种状态下的行为方法实现

ConcreteState

接口State 的实现类，代表某个具体状态的实现用状态模式实现

接口 RoboticState 封装了机器人的行为，也就是上面的提到的 State 角色

public interface RoboticState { public void walk(); public void cook(); public void off(); }

类Robot 实现了接口 RoboticState , 扮演了 Context 角色，包含了一系列状态熟悉和当前状态属性。

public class Robot implements RoboticState{ private RoboticState roboticOn; private RoboticState roboticCook; private RoboticState roboticOff; private RoboticState state; public Robot(){ this.roboticOn = new RoboticOn(this); this.roboticCook = new RoboticCook(this); this.roboticOff = new RoboticOff(this); this.state = roboticOn; } public void setRoboticState(RoboticState state){ this.state = state; } @Override public void walk() { state.walk(); } @Override public void cook() { state.cook(); } @Override public void off() { state.off(); } public RoboticState getRoboticOn() { return roboticOn; } public void setRoboticOn(RoboticState roboticOn) { this.roboticOn = roboticOn; } public RoboticState getRoboticCook() { return roboticCook; } public void setRoboticCook(RoboticState roboticCook) { this.roboticCook = roboticCook; } public RoboticState getRoboticOff() { return roboticOff; } public void setRoboticOff(RoboticState roboticOff) { this.roboticOff = roboticOff; } public RoboticState getState() { return state; } public void setState(RoboticState state) { this.state = state; } }

开机状态实现类 RoboticOn

public class RoboticOn implements RoboticState{ private final Robot robot; public RoboticOn(Robot robot){ this.robot = robot; } @Override public void walk() { System.out.println("Walking..."); } @Override public void cook() { System.out.println("Cooking..."); robot.setRoboticState(robot.getRoboticCook()); } @Override public void off() { robot.setState(robot.getRoboticOff()); System.out.println("Robot is switched off"); } }

做饭状态实现类 RoboticCook

public class RoboticCook implements RoboticState{ private final Robot robot; public RoboticCook(Robot robot){ this.robot = robot; } @Override public void walk() { System.out.println("Walking..."); robot.setRoboticState(robot.getRoboticOn()); } @Override public void cook() { System.out.println("Cooking..."); } @Override public void off() { System.out.println("Cannot switched off while cooking..."); } }

关机状态实现类 RoboticOff

public class RoboticOff implements RoboticState{ private final Robot robot; public RoboticOff(Robot robot){ this.robot = robot; } @Override public void walk() { System.out.println("Walking..."); robot.setRoboticState(robot.getRoboticOn()); } @Override public void cook() { System.out.println("Cannot cook at Off state."); } @Override public void off() { System.out.println("Already switched off..."); } }

测试代码

public class TestStatePattern { public static void main(String[] args) { Robot robot = new Robot(); robot.walk(); robot.cook(); robot.walk(); robot.off(); robot.walk(); robot.off(); robot.cook(); } }

输出结果 Walking… Cooking… Walking… Robot is switched off Walking… Robot is switched off Cannot cook at Off state.

如果此时新增一个待机的需求，该怎么办呢？新增待机状态类 RoboticStandby

public class RoboticStandby implements RoboticState{ private final Robot robot; public RoboticStandby(Robot robot){ this.robot = robot; } @Override public void walk() { System.out.println("In standby state..."); robot.setState(robot.getRoboticOn()); System.out.println("Walking..."); } @Override public void cook() { System.out.println("In standby state..."); robot.setRoboticState(robot.getRoboticCook()); System.out.println("Cooking..."); } @Override public void off() { System.out.println("In standby state..."); robot.setState(robot.getRoboticOff()); System.out.println("Robot is switched off"); } }

类 Robot 新增一个待机状态，代码如下

public class Robot implements RoboticState{ private RoboticState roboticOn; private RoboticState roboticCook; private RoboticState roboticOff; private RoboticState roboticStandby; private RoboticState state; public Robot(){ this.roboticOn = new RoboticOn(this); this.roboticCook = new RoboticCook(this); this.roboticOff = new RoboticOff(this); this.roboticStandby = new RoboticStandby(this); this.state = roboticOn; } public void setRoboticState(RoboticState state){ this.state = state; } @Override public void walk() { state.walk(); setState(getRoboticStandby()); } @Override public void cook() { state.cook(); setState(getRoboticStandby()); } @Override public void off() { state.off(); } public RoboticState getRoboticOn() { return roboticOn; } public void setRoboticOn(RoboticState roboticOn) { this.roboticOn = roboticOn; } public RoboticState getRoboticCook() { return roboticCook; } public void setRoboticCook(RoboticState roboticCook) { this.roboticCook = roboticCook; } public RoboticState getRoboticOff() { return roboticOff; } public void setRoboticOff(RoboticState roboticOff) { this.roboticOff = roboticOff; } public RoboticState getState() { return state; } public void setState(RoboticState state) { this.state = state; } public RoboticState getRoboticStandby() { return roboticStandby; } public void setRoboticStandby(RoboticState roboticStandby) { this.roboticStandby = roboticStandby; } }

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