Smalltalk
Also known as: Smalltalk-80
Smalltalk is a dynamically typed, purely object-oriented language (Xerox PARC; Smalltalk-80 is the usual reference dialect). Everything is an object; control flow is message sends. It pioneered live IDEs, MVC, and the dependents change-notification mechanism that the Gang of Four later catalogued as Observer.
Dependents (subject broadcasts change; views update):
"Model"
anAccount changed: #balance.
"Dependent / view"
MyView>>update: aSymbol
aSymbol = #balance ifTrue: [self refreshBalance].
Modern OOP and many UI toolkits still use the same subscribe / notify shape. See Krasner and Pope (1988) for the Smalltalk-80 MVC write-up.