Proceedings of the Third International Workshop on Component - Oriented Programming ( WCOP ’ 98 ) Editors : Wolfgang Weck

ionComponentUI-Component ConnectionComponent ControlComponent DescriptionComponent Figure 4: Correspondence with PAC and DPACagents.also be employed when de-signing GUIs. Photon is anexample of an object-basedtool for creating GUIs forcontrol systems [7]. Themicro kernel and the wid-get library of the tool pro-vide the core of a user in-terface. The bottom layeris constructed using Pho-ton and graphic widgets inthe primary widget library.Medium-grained componentsof a graphical user interfaceexploit the module widget library and the module database property of the tool. Since acomponent database could be created and used only inside an application, a UI compo-nent has to be built separately for each DPAC agent. A user interface for a DPAC agentconsists of the following components: a GUI interface, a GUI handler component, andGUI components generated by the Photon appication builder. The GUI interface is de-rived from TaskPrimitives, OSPrimitives and QNXInterface classes and implementedby a C code. The GUI handler component is an additional control component for han-dling events and application messages. Finally, the GUI components were integratedwith the GUI handler and the GUI interface component. 4 Results and analysis As an application we used a simplified simulation model of a material transfer manage-ment functionality of a cell-based factory automation system. The control systems forthe cells were implemented as DPAC agents which communicate in a negotiation-basedmanner for the purpose of ordering material components from each other according tothe local needs of a cell agent. Agents inform each other on their external state or anychange in this, and service requests are sent to the partners, defined in a configurationdescription. The partners reply, according to their internal state, as to how they canmeet the request. Physically, the components of a DPAC agent, i.e. the user interfaceand the core component, are tasks which communicate through message queues.