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。

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。

// 每种状态的各个方法的行为都不一样NoQuarterState
{insertQuarter() // 转到HasQuarterStateejectQuarter()  // 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。

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。

// 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";}
}

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