摘要：很明顯，有有分錢(qián)沒(méi)有分錢(qián)售出糖果糖果售罄四個(gè)狀態(tài)，同時(shí)也對(duì)應(yīng)四個(gè)動(dòng)作投入分錢(qián)，退回分錢(qián)，轉(zhuǎn)動(dòng)曲柄和發(fā)放糖果。狀態(tài)模式的類(lèi)圖如下?tīng)顟B(tài)模式是將多個(gè)行為封裝在狀態(tài)對(duì)象中，的行為隨時(shí)可委托到其中一個(gè)狀態(tài)中。

問(wèn)題：有一個(gè)糖果公司需要設(shè)計(jì)一個(gè)糖果售賣(mài)機(jī)，控制流程如下圖，需要怎么實(shí)現(xiàn)？

這是一個(gè)狀態(tài)圖，每個(gè)圓圈都是一種狀態(tài)。很明顯，有有25分錢(qián)、 沒(méi)有25分錢(qián)、 售出糖果、 糖果售罄四個(gè)狀態(tài)，同時(shí)也對(duì)應(yīng)四個(gè)動(dòng)作：投入25分錢(qián)，退回25分錢(qián)，轉(zhuǎn)動(dòng)曲柄和發(fā)放糖果。

那如何從狀態(tài)圖得到真正的代碼呢？

簡(jiǎn)單代碼實(shí)現(xiàn)如下：

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

class GumballMachine:

    # 找出所有狀態(tài)，并創(chuàng)建實(shí)例變量來(lái)持有當(dāng)前狀態(tài)，然后定義狀態(tài)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(qián)
        if self.state == self.STATE_HAS_QUARTER: # 如果已經(jīng)投過(guò)
            print("You can"t insert another quarter")
        elif self.state == self.STATE_NO_QUARTER: # 如果沒(méi)有投過(guò)
            self.state = self.STATE_HAS_QUARTER
            print("You inserted a quarter")
        elif self.state == self.STATE_SOLD_OUT: # 如果已經(jīng)售罄
            print("You can"t insert a quarter, the machine is sold out")
        elif self.state == self.STATE_SOLD: # 如果剛剛買(mǎi)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(zhuǎn)動(dòng)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(fā)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__":
    # 以下是代碼測(cè)試
    gumball_machine = GumballMachine(5) # 裝入5 個(gè)糖果
    print(gumball_machine)

    gumball_machine.insert_quarter() # 投入25分錢(qián)
    gumball_machine.turn_crank() # 轉(zhuǎn)動(dòng)曲柄
    print(gumball_machine)

    gumball_machine.insert_quarter() #投入25分錢(qián)
    gumball_machine.eject_quarter()  # 退錢(qián)
    gumball_machine.turn_crank()     # 轉(zhuǎn)動(dòng)曲柄

    print(gumball_machine)
    
    gumball_machine.insert_quarter() # 投入25分錢(qián)
    gumball_machine.turn_crank() # 轉(zhuǎn)動(dòng)曲柄 
    gumball_machine.insert_quarter() # 投入25分錢(qián) 
    gumball_machine.turn_crank()  # 轉(zhuǎn)動(dòng)曲柄
    gumball_machine.eject_quarter() # 退錢(qián)

    print(gumball_machine)

這段代碼有幾個(gè)問(wèn)題：

沒(méi)有遵守開(kāi)放-關(guān)閉原則

更像是面向過(guò)程的設(shè)計(jì)

狀態(tài)轉(zhuǎn)化被埋藏在條件語(yǔ)句中

未來(lái)加入新的需求，需要改動(dòng)的較多，不易維護(hù)，可能會(huì)出bug

如何改進(jìn)呢？

考慮封裝變化，把每個(gè)狀態(tài)的行為都放在各自的類(lèi)中，每個(gè)狀態(tài)只要實(shí)現(xiàn)自己的動(dòng)作，用加入新類(lèi)的方式來(lái)實(shí)現(xiàn)新?tīng)顟B(tài)的加入。

定義State 父類(lèi)，在這個(gè)類(lèi)中，糖果機(jī)的每個(gè)動(dòng)作都有一個(gè)應(yīng)對(duì)的方法

為機(jī)器中的每個(gè)狀態(tài)實(shí)現(xiàn)狀態(tài)類(lèi)，這些類(lèi)將負(fù)責(zé)在對(duì)應(yīng)的狀態(tài)下進(jìn)行機(jī)器的行為

擺脫舊的條件代碼，將動(dòng)作委托到狀態(tài)類(lèi)

