我有一个库(加载项),其中包含一些在小型应用程序中使用的类。我想为该类提供Save
方法,这将取决于正在运行的应用程序。
为了解决这个问题,我试图使用策略模式(我可能会误解模式),但我对这个主题的理解是缺乏的。在运行时,我提供了一个处理保存的策略类。公共类公开了一个Save
方法,该方法将其中继到提供的策略类。但为了保持一致性,我认为普通类也必须实现策略接口。
IRecord(通用类)接口:
Public Function DoSomething(): End Function
Public Function SetStrategy(ByVal Strategy As IDatabaseStrategy): End Function
记录(公共类)实施:
Private RecordStrategy As IDatabaseStrategy
Implements IRecord
Implements IDatabaseStrategy 'Implements this interface to have Save method
Private Function IRecord_DoSomething():
'does whatever the class is supposed to do
End Function
Private Function IRecord_SetStrategy(ByVal Strategy As IDatabaseStrategy)
Set RecordStrategy = Strategy
End Function
Private Function IDataBaseStrategy_Save()
RecordStrategy.Save
End Function
战略接口和实施:
Public Function Save():End Function
Implements IDatabaseStrategy
Private Function IDataBaseStrategy_Save()
Debug.Print "Saving to database A"
End Function
Implements IDatabaseStrategy
Private Function IDataBaseStrategy_Save()
Debug.Print "Saving to database B"
End Function
应用模块:
Option Explicit
Public Sub ApplicationA()
Dim Item As IRecord
Set Item = New Record
Dim Strategy As IDatabaseStrategy
Set Strategy = New DatabaseStrategyA
Item.SetStrategy Strategy 'this would normally be done with constructor
Dim ItemToSave As IDatabaseStrategy
Set ItemToSave = Item
ItemToSave.Save
End Sub
Public Sub ApplicationB()
Dim Item As IRecord
Set Item = New Record
Dim Strategy As IDatabaseStrategy
Set Strategy = New DatabaseStrategyB
Item.SetStrategy Strategy 'this would normally be done with constructor
Dim ItemToSave As IDatabaseStrategy
Set ItemToSave = Item
ItemToSave.Save
End Sub
通过这种方法,我必须让Record
实施Database strategy
有Save
方法,然后重新Item
从IRecord
到IDatabaseStrategy
to使用它。我认为我使用的模式不正确,所以我的问题是 - 如何向DatabaseStrategy
提供Record
,这样我就不必重新制作对象,也可能没有在IDatabaseStrategy
中实现Record
?我考虑过的替代方案:
DatabaseStrategy
周围的Record
包装Record
和应用程序(DatabaseStrategy.Create(Record).Save
)DatabaseStrategy
作为Record
的成员,但似乎应用程序必须知道DatabaseStrategy
是记录的成员(Record.DatabaseStrategy.Save
)。这是design-patterns
与VBA
的一个非常好的问题。
可以看到strategy pattern
的完整描述here,但总而言之,这是一个描述它的UML类和序列图:
基本上,此策略允许算法独立于使用它的客户端。
推迟决定使用哪种算法直到运行时允许调用代码更灵活和可重用。
你提出了这两个解决方案,我想解释为什么我没有使用它们:
DatabaseStrategy
(DatabaseStrategy.Create(Record).Save
)
使用此解决方案将减少cohesion,因为class
数据库策略将不得不照顾实例化Record
objects
(正如我稍后展示,这可以使用Factory-Pattern
完成)DatabaseStrategy
作为记录的成员,但似乎应用程序必须知道DatabaseStrategy
是记录的成员(Record.DatabaseStrategy.Save
)
这种设计模式必须知道策略,但创新是它可以在运行时更改,传递实现动作方法的desidered策略对象。在这种情况下,我会将DatabaseStrategy
从Record
移开,因为最后提到的是需要保存的object
,但它不是执行动作的代理人(可能是Database
class
,我将在稍后展示)。为了解决如何使用不接受参数的VBA objects
构造函数来实例化这个class
的问题,我使用了另一种模式:Factory Pattern。
下面的代码是为了说明如何使用这两个模式而编写的,因此省略了许多函数,并且没有真正实现数据库方法。现在这是代码的其余部分:
IRecord
class
(接口):
Public Function getValue() As String: End Function
Public Function setValue(stringValue As String): End Function
记录
class
: 该类基本上代表数据库记录。为了简单起见,我们只有一个value
属性,但实际的实现当然会有所不同。
Implements IRecord
Private value As String
Private Function IRecord_getValue() As String
IRecord_getValue = value
End Function
Private Function IRecord_setValue(stringValue As String)
value = stringValue
End Function
IConnectionStrategy
