In our efforts to develop an object-oriented visual programming language, the dataflow model of computation is
extended from its traditional functional model to an object-oriented model. It is argued that the concept of
subroutine in the object-oriented model requires two different types of calling (activation) mechanisam,
synchronous call and asynchronous call. Asynchronous subroutine call offers a new abstraction mechanism for
object-oriented programming, different from the traditional class-based abstraction and functional abstraction.