Merge pull request #354 from seandenigris/bug_property-arg-mismatch

[Bug]: Property Owner Trait - Only Announce Change After Put

source/Magritte-Model/MATPropertyOwner.trait.st

@@ -142,18 +142,20 @@ MATPropertyOwner >> propertyAt: aKey ifPresent: oneArgBlock ifAbsent: absentBloc
 { #category : #accessing }
 MATPropertyOwner >> propertyAt: aKey ifPresent: oneArgBlock ifAbsentPut: absentBlock [
 	"Lookup ==aKey==, if it is present, answer the value of evaluating the oneArgBlock with the value associated with the key, otherwise answer the value of absentBlock."
-	| anOldValue aNewValue |
-	anOldValue := nil.
-	aNewValue := self properties 
+
+	| aValue didPut |
+	didPut := false.
+	aValue := self properties 
 		at: aKey 
-		ifPresent: [ :aValue | 
-			anOldValue := aValue. 
-			oneArgBlock value ]
-		ifAbsentPut: absentBlock.
+		ifPresent: oneArgBlock
+		ifAbsentPut: [
+			didPut := true.
+			absentBlock value ].
 
-	self propertyAnnounceKey: aKey oldValue: anOldValue newValue: aNewValue.
-
-	^ aNewValue
+	didPut ifTrue: [ 
+		self propertyAnnounceKey: aKey oldValue: nil newValue: aValue ].
+
+	^ aValue
 ]
 
 { #category : #accessing }