Michael Papathomas

Video widgets and video actors

Video widgets are user-interface components rendered with video information. The implementation and several usage examples of a family of video widgets, called video actors,, are presented. Video actors rely on two capabilities of digital video: non-linear access, and the layering of video information. Non-linear access allows video frames to be displayed in mbitrary order without loss of continuity, layering allows two or more video streams to be spatially composed. Both capabilities are now becoming available to user-interface designers.

Towards a type theory for active objects

A key property of object-oriented languages is that they promote software reuse through inter-changeability or plug compatibility of software components that conform to the same message-passing interface. A characterization of such an interface is a type, and can be viewed as a constraint on the behaviour of an object. An object that conforms to the type meets the constraint. A subtype, in this view, is simply a stronger constraint: all objects that conform to a subtype also conform to the supertype. The nature of these constraints may vary, however, as may the rules for determining when one type is a subtype of another. The choice of characterization will depend on the computational model of a particular language and the way in which objects interact. We seek to develop a notion of type that will serve to characterize concurrent, active objects whose behaviour may not conform to a strict client/server model of interaction and communication.

Viewing object as patterns of communicating agents

Following our own experience developing a concurrent object-oriented language as well of that of other researchers, we have identified several key problems in the design of a concurrency model compatible with the mechanisms of object-oriented programming. We propose an approach to language design in which an executable notation describing the behaviour of communicating agents is extended by syntactic patterns that encapsulate language constructs. We indicate how various language models can be accommodated, and how mechanisms such as inheritance can be modeled. Finally, we introduce a new notion of types that characterizes concurrent objects in terms of their externally visible behaviour.

A Unifying Framework for Process Calculus Semantics of Concurrent Object-Oriented Languages

A framework for the semantic description of concurrent object-oriented languages based on CCS is outlined. We discuss how the essential object-oriented features, such as encapsulation, object identity, classes, inheritance and concurrency are captured. Then, the proposed framework is used for defining the semantics of significantly different versions of a toy language which supports the above features. The ease with which the different versions of this language are accommodated provides some evidence for the applicability of the framework for a wide range of languages, as well as its usefulness for comparing different language designs and examining the interaction of a rich set of object-oriented features.