設計模式之(十七)狀態模式State

由簡單的開始會比較好理解狀態模式的作用，先來看一個例子，如果您有一個只能順時針轉動的瓦斯開關，轉動一次的狀態爲off、 small fire、medium fire與large fire，您如何在程式中控制狀態的變化與行爲呢？一個最簡單的方式就是用if..else或是switch流程來控制，例如：

• State.java

• public class State { 
      private int state;
  
      public State() { 
          state = 0; 
      } 
  
      public void switchFire() { 
          if (state == 0) { 
              state = 1; 
              System.out.println( "small fire" ); 
          } else if (state == 1) { 
              state = 2; 
              System.out.println( "medium fire" ); 
          } else if (state == 2) { 
              state = 3; 
              System.out.println( "large fire" ); 
          } else { 
              state = 0; 
              System.out.println( "turning off" ); 
          } 
      } 
  }  

• Main.java

• public class Main { 
       public static void main(String[] args) { 
          State state = new State();
  
          state.switchFire(); 
          state.switchFire(); 
          state.switchFire(); 
          state.switchFire(); 
      } 
  } 

  
  

這個方法很簡單，每個人都會，但如果您的狀態變化並不是流水式的變化，而是像TCP連線狀態一樣，會是一個網絡圖的時候，用 if...else或switch來寫的話，您的程式就會亂的不像話了；來考慮如何讓物件控制自己的狀態轉換與所應表現的行爲，這個程式可以這 樣改寫：

• IState.java

• public interface IState { 
      public void switchFire(FireSwitch sw); 
  } 

  
  

• OffState

• public class OffState implements IState { 
      public void switchFire(FireSwitch sw) { 
          sw.setState(new SmallState()); 
          System.out.println( "small fire" ); 
      } 
  } 

• SmallState.java

• public class SmallState implements IState { 
      public void switchFire(FireSwitch sw) { 
          sw.setState(new MediumState()); 
          System.out.println( "medium fire" ); 
      } 
  }  

• MediumState.java

public class MediumState implements IState { 
    public void switchFire(FireSwitch sw) { 
        sw.setState(new LargeState()); 
        System.out.println( "large fire" ); 
    } 
}  

• LargeState.java

public class LargeState implements IState { 
    public void switchFire(FireSwitch sw) { 
        sw.setState(new OffState()); 
        System.out.println( "off fire" ); 
    } 
}  

• FireSwitch.java

public class FireSwitch { 
    private State current;

    public FireSwitch() { 
        current = new OffState(); 
    }

    public void setState(State s) { 
        current = s; 
    }

    public void switchFire() { 
        current.switchFire(this); 
    }
} 

• Main.java

public class Main { 
    public static void main(String[] args) { 
        FireSwitch fireSwitch = new FireSwitch();
        fireSwitch.switchFire(); 
        fireSwitch.switchFire(); 
        fireSwitch.switchFire(); 
        fireSwitch.switchFire(); 
    } 
} 

程式執行結果與上一個例子是一樣的，但這次並沒有用流程控制來進行狀態轉換，而由物件自行控制自己的狀態，與必須表現的行爲，這個方式就是State 模式，將這個例子的 UML 類別結構畫出就如下所示：

再進一步考慮開關可以順時針與逆時針轉動，這時如果您仍以if...else或switch來寫，就會讓流程顯示覆雜，來看看如何使用狀態模式來撰 寫： 

• IState.java

public interface IState { 
    public void switchClockWise(FireSwitch sw); 
    public void switchCountClock(FireSwitch sw); 
}  

• OffState.java

public class OffState implements IState { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new SmallState()); 
        System.out.println("small fire"); 
    }

    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new LargeState()); 
        System.out.println("large fire"); 
    } 
}  

• SmallState.java

public class SmallState implements IState { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new MediumState()); 
        System.out.println("medium fire"); 
    } 

    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new OffState()); 
        System.out.println("off fire"); 
    } 
}  

• MediumState.java

public class MediumState implements IState { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new LargeState()); 
        System.out.println("large fire"); 
    }
 
    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new SmallState()); 
        System.out.println("small fire"); 
    } 
}  

• LargeState.java

public class LargeState implements State { 
    public void switchClockWise(FireSwitch sw) { 
        sw.setState(new OffState()); 
        System.out.println("off fire"); 
    }

    public void switchCountClock(FireSwitch sw) { 
        sw.setState(new MediumState()); 
        System.out.println("medium fire"); 
    } 
}  

• FireSwitch.java

public class FireSwitch { 
    private State current;

    public FireSwitch() { 
        current = new OffState(); 
    }

    public void setState(State s) { 
        current = s; 
    }

    public void switchClockWise() { 
        current.switchClockWise(this); 
    }

    public void switchCountClock() { 
       current.switchCountClock(this); 
    } 
}  

• Main.java

public class Main { 
    public static void main(String[] args) { 
        FireSwitch fireSwitch = new FireSwitch();

        fireSwitch.switchClockWise(); 
        fireSwitch.switchClockWise(); 
        fireSwitch.switchClockWise(); 
        fireSwitch.switchClockWise(); 

        System.out.println();

        fireSwitch.switchCountClock(); 
        fireSwitch.switchCountClock(); 
        fireSwitch.switchCountClock(); 
        fireSwitch.switchCountClock(); 
    } 
} 

接下來您可以任意的轉動開關了，無論是順時針轉動或是逆時針轉動，狀態的轉換都由物件自己來表現，這是雙向狀態轉換下的例子， 如果一個狀態可能轉換至三個以上的狀態，使用State模式就更可以看出它的好處了，就像Gof的TCP連線例子一樣，如果您瞭解TCP連線，可以看看原 書是如何實現TCP連線之間的狀態轉換的。 

State模式的UML結構圖如下： 

轉自http://www.riabook.cn/doc/designpattern/StatePattern.htm