设计模式之十：状态模式

状态模式通过改变对象内部的状态来帮助对象控制自己的行为。

这是一张状态图，其中每个圆圈都是一个状态。

最简单，第一反应的实现就是使用一个变量来控制状态值，并在方法内书写条件代码来处理不同情况。

java 复制代码

package headfirst.designpatterns.state.gumball;

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");
		}
	}
 
	public void turnCrank() {
		if (state == SOLD) {
			System.out.println("Turning twice doesn't get you another gumball!");
		} else if (state == NO_QUARTER) {
			System.out.println("You turned but there's no quarter");
		} else if (state == SOLD_OUT) {
			System.out.println("You turned, but there are no gumballs");
		} else if (state == HAS_QUARTER) {
			System.out.println("You turned...");
			state = SOLD;
			dispense();
		}
	}
 
	private void dispense() {
		if (state == SOLD) {
			System.out.println("A gumball comes rolling out the slot");
			count = count - 1;
			if (count == 0) {
				System.out.println("Oops, out of gumballs!");
				state = SOLD_OUT;
			} else {
				state = NO_QUARTER;
			}
		} else if (state == NO_QUARTER) {
			System.out.println("You need to pay first");
		} else if (state == SOLD_OUT) {
			System.out.println("No gumball dispensed");
		} else if (state == HAS_QUARTER) {
			System.out.println("No gumball dispensed");
		}
	}
 
	public void refill(int numGumBalls) {
		this.count = numGumBalls;
		state = NO_QUARTER;
	}

	public String toString() {
		StringBuffer result = new StringBuffer();
		result.append("\nMighty Gumball, Inc.");
		result.append("\nJava-enabled Standing Gumball Model #2004\n");
		result.append("Inventory: " + count + " gumball");
		if (count != 1) {
			result.append("s");
		}
		result.append("\nMachine is ");
		if (state == SOLD_OUT) {
			result.append("sold out");
		} else if (state == NO_QUARTER) {
			result.append("waiting for quarter");
		} else if (state == HAS_QUARTER) {
			result.append("waiting for turn of crank");
		} else if (state == SOLD) {
			result.append("delivering a gumball");
		}
		result.append("\n");
		return result.toString();
	}
}

以上的代码最大的问题就是没有遵守开发-关闭原则，一遇到新的需求（投币后有10%的概率出现"赢家"状态，给出2颗糖果）就需要修改源代码，重新整理所有代码的逻辑。

重构后的代码理念：

定义一个State接口，糖果机器的每个动作都在接口中有一个对应的方法。
为机器中的每个状态实现一个状态类。这些类将负责在对应状态下进行机器的行为。
将动作委托到状态类。

java 复制代码

// 每种状态的各个方法的行为都不一样

NoQuarterState
{
    insertQuarter() // 转到HasQuarterState
    ejectQuarter()  // 未投入25分钱
    turnCrank()     // 未投入25分钱，转动曲柄无效
    dispense()      // 未投入25分钱，不能分发糖果
}

在新的糖果机中，我们不使用静态整数，而使用state对象。

java 复制代码

public class GumballMachine {
    
    // 所有的状态对象都在构造器中创建并赋值
	State soldOutState;
	State noQuarterState;
	State hasQuarterState;
	State soldState;
 
	State state;
	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;
		} else {
			state = soldOutState;
		}
	}
 
	public void insertQuarter() {
		state.insertQuarter();
	}
 
	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; its new count is: " + this.count);
		state.refill();
	}

	void setState(State state) {
		this.state = state;
	}
    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("\nMighty Gumball, Inc.");
		result.append("\nJava-enabled Standing Gumball Model #2004");
		result.append("\nInventory: " + count + " gumball");
		if (count != 1) {
			result.append("s");
		}
		result.append("\n");
		result.append("Machine is " + state + "\n");
		return result.toString();
	}
}

现在我们已经可以：

将每个状态的行为局部化到它自己的类中。
将容易产生问题的if语句删除，以方便日后的维护。
让每个状态"对修改关闭"，让糖果机"对扩展开放"（可以加入新的状态类）

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。

策略模式和状态模式的类图是一样的（回去翻了下书，好像没瞅到），但

我们把策略模式想成是除了继承之外的一种弹性替代方案。如果使用继承定义一个类的行为，则会被这个行为困住，很难修改。

状态模式是不用在context中放置许多条件判断的替代方案。通过将行为包装进状态对象中，可以通过在context内简单改变状态对象来改变context的行为。

在GumballMachine中，状态决定了下一个状态应该是什么。ConcreteState总是决定接下来的状态是什么吗？

状态转换是固定的时候，就适合放在Context中。转换是更动态的适合，通常就会放在状态类中。

java 复制代码

// GumballMachine的修改和WinnerState的实现是很简单的
// 这里就只将HasQuarterState列出

import java.util.Random;

public class HasQuarterState implements State {
	Random randomWinner = new Random(System.currentTimeMillis());
	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...");
		int winner = randomWinner.nextInt(10);
		if ((winner == 0) && (gumballMachine.getCount() > 1)) {
			gumballMachine.setState(gumballMachine.getWinnerState());
		} else {
			gumballMachine.setState(gumballMachine.getSoldState());
		}
	}

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