結構圖

?

角色

• 環境（Subject）角色：負責定義客戶感興趣的接口，并維護一個具體狀態(ConcreteState)的實例，以及維護狀態轉換或為這種轉換提供支持（因為狀態轉換有時候可能是由上層應用發起的）。
• 狀態（State）角色：定義一個接口以封裝與環境（Context）角色的一個特定狀態相關的行為。
• 具體狀態（ConcreteState）角色：)每一個子類實現一個與環境（Context）角色狀態相關的行為。

動機

　　在軟件構建過程中，某些對象的狀態如果改變，其行為也會隨之而發生變化，比如文檔處于只讀狀態，其支持的行為和讀寫狀態支持的行為就可能完全不同。
???? 如何在運行時根據對象的狀態來透明地更改對象的行為？而不會為對象操作和狀態轉化之間引入緊耦合？

意圖

?? 允許一個對象在其內部狀態改變時改變它的行為。從而使對象看起來似乎修改了其行為。

示意性代碼

示意性代碼
'State?pattern?--?Structural?example

'MainApp?test?application
Module?MainApp
????Public?Sub?Main()
????????'Setup?context?in?a?state
????????Dim?c?As?New?Context(New?ConcreteStateA)

????????'Issue?requests,?which?toggles?state
????????c.Request()
????????c.Request()
????????c.Request()
????????c.Request()

????????'Wait?for?user?
????????Console.ReadLine()
????End?Sub
End?Module

'"State"
Public?MustInherit?Class?State
????Public?MustOverride?Sub?Handle(ByVal?context?As?Context)
End?Class

'"ConcreteStateA"
Public?Class?ConcreteStateA
????Inherits?State
????Public?Overrides?Sub?Handle(ByVal?context?As?Context)
????????context.State?=?New?ConcreteStateB
????End?Sub
End?Class

'"ConcreteStateB"
Public?Class?ConcreteStateB
????Inherits?State
????Public?Overrides?Sub?Handle(ByVal?context?As?Context)
????????context.State?=?New?ConcreteStateA
????End?Sub
End?Class

'"Context"
Public?Class?Context
????'Constructor?
????Public?Sub?New(ByVal?state?As?State)
????????Me._state?=?state
????End?Sub

????'Property
????Private?_state?As?State
????Public?Property?State()?As?State
????????Get
????????????Return?_state
????????End?Get
????????Set(ByVal?value?As?State)
????????????_state?=?value
????????End?Set
????End?Property

????Public?Sub?Request()
????????State.Handle(Me)
????End?Sub
End?Class

?

?一個實例

??? 下面的狀態模式代碼演示了一個帳戶在處于不同狀態時可以進行不同的操作。在行為上的區別委派給以下三個對象紅色狀態（RedState），灰色狀態（SilverState），金色狀態（GoldState）。這些狀態分別代表透支帳戶，新開通的帳戶，和擁有良好信譽的帳戶。

舊系統代碼
'State?pattern?--?Real?World?example

'MainApp?test?application
Module?MainApp
????Public?Sub?Main()
????????'Open?a?new?account
????????Dim?account?As?New?Account("Jim?Johnson")

????????'Apply?financial?transactions
????????account.Deposit(500.0)
????????account.Deposit(300.0)
????????account.Deposit(550.0)
????????account.PayInterest()
????????account.Withdraw(2000.0)
????????account.Withdraw(1100.0)

????????'Wait?for?user
????????Console.ReadLine()
????End?Sub
End?Module

'"Context"
Public?Class?Account
????Private?owner?As?String
????Private?serviceFee?As?Double
????Private?interest?As?Double

????'Constructor
????Public?Sub?New(ByVal?owner?As?String)
????????'New?accounts?are?'Silver'?by?default
????????Me.owner?=?owner
????????_state?=?"SilverState"
????End?Sub

????'Properties
????Protected?_balance?As?Double
????Public?ReadOnly?Property?Balance()?As?Double
????????Get
????????????Return?_balance
????????End?Get
????End?Property

????Private?_state?As?String
????Public?Property?State()?As?String
????????Get
????????????Return?_state
????????End?Get
????????Set(ByVal?value?As?String)
????????????_state?=?value
????????End?Set
????End?Property

????Public?Sub?Deposit(ByVal?amount?As?Double)
????????_balance?+=?amount
????????StateChangeCheck()
????????Console.WriteLine("Deposited?{0:C}?---?",?amount)
????????Console.WriteLine("?Balance?=?{0:C}",?Me.Balance)
????????Console.WriteLine("?Status?=?{0}",?_
?????????_state)
????????Console.WriteLine("")
????End?Sub

????Public?Sub?Withdraw(ByVal?amount?As?Double)
????????If?_state?=?"RedState"?Then
????????????serviceFee?=?15.0
????????????amount?-=?serviceFee
????????????Console.WriteLine("No?funds?available?for?withdrawal!")
????????Else
????????????_balance?-=?amount
????????End?If
????????StateChangeCheck()
????????Console.WriteLine("Withdrew?{0:C}?---?",?amount)
????????Console.WriteLine("?Balance?=?{0:C}",?Me.Balance)
????????Console.WriteLine("?Status?=?{0}",?_
?????????_state)
????????Console.WriteLine("")
????End?Sub

