【设计模式】学习笔记14：状态模式（State）

基本常识: 策略模式和状态模式是双胞胎,在出生时才分开

认识状态模式

假设有一个糖果机, 它的工作状态图如下:

要用代码实现糖果机的功能, 如果不用状态模式:

一种方法是创建一个类,它的作用就是一个状态机,对每一个动作,我们都创建了一个对应的方法,这些方法用条件语句来决定在每一个状态内什么方法是最恰当的.比如对"投入25分钱"这个动作,对应的方法如下:

// 我们根据糖果机的状态,定义4种状态,用整形常量来表示
final static int SOLD_OUT= 0; // 糖果卖完了
final static int NO_QUARTER = 1; // 没有投钱
final static int HAS_QUARTER = 2;  // 已投钱了
final static int SOLD = 3; // 正在出售糖果
public void insertQurter() {
    
    // 根据不同的状态,会有不同的动作反应
    if (state == HAS_QUARTER) {
        // do something     
    } else  if (state == SOLD_OUT) {
        // do something     
    } else if (state == SOLD) {
        // do something     
    } else if (state == NO_QUARTER) {
        // do something     
    }
}

这样做的缺点:
会产生大量的if...else语句, 代码将很不容易改变, 难以拓展.

没有遵守"开放-关闭"原则

不符合面向对象

状态转换隐藏在条件语句中,所以并不明显

没有把会改变的部分包装起来

未来加入的代码可能导致bug

是时候学习新的设计模式了

定义状态模式

状态模式允许对象在内部状态改变时改变它的行为,对象看起来好像修改了它的类

因为这个模式将状态封装成为独立的类，并将动作委托到代表当前状态的对象，我们知道行为会随着内部状态而改变。

Contex（上下文）：是一个类，它可以拥有一些内部状态。

State接口：定义了一个所有具体状态的共同接口；任何状态都实现这个相同的接口，这样一来，状态之间可以互相替换

ConcreteState(具体状态): 处理来自Context的请求.每个ConcreteState都提供了它自己对于请求的实现.所以,当Context改变状态时行为也跟着改变

状态模式的类图和策略模式的基本一模一样, 但它们的"意图"不一样

状态模式允许Context随着状态的改变而改变行为.

策略模式通常会用行为或算法来配置Context类. 通常某个Context类只有一个最适当的策略.

使用状态模式通常那个会导致设计中的类数目大量增加. 因为"一个类,一个责任"原则

用状态模式实现糖果机

状态类图:

1. 实现State接口

public interface State {
 
    // 封装四种动作
	public void insertQuarter(); // 投币
	public void ejectQuarter();  // 退币
	public void turnCrank();     // 转动摇柄
	public void dispense();      // 发糖果
}

2. 实现具体的状态

// 没有投币的状态
public class NoQuarterState implements State {
    GumballMachine gumballMachine;
 
    public NoQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
 
	public void insertQuarter() {
		System.out.println("You inserted a quarter");
		gumballMachine.setState(gumballMachine.getHasQuarterState());
	}
 
	public void ejectQuarter() {
		System.out.println("You haven't inserted a quarter");
	}
 
	public void turnCrank() {
		System.out.println("You turned, but there's no quarter");
	 }
 
	public void dispense() {
		System.out.println("You need to pay first");
	} 
 
	public String toString() {
		return "waiting for quarter";
	}
}

// 已经投币的状态
public class HasQuarterState implements State {
	GumballMachine gumballMachine;
 
	public HasQuarterState(GumballMachine gumballMachine) {
		this.gumballMachine = gumballMachine;
	}
  
	public void insertQuarter() {
		System.out.println("You can't insert another quarter");
	}
 
	public void ejectQuarter() {
		System.out.println("Quarter returned");
		gumballMachine.setState(gumballMachine.getNoQuarterState());
	}
 
	public void turnCrank() {
		System.out.println("You turned...");
		gumballMachine.setState(gumballMachine.getSoldState());
	}

    public void dispense() {
        System.out.println("No gumball dispensed");
    }
 
	public String toString() {
		return "waiting for turn of crank";
	}
}

// 正在售出的状态
public class SoldState implements State {
 
    GumballMachine gumballMachine;
 
    public SoldState(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("Sorry, you already turned the crank");
	}
 
	public void turnCrank() {
		System.out.println("Turning twice doesn't get you another gumball!");
	}
 
	public void dispense() {
		gumballMachine.releaseBall();
		if (gumballMachine.getCount() > 0) {
			gumballMachine.setState(gumballMachine.getNoQuarterState());
		} else {
			System.out.println("Oops, out of gumballs!");
			gumballMachine.setState(gumballMachine.getSoldOutState());
		}
	}
 
	public String toString() {
		return "dispensing a gumball";
	}
}

.....

3. 实现Context

public class GumballMachine {
 
	State soldOutState;
	State noQuarterState;
	State hasQuarterState;
	State soldState;
 
	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);

		this.count = numberGumballs;
 		if (numberGumballs > 0) {
			state = noQuarterState;
		} 
	}
 
	public void insertQuarter() {
		state.insertQuarter();
	}
 
	public void ejectQuarter() {
		state.ejectQuarter();
	}
 
	public void turnCrank() {
		state.turnCrank();
		state.dispense();
	}

	void setState(State state) {
		this.state = state;
	}
 
	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;
		state = noQuarterState;
	}

    public State getState() {
        return state;
    }

    public State getSoldOutState() {
        return soldOutState;
    }

    public State getNoQuarterState() {
        return noQuarterState;
    }

    public State getHasQuarterState() {
        return hasQuarterState;
    }

    public State getSoldState() {
        return soldState;
    }
 
	public String toString() {
		StringBuffer result = new StringBuffer();
		result.append("
Mighty Gumball, Inc.");
		result.append("
Java-enabled Standing Gumball Model #2004");
		result.append("
Inventory: " + count + " gumball");
		if (count != 1) {
			result.append("s");
		}
		result.append("
");
		result.append("Machine is " + state + "
");
		return result.toString();
	}
}

4. 测试类

public class GumballMachineTestDrive {

	public static void main(String[] args) {
		GumballMachine gumballMachine = new GumballMachine(5);

		System.out.println(gumballMachine);

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

		System.out.println(gumballMachine);

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

		System.out.println(gumballMachine);
	}
}

这样做的优点:

1. 将每个状态的行为局部化到自己的类中

2. 将容易产生问题的if语句删除,以方便日后的维护

3. 让每个状态"对修改关闭",让糖果机"对拓展开放",因为可以加入新的状态类

4. 创建一个新的代码基和类结构,这更能映射糖果的图,而且容易阅读和理解