Implementing the observer pattern in Smalltalk/VisualWorks

I'm new to programming with patterns, i spent some hours searching for samples for Smalltalk implementation of the pattern observer but in vain. If someone can provide me with a concrete sample of the implementation of this pattern under Smalltalk, i will be thankful.

Solution

The standard implementataion of the observer pattern to be found in Smalltalk would be the #changed/#update mechanism.

It goes like this:

subject addDependent: anObserver.

subject changed.

and then anObserver gets send #update:

MyObservingObject>>update
  "I got called in a #changed chain"
  super update.
  self doUpdatingStuff

You can have finer control using #changed: and #update: (note the colon):

subject addDependent: anObserver.
subject changed: subject.

and

MyObservingObject>>update: anObject
  "I got called in a #changed: chain"
  super update: anObject.
  self doUpdatingStuffWith: anObject

However, it is commonly found to use a symbol to indicate what changed:

subject addDependent: anObserver.
subject changed: #myNumbers.

and

MyObservingObject>>update: anObject
  "I got called in a #changed: chain"
  anObject == #myNumbers ifTrue: [
    self doUpdatingStuffForNumbers.
    ^ self "inhibits the super"].
  super update: anObject.

When you look at Squeak or Pharo, you'll find at least three other Observer implementations:

The event handling for Morphic (see Morph>>#on:send:to:)
A similar, more general event handling mechanism, see Object>>#when:send:to: and Object>>#triggerEvent:
the Announcements framework, encapsulating messages between subject and observer in classes.

You can find a comparison of these at the Signals project, another implementation, but inspired by Qt.