class
(接口) 这是连接策略的接口。这里的代码几乎没有变化。
Public Function Save(ByVal record As IRecord): End Function
ConnectionStrategyA
class
战略等级A.这里的代码很少改变。
Implements IConnectionStrategy
Private connectionString As String
Private Sub Class_Initialize()
connectionString = "DRIVER=Oracle Server;SERVER=myA.server.com\DatabaseA;"
End Sub
'Implements Strayegy A
Private Function IConnectionStrategy_Save(ByVal record As IRecord)
Debug.Print "Saving to ", connectionString, "Record with value:", record.getValue
End Function
ConnectionStrategyB
class
战略等级A.这里的代码很少改变。
Implements IConnectionStrategy
Private connectionString As String
Private Sub Class_Initialize()
connectionString = "DRIVER=SQL Server;SERVER=myB.server.com\DatabaseB;"
End Sub
'Implements Strategy B
Private Function IConnectionStrategy_Save(ByVal record As IRecord)
Debug.Print "Saving to ", connectionString, "Record with value:", record.getValue
End Function
数据库
class
这是处理与数据库的连接的类。连接策略定义了如何连接到数据库。 请注意,您可以更改策略而无需重新设置对象,只需调用setConnectionStrategy(...)
即可
Private connection As IConnectionStrategy
Private Sub Class_Initialize()
'Initialize everything but strategy.
End Sub
'Funzione che verrà richiamata per inizializzare le propietà della classe
Public Sub InitiateProperties(ByVal connectionStrategy As IConnectionStrategy)
Set connection = connectionStrategy
End Sub
Public Sub saveRecord(ByVal record As IRecord)
connection.Save record
End Sub
Public Sub setConnectionStrategy(ByVal strategy As IConnectionStrategy)
connection = strategy
End Sub
Private Sub Class_Terminate()
'close connections
End Sub
DatabaseFactory
Module
(VBA没有静态类,所以请改用Module) 这个类负责你的Database Objects
的实例化
'Instantiate a Database with given Strategy
Public Function createDatabaseWithStrategy(strategy As IConnectionStrategy) As Database
Set createDatabaseWithStrategy = New Database
CreateFoglioIdro.InitiateProperties connectionStrategy:=strategy
End Function
'Instantiate a Database with Strategy A
Public Function createDatabaseWithStrategyA() As Database
Set createDatabaseWithStrategyA = New Database
createDatabaseWithStrategyA.InitiateProperties connectionStrategy:=New ConnectionStrategyA
End Function
'Instantiate a Database with Strategy B
Public Function createDatabaseWithStrategyB() As Database
Set createDatabaseWithStrategyB = New Database
createDatabaseWithStrategyB.InitiateProperties connectionStrategy:=New ConnectionStrategyB
End Function
最后一个应用示例(App Module
):
Option Explicit
Public Sub ApplicationA()
Dim record As IRecord
Set record = New record
record.setValue ("This is a value")
Dim db As Database
Set db = DatabaseFactory.createDatabaseWithStrategyA
db.saveRecord record
'Prints-> Saving to DRIVER=Oracle Server;SERVER=myA.server.com\DatabaseA; Record with value: This is a value
End Sub
Public Sub ApplicationB()
Dim record As IRecord
Set record = New record
record.setValue ("This is a value")
Dim db As Database
Set db = DatabaseFactory.createDatabaseWithStrategyB
db.saveRecord record
'Prints-> Saving to DRIVER=SQL Server;SERVER=myB.server.com\DatabaseB; Record with value: This is a value
End Sub
正如您所看到的,strategy-pattern
结合Factory-Pattern
,帮助您消除大多数重复代码,以初始化并为您的数据库class
设置策略。
希望这可以帮助。