Vb Classic Object Proxy

Sometimes in VbClassic you want to get hold of an object but not keep a reference to it.

An example would be a Parent property in a collection item. If you do it by holding a reference to the parent in each member of the collection, you end up with circular referenes.

Instead you can hold a Proxy to the parent object.

http://www.vb-faq.com/Articles/Hughson/proxyobjects.asp

This link seems to be dead, here is some example code

You will need the Child, the Parent, and the Proxy Object


        ParentProxy Object

        --------------------------------

        Option Explicit

        Public Event GetRef(ByRef RHS As MyParent)


        Public Function GetParent() As MyParent

        Dim Ref As MyParent

        RaiseEvent GetRef(Ref)

        Set GetParent = Ref

        End Function


        Child Object

        ----------------------

        ..

        Private aParent as ParentProxy

        ..

        Public Property Get Parent() as MyParent

        Set Parent = aParent.GetParent

        End Property


        Public Property Set Parent(RHS as MyParent)

        Set aParent as MyParent.Proxy

        End Property


        Parent Object

        ----------------------------


        Private WithEvents priParentProxy as ParentProxy


        Private Sub Class_Initialize()

        ..

        Set priParentProxy = New ParentProxy

        ..

        End Sub


        Friend Property Get Proxy() As ParentProxy

        Set Proxy = priParentProxy

        End Property


        Private Sub priParentProxy_GetRef(RHS As MyParent)

        Set RHS = Me

        End Sub

Occasionally, you may still be forced to work in Office 97, in which case, you can't define custom events. In these cases, you have 2 alternatives that I know of:

Have a single container object around one or more objects in your cyclic graph that is not, itself part of a cycle. When this object goes out of scope, it calls methods of the objects it contains to tell them to drop their references to other objects in the network, so cyclical references are broken.
Use a non-visible Form object as an event messenger. You can force the Change event handler of a control on the form to fire by changing its Text property (works in MicrosoftAccess anyway).

One question: When can it happen that the proxied object is released, but you still try to reference it through the proxy? Without this knowledge, I am not certain that I dare to use this pattern. -- ThomasEyde

CategoryVbClassic