设计模式学习专栏十一--------状态模式

名称: 状态模式 (State)

价值观念: 通过改变对象内部的状态来帮助对象控制自己的行为

场景

设计一个万能糖果机 , 我们希望设计尽可能有弹性 , 而且将来我们可能要为它增加更多的行为~

image

刚开始的设计方式

public class GumballMachine {
 
    final static int SOLD_OUT = 0;
    final static int NO_QUARTER = 1;
    final static int HAS_QUARTER = 2;
    final static int SOLD = 3;
 
    int state = SOLD_OUT;
    int count = 0;
  
    public GumballMachine(int count) {
        this.count = count;
        if (count > 0) {
            state = NO_QUARTER;
        }
    }
  
    public void insertQuarter() {
        if (state == HAS_QUARTER) {
            System.out.println("You can't insert another quarter");
        } else if (state == NO_QUARTER) {
            state = HAS_QUARTER;
            System.out.println("You inserted a quarter");
        } else if (state == SOLD_OUT) {
            System.out.println("You can't insert a quarter, the machine is sold out");
        } else if (state == SOLD) {
            System.out.println("Please wait, we're already giving you a gumball");
        }
    }
    
    public void ejectQuarter() {
        if (state == HAS_QUARTER) {
            System.out.println("Quarter returned");
            state = NO_QUARTER;
        } else if (state == NO_QUARTER) {
            System.out.println("You haven't inserted a quarter");
        } else if (state == SOLD) {
            System.out.println("Sorry, you already turned the crank");
        } else if (state == SOLD_OUT) {
            System.out.println("You can't eject, you haven't inserted a quarter yet");
        }
    }
    ......
}

我们的第一版设计完成了 , 发现每个动作下都需要判断当前的状态,然后做出相应的动作.

新功能引入 , 我们加入了一个新的游戏状态 , 当曲柄被转动时,有10%的几率掉下来的是两颗糖果(赢家状态)

此时我们会发现第一版设计中. 每个动作下都需要新增条件判断 , 违反了对修改关闭的原则. 程序出错概率大大提升.

如何解决

分析程序扩展时的可变部分与不变部分

	insertCoin	returnCoin	trunCrank	dispense
OnReadyState	-	-	-	-
HasCoin	-	-	-	-
SoldState	-	-	-	-
SoldOutState	-	-	-	-
WinnerState	***	***	***	***

不变部分：从横向来看。用户能执行的操作都是一样的。 (插入硬币，按下退币按钮，拉下把手）

变化部分：从横向看。如果糖果工厂新增的状态，对于用户每一种动作，糖果机的响应都是不同的。都要做出对应的修改

将可变部分抽取出来：每一种状态都会执行4种操作，糖果机具体的操作与当前状态有关。因此将状态与该状态下的对应的动作行为抽取出来形成接口。让每一个状态都实现该接口。

image

状态模式总览

定义:允许对象在内部状态在改变时改变它的行为,对象看起来好像改变了它的类
(将状态与该状态下的行为封装成独立的类,并将动作委托到代表当前状态的对象)

模式的理解
- 类图
  
  image
- 角色
  - 上下文Context
  - 封装状态及该状态下行为的 State
  - 具体的状态实习那类ConcreteState
- 细节
  - 状态模式允许一个对象基于内部状态而拥有不同的行为
  - 通过把每个状态封装进一个类, 我们把以后需要做的任何改变都局部化了 (改变这个状态下的行为)
  - 状态模式与策略模式有相同的类图 ,但是它们的意图不同
    - 策略模式通常会用行为或算法来配置Context类
    - 状态模式允许Context随着状态的改变而改变行为
  - 状态转换可以由State类(某个行为后改变状态)或者Context(外部设置状态setState)类控制.
  - 使用状态模式通常会导致设计中的类的数据大量增量(状态类)

核心代码部分

上下文

public class GumballMachine {
 
    //上下文持有不同的状态引用
  State soldOutState;     
  State noQuarterState;
  State hasQuarterState;
  State soldState;
  State winnerState;
 
  State state = soldOutState;
  int count = 0;
 
  public GumballMachine(int numberGumballs) {
      soldOutState = new SoldOutState(this);
      noQuarterState = new NoQuarterState(this);
      hasQuarterState = new HasQuarterState(this);
      soldState = new SoldState(this);
      winnerState = new WinnerState(this);

      this.count = numberGumballs;
      if (numberGumballs > 0) {
          state = noQuarterState;
      } 
  }
    //上下文提供改变状态的接口
    void setState(State state) {
      this.state = state;
  }
 
  public void insertQuarter() {
      state.insertQuarter();  //将行为委托给状态对象来处理
  }
 
  public void ejectQuarter() {
      state.ejectQuarter();
  }
 
  public void turnCrank() {
      state.turnCrank();
      state.dispense();
  }
 
  void releaseBall() {
      System.out.println("A gumball comes rolling out the slot...");
      if (count != 0) {
          count = count - 1;
      }
  }
 
  int getCount() {
      return count;
  }
 
  void refill(int count) {
      this.count += count;
      System.out.println("The gumball machine was just refilled; it's new count is: " + this.count);
      state.refill();
  }

    getter and setter...
}

状态接口

public interface State {
 
  public void insertQuarter();
  public void ejectQuarter();
  public void turnCrank();
  public void dispense();
  
  public void refill();
}

具体的状态

public class WinnerState implements State {
    GumballMachine gumballMachine;
 
    public WinnerState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
 
    public void insertQuarter() {
        System.out.println("Please wait, we're already giving you a Gumball");
    }
 
    public void ejectQuarter() {
        System.out.println("Please wait, we're already giving you a Gumball");
    }
 
    public void turnCrank() {
        System.out.println("Turning again doesn't get you another gumball!");
    }
 
    public void dispense() {
        gumballMachine.releaseBall();
        if (gumballMachine.getCount() == 0) {
            //通过上下文改变状态
            gumballMachine.setState(gumballMachine.getSoldOutState());
        } else {
            gumballMachine.releaseBall();
            System.out.println("YOU'RE A WINNER! You got two gumballs for your quarter");
            if (gumballMachine.getCount() > 0) {
                gumballMachine.setState(gumballMachine.getNoQuarterState());
            } else {
                System.out.println("Oops, out of gumballs!");
                gumballMachine.setState(gumballMachine.getSoldOutState());
            }
        }
    }
 
    public void refill() { }
    
    public String toString() {
        return "despensing two gumballs for your quarter, because YOU'RE A WINNER!";
    }
}

主程序

public class GumballMachineTestDrive {

  public static void main(String[] args) {
      GumballMachine gumballMachine =
          new GumballMachine(10);

      System.out.println(gumballMachine);

      gumballMachine.insertQuarter();
      gumballMachine.turnCrank();
      gumballMachine.insertQuarter();
      gumballMachine.turnCrank();

      System.out.println(gumballMachine);
  }
}

输出结果

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 10 gumballs
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
You inserted a quarter
You turned...
A gumball comes rolling out the slot...

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 8 gumballs
Machine is waiting for quarter

参考

书籍: HeadFirst设计模式

代码参考地址: 我就是那个地址

设计模式学习专栏十一--------状态模式

设计模式学习专栏十一--------状态模式

场景

刚开始的设计方式

如何解决

分析程序扩展时的 可变部分与不变部分

状态模式总览

核心代码部分

参考

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分析程序扩展时的可变部分与不变部分