Vb Classic Singleton

Problem:

VbClassic Classes don't support static methods, or a mechanism equivalent to overriding new(), so singletons can't be implemented in the way you would in C++.

Quoting from the MicroSoft VisualBasicDesignPatterns? book...: "Implementing a Singleton design pattern in VisualBasic is not possible with the current language features." (And then it goes on to show how to do it anyway. ;-)

MicrosoftDotNet appear to have Singleton built in to the framework, and an article on MSDN is located at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnbda/html/singletondespatt.asp

Context:

You are in a context where you would normally use a singleton but the implementation is in VisualBasic

Forces:

Resorting to global variables has all the problems of correctness and maintainability that singletons were designed to prevent
Rewriting the code in something other than VisualBasic is elegant and conducive to sanity yet all too often infeasible due to outside constraints
Implementation details that depend on the programmer rather than language features need to be made transparent yet avoid overload.

Solutions:

Make a "private" implementation class, and use a FactoryMethod on a general "singleton factory" class to create it.
AbstractFactory class controls creation of implementation class, in ActiveX DLL for enforcement. (from the MicroSoft VisualBasicDesignPatterns? book)
HandleBody? -- multiple "handle" instances all point to a single "body" instance; the singleton.
Procedural ClassFactory function (not an object), with ActiveX protection of singleton class

Related Patterns:

Singleton, FactoryMethod, IntentionSuggestingName?, QualifiedClassName?

Contributors: Stefan Kapusniak, JeffGrigg

But singletons can be implemented in VbClassic. It's just a different technique but the principle it's the same. All it really takes is a GlobalMultiuse? class in the same ActiveX DLL as your Singleton class and a static module. I've described it below. :)

I have found that you can fake a lot of things using the different instancing properties of classes and standard modules in in VbClassic -- AlfredoChavez

Problem: Ensure a class only has one instance, and provide a global point of access to it.

Context: You are in a context where you would normally use a singleton but the implementation is in VisualBasic

Implementation: I don't know if someone came up with this one before me, but I recently had a programming problem in a project that quite fits the context and forces described for the singleton pattern in many online catalogs. So here's my take on it:

If your target class isn't already in an ActiveX DLL project, create one and move your class into it.
Make your singleton-to-be class PublicNotCreatable
Add a new standard module to your ActiveX DLL and declare a new variable. i.e. if I'm dealing with a CSesion class, then I'd use:

 Public g_oSesion As CSesion

Create a new class (which I personally call "CAccesor", unless I have previously created a "CCreator" one, in which case I'll "recycle" the previous one) and make it GlobalMultiuse?
Add a new public Property Get procedure, which will grant public access to your singleton class. Using the example above, I'd use something like:

 Public Property Get Sesion() As CSesion
     If g_oSesion is Nothing Then
         Set g_oSesion = New CSesion
         ' add any initialization code here,
         ' or better yet, IntroduceCreationObject here
     End If
     Set Sesion = g_oSesion
 End Property

Finally, make sure you destroy your Singleton properly. A new Class_Terminate event handler will do nicely here:

 Private Sub Class_Terminate()
     Set g_oSesion = Nothing
 End Sub

-- AlfredoChavez

Private implementation class, with a FactoryMethod:

Create the classes which should be singletons as standard classes named in the form privateSomeClass
Create a module called Singletons and create a GlobalAccessor? for each singleton thusly:

public property get publicSomeClass() as privateSomeClass

static oInstance as privateSomeClass

if oInstance is nothing then
set oInstance = new privateSomeClass
end if

set publicClassName = oInstance

end property

Heavily document that classes named in the form privateSomeClass, should only be accessed through their corresponding publicSomeClass interface.
Call methods and set properties of publicSomeClass in your code in the usual way:

publicSomeClass.someMethod publicSomeClass.someProperty = "someString" Set someReference = publicSomeClass.someObjectProperty

Additional thought: Make this an ActiveX DLL, and you can set the Instancing property to PublicNotCreatable, preventing DLL users from violating your creation rules. Consider setting the ThreadingModel to SingleThread? too.

Contributors: StefanKapusniak?, JeffGrigg

AbstractFactory class controls creation of implementation class, in ActiveX DLL for enforcement:

Like the above, except that it...

Uses a different TargetFactory? (AbstractFactory) class for each singleton class -- with a CreateInstance? function.
Uses ActiveX Instancing property of PublicNotCreatable to prevent cleints from creating singleton class instances themselves.
Sets the ThreadingModel to SingleThread? to avoid one-instance-per-apartment.

HandleBody? approach:

The problem with this approach is it still relies on programmer's discipline not to instantiate the actual "private" class. I turn it around, and basically use the handle/body approach.

Implement the actual class as module (MySingletonBody?), with all data members private to the module. Provide access functions for all "public" properties.
Create a class MySingleton?, and delegate all methods and properties to those in the body module.
If garbage collection is required, perform reference counting in the Initialize and terminate events of the class. The body can release its resources on the count going to zero, and re-acquire them when the count becomes non-zero.

The result has many advantages

Programmers can not create more than one instance as all the data is effectively static to the body module.
Garbage collection is still possible
You can get the "instance" reference by simply creating a instance of the handle class.
It is relatively simple to move from a singleton to an ordinary class at a later date.
The Implements feature is now available to allow you to have polymorphic singletons.
If you define this in an OLE server, you can export the handle class and get a PC-wide singleton (useful for sharing COM ports between apps).

The only thing I can not find an easy way to do is to allow the classes to raise events. The only solution I have is for the body to maintain a collection of all created handles and call a"private" function in each instance telling them to raise their event.

--KeithDerrick

Procedural ClassFactory function (not an object), with ActiveX protection of singleton class:

Here's how you do it. Slightly long winded, perhaps, but it works.

Create a new ActiveX DLL. Create in this your Singleton class with all its methods etc. Make sure you mark it 'Public not creatable'.

Add a .BAS module with one function:

Public Function getSingleton() as NameOfSingletonClass?

  Static Singleton as NameOfSingletonClass?
  If Singleton is Nothing then
    set Singleton= New NameOfSingletonClass?
    ' add any initialisation calls you want
  endif
  Set getSingleton = Singleton
  Exit Function

End Function

Create a new class; ensure it is marked MultiUse?. Give it one method:

public property Get SingletonInstance?() as NameOfSingletonClass?

  Set SingletonInstance? = getSingleton

end property

Add a reference to this AxDLL in your VB project and there's your singleton. What's so hard about that?

Took me < 5 mins to type into Wiki; add in compiles etc and there is about 10 mins overhead. -- SimonSmith

CategoryVbClassic