Global state no more global

Classes are object as well, which means that global state isn't really global anymore.

CtxBuffer class
    instanceVariableNames: 'defaultSize'

CtxBuffer class>> new
^ super new size: self defaultSize .

The context is an object, which can be parametrized with the new value to fill in the class-side variable.

Context subclass: #ConfigContext
    instanceVariableNames: 'defaultSize'

ConfigContext>>transitionKeysFrom: instVarFrom keysTo: instVarTo
    | cls |
    cls := instVarFrom at: 'class'.
    ( cls = CtxBuffer class ) 
        ifTrue: [  
            instVarTo at: 'defaultSize' put: self defaultSize . 
            instVarTo at: 'class' put: cls. 
        ] 
        ifFalse: [ 
            super transitionKeysFrom: instVarFrom  keysTo: instVarTo   
        ].

And here is a sample test case:

ConfigeTest>>testBufferSize
    self assert: [ CtxBuffer new  size = 10 ].
    ( ConfigContext defaultSize: 20  ) do: [
        self assert: [ CtxBuffer new  size = 20 ].
    ]

A naive approach where the value of the class-side variable is changed to 20 then back to 10 manually would work in this particular test case but would fail in a multi-threaded scenario. While with active context, state isolation is correctly achieved in multi threaded scenario.