python設計模式-狀態模式

問題：有一個糖果公司須要設計一個糖果售賣機，控制流程以下圖，須要怎麼實現？

這是一個狀態圖，每一個圓圈都是一種狀態。很明顯，有有25分錢、 沒有25分錢、 售出糖果、 糖果售罄四個狀態，同時也對應四個動做：投入25分錢，退回25分錢，轉動曲柄和發放糖果。

那如何從狀態圖獲得真正的代碼呢？

簡單代碼實現以下：

#! -*- coding: utf-8 -*-

class GumballMachine:

    # 找出全部狀態，並建立實例變量來持有當前狀態，而後定義狀態的值
    STATE_SOLD_OUT = 0
    STATE_NO_QUARTER = 1
    STATE_HAS_QUARTER = 2
    STATE_SOLD = 3

    state = STATE_SOLD_OUT

    def __init__(self, count=0):
        self.count = count
        if count > 0:
            self.state = self.STATE_NO_QUARTER

    def __str__(self):
        return "Gumball machine current state: %s" % self.state

    def insert_quarter(self):
        # 投入25分錢
        if self.state == self.STATE_HAS_QUARTER: # 若是已經投過
            print("You can't insert another quarter")
        elif self.state == self.STATE_NO_QUARTER: # 若是沒有投過
            self.state = self.STATE_HAS_QUARTER
            print("You inserted a quarter")
        elif self.state == self.STATE_SOLD_OUT: # 若是已經售罄
            print("You can't insert a quarter, the machine is sold out")
        elif self.state == self.STATE_SOLD: # 若是剛剛買了糖果
            print("Please wait, we're already giving you a gumball")

    def eject_quarter(self):
        # 退回25分
        if self.state == self.STATE_HAS_QUARTER:
            print("Quarter returned")
            self.state = self.STATE_NO_QUARTER
        elif self.state == self.STATE_NO_QUARTER:
            print("You haven't inserted a quarter")
        elif self.state == self.STATE_SOLD:
            print("Sorry, you alread turned the crank")
        elif self.state == self.SOLD_OUT:
            print("You can't eject, you haven't inserted")

    def turn_crank(self):
        # 轉動曲柄
        if self.state == self.STATE_SOLD:
            print("Turning twice doesn't get you another gumball")
        elif self.state == self.STATE_NO_QUARTER:
            print("You turned but there's no quarter")
        elif self.state == self.STATE_SOLD_OUT:
            print("You turned, but there are no gumballs")
        elif self.state == self.STATE_HAS_QUARTER:
            print("You turned...")
            self.state = self.STATE_SOLD
            self.dispense()
    
    def dispense(self):
        # 發放糖果
        if self.state == self.STATE_SOLD:
            print("A gumball comes rolling out the slot")
            self.count -= 1
            if self.count == 0:
                self.state = self.STATE_SOLD_OUT
            else:
                self.state = self.STATE_NO_QUARTER
        elif self.state == self.STATE_NO_QUARTER:
            print("You need to pay first")
        elif self.state == self.STATE_SOLD_OUT:
            print("No gumball dispensed")
        elif self.state == self.STATE_HAS_QUARTER:
            print("No gumball dispensed")


if __name__ == "__main__":
    # 如下是代碼測試
    gumball_machine = GumballMachine(5) # 裝入5 個糖果
    print(gumball_machine)

    gumball_machine.insert_quarter() # 投入25分錢
    gumball_machine.turn_crank() # 轉動曲柄
    print(gumball_machine)

    gumball_machine.insert_quarter() #投入25分錢
    gumball_machine.eject_quarter()  # 退錢
    gumball_machine.turn_crank()     # 轉動曲柄

    print(gumball_machine)
    
    gumball_machine.insert_quarter() # 投入25分錢
    gumball_machine.turn_crank() # 轉動曲柄 
    gumball_machine.insert_quarter() # 投入25分錢 
    gumball_machine.turn_crank()  # 轉動曲柄
    gumball_machine.eject_quarter() # 退錢

    print(gumball_machine)

這段代碼有幾個問題：

1. 沒有遵照開放-關閉原則
2. 更像是面向過程的設計
3. 狀態轉化被埋藏在條件語句中
4. 將來加入新的需求，須要改動的較多，不易維護，可能會出bug

如何改進呢？

考慮 封裝變化，把每一個狀態的行爲都放在各自的類中，每一個狀態只要實現本身的動做，用加入新類的方式來實現新狀態的加入。

1. 定義State 父類，在這個類中，糖果機的每一個動做都有一個應對的方法
2. 爲機器中的每一個狀態實現狀態類，這些類將負責在對應的狀態下進行機器的行爲
3. 擺脫舊的條件代碼，將動做委託到狀態類

新的實現代碼以下：

#! -*- coding: utf-8 -*-

class State:
    # 定義state基類
    def insert_quarter(self):
        pass

    def eject_quarter(self):
        pass

    def turn_crank(self):
        pass

    def dispense(self):
        pass


class SoldOutState(State):
    # 繼承State 類
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine

    def __str__(self):
        return "sold_out"

    def insert_quarter(self):
        print("You can't insert a quarter, the machine is sold out")

    def eject_quarter(self):
        print("You can't eject, you haven't inserted a quarter yet")

    def turn_crank(self):
        print("You turned, but ther are no gumballs")

    def dispense(self):
        print("No gumball dispensed")


class SoldState(State):
    # 繼承State 類
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine

    def __str__(self):
        return "sold"

    def insert_quarter(self):
        print("Please wait, we're already giving you a gumball")

    def eject_quarter(self):
        print("Sorry, you already turned the crank")

    def turn_crank(self):
        print("Turning twice doesn't get you another gumball")

