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Presentation-abstraction-control (PAC) is a software architectural pattern, somewhat similar to model-view-controller (MVC). PAC is used as a hierarchical structure of agents, each consisting of a triad of presentation, abstraction and control parts. The agents (or triads) communicate with each other only through the control part of each triad. It also differs from MVC in that within each triad, it completely insulates the presentation (view in MVC) and the abstraction (model in MVC), this provides the option to separately multithread the model and view which can give the user experience of very short program start times, as the user interface (presentation) can be shown before the abstraction has fully initialized.

The structure of an application with PAC.

A subset or variation of PAC under the name Hierarchical-Model-View-Controller (HMVC) was published in an article^[1] in JavaWorld Magazine, the authors apparently unaware^[2] of PAC which was published 13 years earlier. The main difference between HMVC and PAC is that HMVC is less strict in that it allows the view and model of each agent to communicate directly, thus bypassing the controller.

• Model-view-controller
• PAC-Amodeus^[3], ^[4], ^[5]
• PAC*

• Coutaz, Joëlle (1987). "PAC: an Implementation Model for Dialog Design". H-J. Bullinger, B. Shackel (ed.) Proceedings of the Interact'87 conference, September 1-4, 1987, Stuttgart, Germany: pp. 431-436, North-Holland. 
• Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, Machael Stal (1996). Pattern-Oriented Software Architecture. John Wiley and Sons, pp. 145-168. ISBN 0-471-95869-7. 
• Gaëlle Calvary; Joëlle Coutaz, Laurence Nigay (1997). "From Single-User Architectural Design to PAC*: a Generic Software Architecture Model for CSCW". Pemberton, Steven (ed.) Proceedings of the ACM CHI 97 Human Factors in Computing Systems Conference. March 22-27, 1997, Atlanta, Georgia.: pp. 242-249. 
• Joëlle Coutaz (1997). "PAC-ing the Architecture of Your User Interface". DSV-IS’97, 4th Eurographics Workshop on Design, Specification and Verification of Interactive Systems: pp. 15-32, Springer Verlag. 
• Markopoulos, Panagiotis (1997). A compositional model for the formal specification of user interface software (pdf), PhD thesis, Queen Mary and Westfield College, University of London, p. 26. Retrieved on 2006-05-25. 
• Paris Avgeriou (2005). "Architectural patterns revisited - a pattern language". Proceedings of 10th European Conference on Pattern Languages of Programs (EuroPlop 2005), Irsee, Germany, July 2005: pp. 1-39. 

• Presentation-Abstraction-Control introduction by Patrick van Bergen.
• PAC description in the Portland Pattern Repository
• WengoPhone is a free software VoIP application that is written using the PAC design pattern.
  • description of PAC and motivation for use in WengoPhone.
  • demonstration code, courtesy of the OpenWengo community.

1. ^ Jason Cai, Ranjit Kapila, and Gaurav Pal (July, 2000). "HMVC: The layered pattern for developing strong client tiers". JavaWorld Magazine. Retrieved on 2006-05-25. 
2. ^ "TP" (2000). "Is HMVC PAC? (letter to the editor)". JavaWorld Magazine. Retrieved on 2006-05-25. 
3. ^ Laurence Nigay (1991). "Building User Interfaces: Organizing Software Agents.". ESPRIT '91 Conference. Brussels, Belgium: November 1991. 
4. ^ Laurence Nigay (January 1994). Conception et modélisation logicielles des systèmes interactifs : application aux interfaces multimodales (in French, with abstract in English). PhD dissertation, 315 pages, University of Grenoble, France, pp. 131-268. 
5. ^ Nigay, Laurence; Joëlle Coutaz (1997). "Software Architecture Modelling: Bridging Two Worlds Using Ergonomics and Software Properties.", Formal Methods in Human-Computer Interaction. Springer-Verlag, chapter 3, pp. 49-73. ISBN 3-540-76158-6.