Chair of Software Engineering JMLC 2003, 26.08.2003, Klagenfurt Austria Event Library: an object-oriented library for event-driven design Volkan Arslan,

JMLC 2003, 26.08.2003, Klagenfurt AustriaChair of Software Engineering

Event Library: an object-oriented library for event-driven design

Volkan Arslan, Piotr Nienaltowski, Karine Arnout

Chair of Software Engineering, ETH Zurich, Switzerland

http://se.inf.ethz.ch

{Volkan.Arslan, Piotr.Nienaltowski, Karine.Arnout}@inf.ethz.ch


Overview

Motivation

Use of the Event Library

Example: Event Library in action

Architecture of the Event Library

Conclusion and future work


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


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


Use of the Event Library

Using the Event Library consists only of 3 steps:

1. PUBLISHERdefines an event type

2. SUBSCRIBERsubscribes a feature of an object to an event type

3. PUBLISHERpublishes an event (of a certain event type)


Example: Event Library in action


Class SENSOR

class SENSOR

feature -- Access

temperature: INTEGER-- Container temperature

humidity: INTEGER-- Container humidity

pressure: INTEGER-- Container pressure

end -- class SENSOR


Class SENSOR

class SENSOR

feature -- Access


feature -- Element change

set_temperature (a_temperature: INTEGER) is-- Set temperature to a_temperature.

requirevalid_temperature: a_temperature > -100 and a_temperature <

1000 do

temperature := a_temperature ensure

temperature_set: temperature = a_temperature end

end -- class SENSOR


(1) Publisher defines an event type

class SENSOR

feature -- Access



set_temperature (a_temperature: INTEGER) is-- Set temperature to a_temperature....

feature -- Events

temperature_event: EVENT_TYPE [TUPLE [INTEGER]]-- Event associated with attribute temperature.

invarianttemperature_event_not_void: temperature_event /= Void

end -- class SENSOR

EVENT_TYPE [EVENT_DATA -> TUPLE]

[40, 60]

[25]

[]

conforms to

conforms to


(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


(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.

requirean_action_not_void:

an_action /= Voidan_action_not_already_subscribed:

not has (an_action)ensure

an_action_subscribed: count = old count + 1 and has (an_action)



(3) Publisher publishes an event

class SENSOR

feature -- Access




requirevalid_temperature: a_temperature > -100 and a_temperature < 1000

dotemperature := a_temperature temperature_event.publish ([temperature])

ensuretemperature_set: temperature = a_temperature

end

feature – Eventstemperature_event: EVENT_TYPE [TUPLE [INTEGER]]

end -- class SENSOR



class SENSOR…






end


end -- class SENSOR

Class EVENT_TYPE

publish (arguments: EVENT_DATA) is-- Publish all not suspended actions from the subscription list.

requirearguments_not_void: arguments /= Void




class SENSOR…






end


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



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)


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


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)


References

1. Arslan V.: Event Library, at http://se.inf.ethz.ch/people/arslan

2. Eiffel Software Inc.: Agents, iteration and introspection, athttp://archive.eiffel.com/doc/manuals/language/agent/agent.pdf

3. Eugster P. Th., Felber P., Guerraoui R., Kermarrec A.-M.: The Many Faces of Publish/Subscribe, Technical Report 200104 athttp://icwww.epfl.ch/publications/documents/IC_TECH_REPORT_200104.pdf

4. Gamma E., Helm R., Johnson R., Vlissides J.: Design Patterns: Elements of Reusable Object-Oriented Software, 1st edition, Addison-Wesley, 1995.

5. Meyer B.: The power of abstraction, reuse and simplicity: an object-oriented library for event-driven design, at http://www.inf.ethz.ch/~meyer/ongoing/events.pdf.

6. 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.


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Documents

Chair of Software Engineering JMLC 2003, 26.08.2003, Klagenfurt Austria Event Library: an object-oriented library for event-driven design Volkan Arslan,