    def dispense(self):
        self.gumball_machine.release_ball()
        if gumball_machine.count > 0:
            self.gumball_machine.state = self.gumball_machine.no_quarter_state
        else:
            print("Oops, out of gumballs!")
            self.gumball_machine.state = self.gumball_machine.soldout_state


class NoQuarterState(State):
    # 繼承State 類
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine

    def __str__(self):
        return "no_quarter"

    def insert_quarter(self):
        # 投幣 而且改變狀態
        print("You inserted a quarter")
        self.gumball_machine.state = self.gumball_machine.has_quarter_state

    def eject_quarter(self):
        print("You haven't insert a quarter")

    def turn_crank(self):
        print("You turned, but there's no quarter")

    def dispense(self):
        print("You need to pay first")


class HasQuarterState(State):
    # 繼承State 類
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine

    def __str__(self):
        return "has_quarter"

    def insert_quarter(self):
        print("You can't insert another quarter")

    def eject_quarter(self):
        print("Quarter returned")
        self.gumball_machine.state = self.gumball_machine.no_quarter_state

    def turn_crank(self):
        print("You turned...")
        self.gumball_machine.state = self.gumball_machine.sold_state

    def dispense(self):
        print("No gumball dispensed")


class GumballMachine:

    def __init__(self, count=0):
        self.count = count
        # 找出全部狀態，並建立實例變量來持有當前狀態，而後定義狀態的值
        self.soldout_state = SoldOutState(self)
        self.no_quarter_state = NoQuarterState(self)
        self.has_quarter_state = HasQuarterState(self)
        self.sold_state = SoldState(self)
        if count > 0:
            self.state = self.no_quarter_state
        else:
            self.state = self.soldout_state

    def __str__(self):
        return "Gumball machine current state: %s" % self.state

    def insert_quarter(self):
        # 投入25分錢
        self.state.insert_quarter()

    def eject_quarter(self):
        # 退回25分
        self.state.eject_quarter()
       
    def turn_crank(self):
        # 轉動曲柄
        self.state.turn_crank()
        self.state.dispense()
    
    def release_ball(self):
        # 發放糖果
        print("A gumball comes rolling out the slot...")
        if self.count > 0:
            self.count -= 1
        
        
if __name__ == "__main__":
    # 如下是代碼測試
    gumball_machine = GumballMachine(5) # 裝入5 個糖果
    print(gumball_machine)

    gumball_machine.insert_quarter() # 投入25分錢
    gumball_machine.turn_crank() # 轉動曲柄
    print(gumball_machine)

    gumball_machine.insert_quarter() #投入25分錢
    gumball_machine.eject_quarter()  # 退錢
    gumball_machine.turn_crank()     # 轉動曲柄

    print(gumball_machine)
    
    gumball_machine.insert_quarter() # 投入25分錢
    gumball_machine.turn_crank() # 轉動曲柄 
    gumball_machine.insert_quarter() # 投入25分錢 
    gumball_machine.turn_crank()  # 轉動曲柄
    gumball_machine.eject_quarter() # 退錢

    print(gumball_machine)

重構後的代碼相對於以前的代碼作了哪些事情呢？

1. 將每一個狀態的行爲局部話到本身的類中
2. 刪除if 語句
3. 將狀態類對修改關閉，對糖果季類對擴展開放

下圖是剛初始狀態圖示：

上面重構部分代碼使用的就是狀態模式：

定義

狀態模式: 狀態模式容許對象在內部狀態改變時改變它的行爲，對象看起來好像修改了它的類。

狀態模式的類圖以下：

狀態模式是將多個行爲封裝在狀態對象中， context 的行爲隨時可委託到其中一個狀態中。當前狀態在不一樣的狀態對象中改變，以反映出context 內部的狀態，context 的行爲也會隨之改變。

擴展

若是，如今要在這四個狀態的基礎上再加一個狀態（購買糖果後，有10%的機率再得一個），該如何實現呢？

# 添加WinnerState 類，只有dispense 方法不一樣，能夠從SoldState 類繼承
class WinnerState(SoldState):
    
    def __str__(self):
        return "winner"

    def dispense(self):
        print("You're a WINNER! You get two gumballs for your quarter")
        self.gumball_machine.release_ball()
        if gumball_machine.count == 0:
            self.gumball_machine.state = self.gumball_machine.soldout_state
        else:
            self.gumball_machine.release_ball()
            if gumball_machine.count > 0:
                self.gumball_machine.state = self.gumball_machine.no_quarter_state
            else:
                print("Oops, out of gumballs!")
                self.gumball_machine.state = self.gumball_machine.soldout_state

# 修改turn_crank 方法
class HasQuarterState(State):
    ...
    def turn_crank(self):
        print("You turned...")
        winner = random.randint(0, 9)
        if winner == 4 and self.gumball_machine.count > 1: # 若是庫存大於 1 而且隨機數等於4（能夠是0到9任意值）
            self.gumball_machine.state = self.gumball_machine.winner_state
        else:
            self.gumball_machine.state = self.gumball_machine.sold_state


# 在 GumballMachine 中初始化
class GumballMachine:

    def __init__(self, count=0):
        self.count = count
        # 找出全部狀態，並建立實例變量來持有當前狀態，而後定義狀態的值
        ...
        self.winner_state = WinnerState(self)
        ...

總結

1. 狀態模式容許一個對象給予內部狀態而擁有不一樣的行爲
2. 狀態模式用類表明狀態
3. Context 會將行爲委託給當前狀態對象
4. 經過將每狀態封裝進一個類，把改變局部化
5. 狀態裝歡能夠由State 類或Context 類控制
6. 使用狀態模式會增長類的數目
7. 狀態類能夠被多個Context 實例共享

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本文例子來自《Head First 設計模式》。

最後，感謝女友支持和包容，比❤️

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