Nmingott: imported material

2025-05-12T20:42:52Z

imported material

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* 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'''.

The Dependency Mechanism - part 3 - Revision history

Nmingott: imported material