After my blog post last week on LotusScript classes and using
Static Property Get to create singletons, there was some discussion on OpenNTF’s Discord about the challenges of forcing use of the singleton. “Singleton” is a misnomer really, because it’s not scoped to the JVM asaJava singleton would be. But I can’t think of a better name yet, so I’ll stick with that terminology, but be aware of the specific scope for static instances in LotusScript / VoltScript.
Firstly, it’s not possible to create a public static property of a private class. So you cannot hide the class in the script file. And the
New method is a Sub, so I couldn’t find a way to define what the current instance is during the New sub, and so always return the same instance. That of course is possible in an LSX, as shown with the NotesSession class. No matter how many times you use
Dim session as New NotesSession or
Dim s as New NotesSession, every time you are returning the same instance of the NotesSession class - it is truly a LotusScript singleton.
I played about with a few scenarios, and the best I could come up with was a way to prevent users creating new instances. First the script file needs a private global variable for whether or not the call is from the static instance:
Private fromInstance as Boolean.
Next, we need the static public property of our class - we’ll use a class called
Config as in the previous blog post. The code for our public property is this:
Public Property Get Config As Config Static this As Config If (this Is Nothing) Then fromInstance = true Set this = New Config() fromInstance = false End If Set Config = this End Property
This looks very similar to what we had before, except before instantiating the
Config class, we set the global
fromInstance variable to true, then reset it to false. It’s a global and global variables persist, so we need to reset it.
Now, in the constructor for the Config class, we just need to check the
fromInstance variable, and throw an error accordingly:
Sub New If (Not fromInstance) Then Error 1403, "Config cannot be instantiated in this way, use FooInstance variable" End If End Sub
Now if you use
Dim Config as New Config(), you will get a runtime error. If you use
Print Config.version, you will be fine.