Dynamic Variable

Smalltalk has a mechanism to simulate dynamic variables.

Smalltalk DynamicVariable

DynamicVariable subclass: #RegularDynamicVariable
    instanceVariableNames: ''
    classVariableNames: ''
    poolDictionaries: ''
    category: 'ActiveContext-Examples'

[
    RegularDynamicVariable value: 1 during:[
        self checkRegularDynamic: 1.
        (Delay forMilliseconds: 30) wait.
        self checkRegularDynamic: 1.            
        RegularDynamicVariable value: 3 during:[        
            (Delay forMilliseconds: 30) wait.
            self checkRegularDynamic: 3.            
        ].          
        self checkRegularDynamic: 1.
    ].
    self checkRegularDynamic: nil.
    p1stopped := true.
    s1 signal.
] fork.

With Active Context

Context subclass: #DynamicContextTest
    instanceVariableNames: 'targetVar targetVal'
    classVariableNames: ''
    poolDictionaries: ''
    category: 'ActiveContext-Examples'

The transfer function copy everything except targetVar which is initialized to targetVal.

DynamicContextTest>>transitionKeysFrom: keysFrom keysTo: keysTo
keysFrom keysAndValuesDo: [ :key :val |      
    ( keysTo at: key ) ifNil: [ 
        (key = targetVar ) 
            ifTrue: [ keysTo at: key put: targetVal ] 
            ifFalse: [ keysTo at: key put: (keysFrom at: key ) ].
    ].
]. 

The code is then similar, except that the dynamic variable can be accessed with value as usual.

[
    ( DynamicContextTest with: 1 ) do: [ 
        self checkContextDynamic: 1.
        (Delay forMilliseconds: 30) wait.
        self checkContextDynamic: 1.
        ( DynamicContextTest with: 3 ) do:[
            (Delay forMilliseconds: 30) wait.
            self checkContextDynamic: 3
        ].
        self checkContextDynamic: 1.
    ].
    self checkContextDynamic: 0.
    p1stopped := true.
    s1 signal.
] fork.