????Public?Sub?PayInterest()
????????Select?Case?_state
????????????Case?"RedState"
????????????????'No?Interest?is?paid
????????????Case?"SilverState"
????????????????interest?=?0.0
????????????????_balance?+=?interest?*?_balance
????????????Case?"GoldState"
????????????????interest?=?0.05
????????????????_balance?+=?interest?*?_balance
????????End?Select

????????Console.WriteLine("Interest?Paid?---?")
????????Console.WriteLine("?Balance?=?{0:C}",?Me.Balance)
????????Console.WriteLine("?Status?=?{0}",?_
?????????_state)
????????Console.WriteLine("")
????End?Sub

????Private?Sub?StateChangeCheck()
????????If?Balance?<?0.0?Then
????????????_state?=?"RedState"
????????ElseIf?_balance?<?1000?Then
????????????_state?=?"SilverState"
????????Else
????????????_state?=?"GoldState"
????????End?If
????End?Sub
End?Class

在開發中時常遇到的根據不同的狀態需要進行不同的處理操作的問題,而這樣的問題,大部分人是采用Select-Case/If-Else語句進行處理的,這樣會造成一個問題:分支過多,而且如果加入一個新的狀態就需要對原來的代碼進行編譯.State模式采用了對這些不同的狀態進行封裝的方式處理這類問題,當狀態改變的時候進行處理然后再切換到另一種狀態,也就是說把狀態的切換責任交給了具體的狀態類去負責.具體的狀態類可以通過派生的方式不依賴于其他對象而獨立變化。

運用模式后的代碼
'State?pattern?--?Real?World?example

'MainApp?test?application
Module?MainApp
????Public?Sub?Main()
????????'Open?a?new?account
????????Dim?account?As?New?Account("Jim?Johnson")

????????'Apply?financial?transactions
????????account.Deposit(500.0)
????????account.Deposit(300.0)
????????account.Deposit(550.0)
????????account.PayInterest()
????????account.Withdraw(2000.0)
????????account.Withdraw(1100.0)

????????'Wait?for?user
????????Console.ReadLine()
????End?Sub
End?Module

'"State"
Public?MustInherit?Class?State
????'Fields
????Protected?interest?As?Double
????Protected?lowerLimit?As?Double
????Protected?upperLimit?As?Double

????'Properties
????Protected?_account?As?Account
????Public?Property?Account()?As?Account
????????Get
????????????Return?_account
????????End?Get
????????Set(ByVal?value?As?Account)
????????????_account?=?value
????????End?Set
????End?Property

????Protected?_balance?As?Double
????Public?Property?Balance()?As?Double
????????Get
????????????Return?_balance
????????End?Get
????????Set(ByVal?value?As?Double)
????????????_balance?=?value
????????End?Set
????End?Property

????Public?MustOverride?Sub?Deposit(ByVal?amount?As?Double)
????Public?MustOverride?Sub?Withdraw(ByVal?amount?As?Double)
????Public?MustOverride?Sub?PayInterest()
End?Class

'"ConcreteState"
'Account?is?overdrawn
Public?Class?RedState
????Inherits?State

????Private?serviceFee?As?Double

????'Constructor
????Public?Sub?New(ByVal?state?As?State)
????????Me.Balance?=?state.Balance
????????Me.Account?=?state.Account
????????Initialize()
????End?Sub

????Private?Sub?Initialize()
????????'Should?come?from?a?datasource
????????interest?=?0.0
????????lowerLimit?=?-100.0
????????upperLimit?=?0.0
????????serviceFee?=?15.0
????End?Sub

????Public?Overrides?Sub?Deposit(ByVal?amount?As?Double)
????????Balance?+=?amount
????????StateChangeCheck()
????End?Sub
????Public?Overrides?Sub?Withdraw(ByVal?amount?As?Double)
????????amount?-=?serviceFee
????????Console.WriteLine("No?funds?available?for?withdrawal!")
????End?Sub
????Public?Overrides?Sub?PayInterest()
????????'No?intereset?is?paid
????End?Sub
????Private?Sub?StateChangeCheck()
????????If?Balance?>?upperLimit?Then
????????????Account.State?=?New?SilverState(Me)
????????End?If
????End?Sub
End?Class

'"ConcreteState"
'Silver?is?non-interest?bearing?state
Public?Class?SilverState
????Inherits?State

????'Overloaded?constructors
????Public?Sub?New(ByVal?state?As?State)
????????Me.new(state.Balance,?state.Account)
????End?Sub

????Public?Sub?New(ByVal?balance?As?Double,?ByVal?account?As?Account)
????????Me.Balance?=?balance
????????Me.Account?=?account
????????Initialize()
????End?Sub
????Private?Sub?Initialize()
????????'Should?come?from?a?datasource
????????interest?=?0.0
????????lowerLimit?=?0.0
????????upperLimit?=?1000.0
????End?Sub

