Category: Programming

VBScript는 객체 지향적인 측면에서는 많이 부족한 언어다. 상속이나 다형성을 전혀 제공하고 있지 않아서 클래스를 만드는게 오히려 불현할 때가 더 많은 게 사실이다. 조금이나마 이런 단점을 보안해 줄 수 있는 함수가 하나 있는데 GetRef 이다.

GetRef는 인자로 넘긴 문자열에 해당하는 함수 포인터를 반환하는 기능을 하는 함수이다.  이 함수를 이용하여 VBscript에서 간단하게 이벤트 드리븐(event driven) 프로그래밍하는 좋은 예제가 있어서 소개할려고 한다.

VBScript doesn’t have an event implementation so if you fancy having features like attaching handlers which will respond to specific events on your object you can do it simply by using the GetRef function and a bit of “syntactic sugar”.

I’m using ASP in these examples cos it’s easy.

Example 1 – Simple Events

‘Create a handler

Function MyHandler()

   Response.Write “Hello from the handler!”

End Function

‘Create an event

Dim OnLoad

Set OnLoad = GetRef(“MyHandler”)
‘Fire the event

OnLoad()

Here we’ve created a simple event which takes one handler function and fired the event which in turn has called the function we attached.

To turn this in to a more useful event system we can use an array for the OnLoad event variable thus…

‘Create some handlers

Function MyHandler1()

   Response.Write “Hello from handler 1!”

End Function

Function MyHandler2()

   Response.Write “Hello from handler 2!”

End Function
‘Create an event

Dim OnLoad

OnLoad = Array(GetRef(“MyHandler1”), GetRef(“MyHandler2”))
‘Fire the event

For Each handler In OnLoad

   handler()

Next

Example 2 – Event Arguments

In most event implementations the event handlers take one argument, passed to them by the fired event, which contains things like the type of event and a reference to the object on which it was fired etc.

‘Create a handler which takes one argument

Function MyHandler(e)

   Response.Write “Hello from the handler – i was called by ” & e

End Function

‘Create two events

Dim OnLoad

Set OnLoad = GetRef(“MyHandler”)
Dim OnUnload

Set OnUnload = GetRef(“MyHandler”)
‘Fire the events

OnLoad(“Load”)

OnUnload(“Unload”)

Wrapping it up

We’ve established we can do all the basics of events, now all we need to do is wrap it up in a few classes to make it usable.

First we need an Event class that we can instantiate for each event we want. This will have to expose an event arguments property and methods for attaching handlers and firing the event. It will also have to keep track internally of the attached handlers. Lets have a go…

Class clsEvent

   ‘An array to keep track of our handlers

   Private aryHandlers()
   ‘Our event arguments object to be passed

   ‘to the handlers

   Public EventArgs
   Private Sub Class_Initialize()

      ReDim aryHandlers(-1)

      Set EventArgs = New clsEventArgs

   End Sub
   Private Sub Class_Terminate()

      Set EventArgs = Nothing

      Erase aryHandlers

   End Sub
   ‘Method for adding a handler

   Public Function AddHandler(strFunctionName)

      ReDim Preserve aryHandlers(UBound(aryHandlers) + 1)

      Set aryHandlers(UBound(aryHandlers)) = _

         GetRef(strFunctionName)

   End Function
   ‘Method for firing the event

   Public Function Fire(strType, objCaller)

      EventArgs.EventType = strType

      Set EventArgs.Caller = objCaller

      For Each f In aryHandlers

         f(EventArgs)

      Next

   End Function
End Class

Next we need an EventArgs class for passing data about the event to the handlers. This just needs three properties; event type, caller and an arguments collection for event type specific things.

Class clsEventArgs

   Public EventType, Caller, Args
   Private Sub Class_Initialize()

      Set Args = CreateObject(“Scripting.Dictionary”)

   End Sub
   Private Sub Class_Terminate()

      Args.RemoveAll

      Set Args = Nothing

   End Sub
End Class

Next our class that has an event, in this case an OnLoad which fires after the object’s Load method is called. We’ll also create a few handlers and do a trial run.

Class MyClass

   Public OnLoad
   Private Sub Class_Initialize()

      ‘Setting up our event

      Set OnLoad = New clsEvent
      ‘Adding an argument

      OnLoad.EventArgs.Args.Add “arg1”, “Hello”

   End Sub
   Public Function Load()

      Response.Write “loading the object here!<br />”
      ‘Firing the event

      OnLoad.Fire “load”, Me

   End Function
End Class
‘A couple of handling function for the events

Function EventHandler(e)

   Response.Write “<h2>EventHandler</h2>”

   Response.Write “<p>Event “”” & e.EventType & “”” fired by object

of type ” & TypeName(e.Caller) & “.</p>”

End Function
Function EventHandler2(e)

   Response.Write “<h2>EventHandler2</h2>”

   For Each x In e.Args

      Response.Write x & “: ” & e.Args(x) & “<br />”

   Next

End Function
‘instantiate the object, attach the handlers and call the load

Set myObj = New MyClass

myObj.OnLoad.AddHandler(“EventHandler”)

myObj.OnLoad.AddHandler(“EventHandler2”)

myObj.Load()

Event based programming reverses the responsibility for code execution within your program. In conventional procedural programming it would be the responsibility of the myObj class to make sure the two event handlers were fired when it’s Load method was called. By using an OnLoad event instead myObj doesn’t have to know anything about the environment in which its executing, it just blindly fires the event and any attached handlers will be called. In this way you can add additional functions which run when myObj’s Load method is called without modifying MyClass.

In more complex systems being able to add functionality with a minimum of intrusion into other parts of the system is a big bonus and event based programming is an easy way of achieving it.

출처: http://derek-says.blogspot.com/2006/10/simple-event-driven-programming-using.html