新的實(shí)現(xiàn)代碼如下：

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

class State:
    # 定義state基類(lèi)
    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(lèi)
    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(lèi)
    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(lèi)
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine

    def __str__(self):
        return "no_quarter"

    def insert_quarter(self):
        # 投幣 并且改變狀態(tài)
        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(lèi)
    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
        # 找出所有狀態(tài)，并創(chuàng)建實(shí)例變量來(lái)持有當(dāng)前狀態(tài)，然后定義狀態(tài)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(qián)
        self.state.insert_quarter()

    def eject_quarter(self):
        # 退回25分
        self.state.eject_quarter()
       
    def turn_crank(self):
        # 轉(zhuǎn)動(dòng)曲柄
        self.state.turn_crank()
        self.state.dispense()
    
    def release_ball(self):
        # 發(fā)放糖果
        print("A gumball comes rolling out the slot...")
        if self.count > 0:
            self.count -= 1
        
        
if __name__ == "__main__":
    # 以下是代碼測(cè)試
    gumball_machine = GumballMachine(5) # 裝入5 個(gè)糖果
    print(gumball_machine)

    gumball_machine.insert_quarter() # 投入25分錢(qián)
    gumball_machine.turn_crank() # 轉(zhuǎn)動(dòng)曲柄
    print(gumball_machine)

    gumball_machine.insert_quarter() #投入25分錢(qián)
    gumball_machine.eject_quarter()  # 退錢(qián)
    gumball_machine.turn_crank()     # 轉(zhuǎn)動(dòng)曲柄

    print(gumball_machine)
    
    gumball_machine.insert_quarter() # 投入25分錢(qián)
    gumball_machine.turn_crank() # 轉(zhuǎn)動(dòng)曲柄 
    gumball_machine.insert_quarter() # 投入25分錢(qián) 
    gumball_machine.turn_crank()  # 轉(zhuǎn)動(dòng)曲柄
    gumball_machine.eject_quarter() # 退錢(qián)

    print(gumball_machine)

重構(gòu)后的代碼相對(duì)于之前的代碼做了哪些事情呢？

將每個(gè)狀態(tài)的行為局部話到自己的類(lèi)中

刪除if 語(yǔ)句

將狀態(tài)類(lèi)對(duì)修改關(guān)閉，對(duì)糖果季類(lèi)對(duì)擴(kuò)展開(kāi)放

下圖是剛初始狀態(tài)圖示：

上面重構(gòu)部分代碼使用的就是狀態(tài)模式：

狀態(tài)模式: 狀態(tài)模式允許對(duì)象在內(nèi)部狀態(tài)改變時(shí)改變它的行為，對(duì)象看起來(lái)好像修改了它的類(lèi)。

狀態(tài)模式的類(lèi)圖如下：

狀態(tài)模式是將多個(gè)行為封裝在狀態(tài)對(duì)象中， context 的行為隨時(shí)可委托到其中一個(gè)狀態(tài)中。當(dāng)前狀態(tài)在不同的狀態(tài)對(duì)象中改變，以反映出context 內(nèi)部的狀態(tài)，context 的行為也會(huì)隨之改變。

如果，現(xiàn)在要在這四個(gè)狀態(tài)的基礎(chǔ)上再加一個(gè)狀態(tài)（購(gòu)買(mǎi)糖果后，有10%的概率再得一個(gè)），該如何實(shí)現(xiàn)呢？

# 添加WinnerState 類(lèi)，只有dispense 方法不同，可以從SoldState 類(lèi)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: # 如果庫(kù)存大于 1 并且隨機(jī)數(shù)等于4（可以是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
        # 找出所有狀態(tài)，并創(chuàng)建實(shí)例變量來(lái)持有當(dāng)前狀態(tài)，然后定義狀態(tài)的值
        ...
        self.winner_state = WinnerState(self)
        ...

狀態(tài)模式允許一個(gè)對(duì)象給予內(nèi)部狀態(tài)而擁有不同的行為

狀態(tài)模式用類(lèi)代表狀態(tài)

Context 會(huì)將行為委托給當(dāng)前狀態(tài)對(duì)象

通過(guò)將每狀態(tài)封裝進(jìn)一個(gè)類(lèi)，把改變局部化

狀態(tài)裝歡可以由State 類(lèi)或Context 類(lèi)控制

使用狀態(tài)模式會(huì)增加類(lèi)的數(shù)目

狀態(tài)類(lèi)可以被多個(gè)Context 實(shí)例共享

本文例子來(lái)自《Head First 設(shè)計(jì)模式》。

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