????Public?Overrides?Sub?Deposit(ByVal?amount?As?Double)
????????Balance?+=?amount
????????StateChangeCheck()
????End?Sub
????Public?Overrides?Sub?Withdraw(ByVal?amount?As?Double)
????????Balance?-=?amount
????????StateChangeCheck()
????End?Sub
????Public?Overrides?Sub?PayInterest()
????????Balance?+=?interest?*?Balance
????????StateChangeCheck()
????End?Sub
????Private?Sub?StateChangeCheck()
????????If?Balance?<?lowerLimit?Then
????????????Account.State?=?New?RedState(Me)
????????ElseIf?Balance?>?upperLimit?Then
????????????Account.State?=?New?GoldState(Me)
????????End?If
????End?Sub
End?Class

'"ConcreteState"
'Interest?bearing?state
Public?Class?GoldState
????Inherits?State

????'Overloaded?constructors
????Public?Sub?New(ByVal?state?As?State)
????????Me.new(state.Balance,?state.Account)
????End?Sub

????Public?Sub?New(ByVal?balance?As?Double,?ByVal?account?As?Account)
????????Me.Balance?=?balance
????????Me.Account?=?account
????????Initialize()
????End?Sub
????Private?Sub?Initialize()
????????'Should?come?from?a?datasource
????????interest?=?0.05
????????lowerLimit?=?1000.0
????????upperLimit?=?10000000.0
????End?Sub

????Public?Overrides?Sub?Deposit(ByVal?amount?As?Double)
????????Balance?+=?amount
????????StateChangeCheck()
????End?Sub
????Public?Overrides?Sub?Withdraw(ByVal?amount?As?Double)
????????Balance?-=?amount
????????StateChangeCheck()
????End?Sub
????Public?Overrides?Sub?PayInterest()
????????Balance?+=?interest?*?Balance
????????StateChangeCheck()
????End?Sub
????Private?Sub?StateChangeCheck()
????????If?Balance?<?0.0?Then
????????????Account.State?=?New?RedState(Me)
????????ElseIf?Balance?<?lowerLimit?Then
????????????Account.State?=?New?SilverState(Me)
????????End?If
????End?Sub
End?Class

'"Context"
Public?Class?Account
????Private?owner?As?String

????'Constructor
????Public?Sub?New(ByVal?owner?As?String)
????????'New?accoounts?are?'Silver'?by?default
????????Me.owner?=?owner
????????_state?=?New?SilverState(0.0,?Me)
????End?Sub

????'Properties
????Public?ReadOnly?Property?Balance()?As?Double
????????Get
????????????Return?_state.Balance
????????End?Get
????End?Property

????Private?_state?As?State
????Public?Property?State()?As?State
????????Get
????????????Return?_state
????????End?Get
????????Set(ByVal?value?As?State)
????????????_state?=?value
????????End?Set
????End?Property

????Public?Sub?Deposit(ByVal?amount?As?Double)
????????State.Deposit(amount)
????????Console.WriteLine("Deposited?{0:C}?---?",?amount)
????????Console.WriteLine("?Balance?=?{0:C}",?Me.Balance)
????????Console.WriteLine("?Status?=?{0}",?_
?????????_state.GetType().Name)
????????Console.WriteLine("")
????End?Sub

????Public?Sub?Withdraw(ByVal?amount?As?Double)
????????State.Withdraw(amount)
????????Console.WriteLine("Withdrew?{0:C}?---?",?amount)
????????Console.WriteLine("?Balance?=?{0:C}",?Me.Balance)
????????Console.WriteLine("?Status?=?{0}",?_
?????????_state.GetType().Name)
????????Console.WriteLine("")
????End?Sub

????Public?Sub?PayInterest()
????????_state.PayInterest()
????????Console.WriteLine("Interest?Paid?---?")
????????Console.WriteLine("?Balance?=?{0:C}",?Me.Balance)
????????Console.WriteLine("?Status?=?{0}",?_
?????????Me.State.GetType().Name)
????????Console.WriteLine("")
????End?Sub
End?Class

?

State?Pattern模式的幾個要點：
　 　１、State模式將所有與一個特定狀態相關的行為都放入一個State的子類對象中，在對象狀態切換時，切換相應的對象；但同時維持State的接口，這樣實現了具體操作與狀態轉換之間的解耦。
　 　2、為不同的狀態引入不同的對象使得狀態轉換變得更加明確，而且可以保證不會出現狀態不一致的情況，因為轉換是原子性的——即要么徹底轉換過來，要么不轉換。
?????3、如果State對象沒有實例變量，那么各個上下文可以共享一個State對象，從而節省對象開銷。

我的理解

封裝與狀態相關的行為，支持狀態的變化。

參考資料
《C#面向對象設計模式縱橫談系列課程（21）》?????李建中老師

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