Skip to content
Pharo language extension to have slots that announce changes
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


Pharo language extension to have slots that announce changes


Execute the following lines in a playground:

Metacello new
    baseline: 'ObservableSlot';
    repository: 'github://guillep/ObservableSlots/src';


Defining Observable Slots

Once the package is loaded, you will be able to create new classes with observable slots like this:

Object subclass: #ObservablePoint
	slots: { #x => ObservableSlot. 
				#y }
	classVariables: {  }
	package: 'ObservableSlot'

An observable slot works like a normal slot, and hides completely the announcement mechanism that is behind:

"Create Accessors"
ObservablePoint >> x: aNumber [
  x := aNumber
ObservablePoint >> x [
  ^ x

"Create an observable point and use it normally"
point := ObservablePoint new.
point x: 17.
self assert: point x equals: 17.

Subscribing to Slots Changes

To subscribe to the ObservableSlot changes, your object can use the TObservable trait, that contains some methods for subscription and explicit notification.

Object subclass: #ObservablePoint
  uses: TObservable
	slots: { #x => ObservableSlot. 
				#y }
	classVariables: {  }
	package: 'ObservableSlot'

Then we can subscribe to it doing

"Create an observable point and use it normally"
point property: #x whenChangedDo: [ "your code" ].

The method property:whenChangedDo: receives as first argument the name of the instance variable (as a symbol) and as second argument a block. The block has either a single parameter (where it will be passed the value that has just changed) or no parameter at all.

Manually Announce Slots Changes

Sometimes we want to have control on the announcements and manually announce that a value changed. For example, imagine a (not so) more complex graph like:

abObservablePoint -- x (observable) --> someObject -- someOtherInstVar (non-observable) --> someOtherObject

If someObject has its someOtherInstVar changed, we could want to announce that x changed too. That could be done by doing:

point notifyPropertyChanged: #x.


If the property we want to subscribe or notify to does not exist a SlotNotFound will be risen. If the property exists but is not observable a NonObservableSlotError will be risen.

Implementation Details

This implementation uses a ValueHolder to capture changes, and Slots to intercept reading and writing.

You can’t perform that action at this time.