Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Event Library: an object-oriented library for event-driven design Volkan Arslan, Piotr Nienaltowski, Karine Arnout Chair of Software Engineering, ETH Zurich, Switzerland {Volkan.Arslan, Piotr.Nienaltowski,

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Overview  Motivation  Use of the Event Library  Example: Event Library in action  Architecture of the Event Library  Conclusion and future work

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Event-driven programming  Event-driven programming is very popular, especially for GUI applications  Facilitates the separation of concerns: application layer and GUI  Publish/Subscribe and Observer pattern follow the event-driven approach

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Event Library Make things as simple as possible, but not simpler. (A. Einstein) Turn the Observer pattern to a simple, reusable, but powerful library  Event Library

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Use of the Event Library Using the Event Library consists only of 3 steps: 1.PUBLISHER defines an event type 2.SUBSCRIBER subscribes a feature of an object to an event type 3.PUBLISHER publishes an event (of a certain event type)

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Example: Event Library in action

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Class SENSOR class SENSOR feature -- Access temperature: INTEGER -- Container temperature humidity: INTEGER -- Container humidity pressure: INTEGER -- Container pressure end -- class SENSOR

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Class SENSOR class SENSOR feature -- Access temperature: INTEGER -- Container temperature feature -- Element change set_temperature (a_temperature: INTEGER) is -- Set temperature to a_temperature. require valid_temperature: a_temperature > -100 and a_temperature < 1000 do temperature := a_temperature ensure temperature_set: temperature = a_temperature end end -- class SENSOR

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria (1) Publisher defines an event type class SENSOR feature -- Access temperature: INTEGER -- Container temperature feature -- Element change set_temperature (a_temperature: INTEGER) is -- Set temperature to a_temperature.... feature -- Events temperature_event: EVENT_TYPE [TUPLE [INTEGER]] -- Event associated with attribute temperature. invariant temperature_event_not_void: temperature_event /= Void end -- class SENSOR EVENT_TYPE [EVENT_DATA -> TUPLE] [40, 60] [25] [] conforms to

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria (2) Subscription to an event type In subscriber class MAIN_WINDOW Sensor.temperature_event.subscribe (agent application_window_1.display_temperature (?)) In the subscribed class APPLICATION_WINDOW display_temperature (a_temperature: INTEGER) is -- Update the text of temperature_value_label -- with a_temperature. do … end

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria (2) Subscription to an event type In class MAIN_WINDOW (Subscriber) Sensor.temperature_event.subscribe (agent application_window_1.display_temperature (?)) Class EVENT_TYPE subscribe (an_action: PROCEDURE [ANY, EVENT_DATA]) is -- Add an_action to the subscription list. require an_action_not_void: an_action /= Void an_action_not_already_subscribed: not has (an_action) ensure an_action_subscribed: count = old count + 1 and has (an_action) EVENT_TYPE [EVENT_DATA -> TUPLE]

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria (3) Publisher publishes an event class SENSOR feature -- Access temperature: INTEGER -- Container temperature feature -- Element change set_temperature (a_temperature: INTEGER) is -- Set temperature to a_temperature. require valid_temperature: a_temperature > -100 and a_temperature < 1000 do temperature := a_temperature temperature_event.publish ([temperature]) ensure temperature_set: temperature = a_temperature end feature – Events temperature_event: EVENT_TYPE [TUPLE [INTEGER]] end -- class SENSOR

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria (3) Publisher publishes an event class SENSOR … feature -- Element change set_temperature (a_temperature: INTEGER) is -- Set temperature to a_temperature. require valid_temperature: a_temperature > -100 and a_temperature < 1000 do temperature := a_temperature temperature_event.publish ([temperature]) ensure temperature_set: temperature = a_temperature end feature – Events temperature_event: EVENT_TYPE [TUPLE [INTEGER]] end -- class SENSOR Class EVENT_TYPE publish (arguments: EVENT_DATA) is -- Publish all not suspended actions from the subscription list. require arguments_not_void: arguments /= Void EVENT_TYPE [EVENT_DATA -> TUPLE]

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria (3) Publisher publishes an event class SENSOR … feature -- Element change set_temperature (a_temperature: INTEGER) is -- Set temperature to a_temperature. require valid_temperature: a_temperature > -100 and a_temperature < 1000 do temperature := a_temperature temperature_event.publish ([temperature]) ensure temperature_set: temperature = a_temperature end feature – Events temperature_event: EVENT_TYPE [TUPLE [INTEGER]] end -- class SENSOR Class EVENT_TYPE publish (arguments: EVENT_DATA) is -- Publish all not suspended actions from the subscription list. do do_all (agent {PROCEDURE [ANY, EVENT_DATA]}.call (arguments)) end EVENT_TYPE [EVENT_DATA -> TUPLE]

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Architecture Classification according to Eugster P. Th. [3]  Space decoupling The publisher and the subscribed class do not know each other  Flow decoupling Publishers should be not blocked, while publishing events  Time decoupling Publishers and subscribed objects do not have to participate actively in the interaction at the same time (e.g. distributed setting)

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Conclusion Event Library is simple to use  Consists of only one class EVENT_TYPE  Publisher, Subscriber and Subscribed object  Supports full event-driven programming  Type safe  For advanced needs, class EVENT_TYPE is extendible It relies on powerful language mechanism  Genericity (including constrained genericity)  Tuples  Agents  Inheritance

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Future work Extension of the Event Library  “Conditional Event subscription” possible when inline agents are introduced to the language  Use of concurrency (SCOOP) in order to obtain flow and time decoupling  Guaranteeing response times for subscribed objects when an event of a certain event type is triggered.  General publish-subscribe mechanism available as Web Service (time decoupling)

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria References 1.Arslan V.: Event Library, at 2.Eiffel Software Inc.: Agents, iteration and introspection, at Eugster P. Th., Felber P., Guerraoui R., Kermarrec A.-M.: The Many Faces of Publish/Subscribe, Technical Report at df df 4.Gamma E., Helm R., Johnson R., Vlissides J.: Design Patterns: Elements of Reusable Object-Oriented Software, 1st edition, Addison-Wesley, Meyer B.: The power of abstraction, reuse and simplicity: an object- oriented library for event-driven design, at Nienaltowski P., Arslan V.: SCOOPLI: a library for concurrent object- oriented programming on.NET, in Proceedings of the 1st International Workshop on C# and.NET Technologies, University of West Bohemia, 5-8 February 2003, Pilsen, Czech Republic.

Chair of Software Engineering JMLC 2003, , Klagenfurt Austria Questions ? Thank you for your attention!