View source for The Dependency Mechanism - part 3

* In Cuis the dependency is handled with the  Observer pattern
* In the observer pattern an object called '''subject''' maintains a list of '''observers'''. The '''subject''' notifies its '''observers''' when it changes.
* In Cuis the Parent-Dependent pattern is implemented with the Subject-Observer this way

{| class="wikitable"
|
|'''Parent-Dependent'''
|'''Subject-Observer'''
|-
|connection making. '''objB''' wants to be informed of changes in '''objA'''.
|<code>objA addDependent: objB</code>
|<code>objA when: #changed send: #update to: objB</code>
|-
|'''objA''' tells the objects who are watching it that it changed state.
|<code>objA changed: #param</code>
|<code>objA triggerEvent: #changed with: #param</code>
|-
|when  '''objA''' state changes the message <code>update</code> is sent to '''objB''' with a parameter.
|<code>objB update: #param</code>
|<code>objB update: #param</code>
|-
|we want '''objB''' to stop watching '''objA'''
|<code>objA removeDependent: objB</code>
|<code>objA removeActionWithReceiver: objB forEvent: #changed</code>
|}

* Observe that the parameter <code>#changed</code> is used explicitly in Cuis to build Parent-Dependent pattern, but that is actually a free parameter in the Observer model.
* In Cuis6 '''Model''' is substituted by '''ActiveModel''', if you look into the implementation of the Parent-Dependent pattern you will see that dependencies between objects go by default stored in class variable '''ActionMaps''' of '''ActiveModel''' . This instead of using class variable '''DependentsFields''' of class '''Object''', as done in the classic implementation, for example in VisualWorks and Squeak.
* '''Exercise'''. Use <code>ActiveModel actionMaps</code> to explore the '''ActionMaps''' class variable content.
* Mirroring our previous example in the Parent-Dependent model, we impolement of two simple classes (in the Subject-Observer model) '''MyActiveNumber''' and '''MyObserverNumber''' for objects '''objA''' and '''objB''', where '''objB''' is observer for '''objA'''. We want to keep the dependency mechanism localized so '''MyActiveNumber''' will be subclass of '''ActiveModel''' (instead of '''Object'''). When '''objA''' changes we want '''objB''' to receive the message <code>update</code>, we don't want to pass any parameter, '''objB''' will ask what it need to '''objA'''. You can get the fileout here: MyActiveNumber.st and MyObserverNumber.st .

 ActiveModel subclass: #MyActiveNumber
     instanceVariableNames: 'value'
 
 MyActiveNumber >> initialize
     value := 0. 
     ^ self 
 
 MyActiveNumber >> value 
         ^ value 
 
 MyActiveNumber >> value: aNumber        
         value := aNumber
         self triggerEvent: #iChanged 
 
 Object subclass: #MyObserverNumber
     instanceVariableNames: 'value model'  
 
 MyObserverNumber class >> newWithModel: anObject
     ^ super new initializeWithModel: anObject
 
 MyObserverNumber >> initializeWithModel: anObject
     value := 0.
     model := anObject . 
     model when: #iChanged send: #update  to: self .  
     ^ self     
 
 MyObserverNumber update
     |vPar vMe |
      vPar := model value. 
      vMe := vPar * 2.
      value := vMe.
      Transcript show: ('my MODEL value is:{1} and mine is value:{2}' format: {vPar. vMe} ). 
 
 MyObserverNumber value
          ^ value                                            

* Then try everything works with a little interaction from the Workspace

 objA := MyActiveNumber new. 
 objB := MyObserverNumber newWithModel: objA. 
 
 objA value .  "=> 0"
 objB value.   "=> 0"
 
 objA value: 10.  
 
 objB value. "=> 20" 

* '''Exercise'''. explore '''objA''' and convince your self it contains the map toward object '''objB'''.