設計模式之狀態模式（一）

我本來覺得在對鄉村的一切事物都很容易，可是每次我一回頭就有更多變動的請求紛至沓來。我快崩潰了！----當小編讀到這段話的時候，甚是感慨，設計模式的世界真的不是那麼容易弄懂的，好在我還在學習。

基本常識：策略模式和狀態模式是雙胞胎，在出生時才分開。策略模式是圍繞能夠互換的算法來建立成功業務的。然而，狀態走的是更崇高的路，它經過改變對象內部的狀態來幫助對象控制本身的行爲。

咱們先來看下對象村的你們，此次又想出了什麼。他們發現只要在糖果機裏裝上CPU，就能夠增長銷量、經過網絡監測庫存，而且能準確地得知客戶的滿意度。全部，就有了下面的這個設想：

狀態機101

咱們如何從狀態圖獲得真正的代碼呢？下面是一個實現狀態機的簡單介紹

1. 首先，找出全部的狀態：

咱們有：沒有25分錢、有25分錢、糖果售罄、售出糖果四個狀態

1. 接下來，建立一個實例變量來持有目前的狀態，而後定義每一個狀態的值:

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;

1. 如今，咱們將全部系統中能夠發生的動做整合起來：

投入25分錢、退回25分錢、轉動曲柄、發放糖果

1. 如今，咱們建立一個類，它的做用就像是一個狀態機。對每個動做，咱們都建立了一個對應的方法，這些方法利用條件語句來決定在每一個狀態內什麼行爲是恰當的。好比對「投入25分錢」這個動做來講，咱們能夠把對應方法寫成下面的樣子:

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 old out");
    } else if (state == SOLD) {
        System.out.println("Please wait, we're already giving you a gumball");
    }
}

寫下代碼

如今咱們來實現糖果機。咱們知道要利用實例變量持有當前的狀態，而後須要處理全部可能發生的動做、行爲和狀態的轉換。咱們須要實現的動過包括：投入25分錢、退回25分錢、轉動曲柄和發放糖果；也要檢查糖果是否售罄。

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;
        }
    }
    // 當有25分錢投進來，就會執行這裏
    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");
        }
    }

    // 如今，若是顧客試着退回25分錢
    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");
        }
    }
 
    // 顧客試着轉動曲柄
    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();
        }
    }
 
    // 調用此方法，發放糖果
    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();
    }
}

如今，讓咱們進入一個小小的內部測試，看下是否符合了要求。

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) {
            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!";
    }
}

運行結果比較長，在這裏就不進行打印啦，須要的朋友請自行根據代碼進行執行。

變動需求來了，須要新設計

如今，客戶要求當曲柄轉動時，有10%的概率掉下來的是兩顆糖果（多送你一顆），那你應該怎麼作呢？

咱們的計劃是這樣的：不要維護咱們現有的代碼，咱們重寫它以便於將狀態對象封裝在各自的類中，而後在動做發生時委託給當前狀態。因此，咱們要作的事情就是：

1. 首先，咱們定義一個State接口。在這個接口中，糖果機的每一個動做都有一個對應的方法
2. 而後爲機器中的每一個狀態實現狀態類。這些將負責在對應的狀態下進行機器的行爲
3. 最後，咱們要擺脫舊的條件代碼，取而代之的方式是，將動做委託到狀態類

你將會看到，咱們不只遵照了設計原則，實際上咱們還實現了狀態模式。接下來，咱們定義狀態接口和類，以下：

好了，到如今爲止，咱們下一步確定是要去實現怎麼改造了，對吧。在這裏呢，給你們先留着，先去思考思考，如何進行改造。思路有了，類圖有了，就看你怎麼想啦。咱們下一次揭曉。

愛生活，愛學習，愛感悟，愛挨踢