1. Chair of Software Engineering 1 Introduction to Programming Tuples and Agents Exercise Session 1 and 2 December 2008

2. Chair of Software Engineering 2 This week  Tuples  Agents  Observer Pattern (revisited)

3. Chair of Software Engineering Tuples  A tuple of type TUPLE [A, B, C] is a sequence of at least three values, first of type A, second of type B, third of type C.  In this case possible tuple values that conform are:  [a 1, b 1, c 1 ], [a 1, b 1, c 1, x 1 ],... where a i is of type A, b i of type B, c i of type C and x i of some type X  Tuple types (for any types A, B, C,... ): TUPLE TUPLE [A] TUPLE [A, B] TUPLE [A, B, C]...

4. Chair of Software Engineering Labeled Tuples  Tuples may be declared with labeled arguments: tuple: TUPLE [food: STRING; quantity: INTEGER]  Same as an unlabeled tuple: TUPLE [STRING, INTEGER] but provides easier (and safer!) access to its elements: May use io.print (tuple.food) ‏ instead of io.print (tuple.item(1)) ‏

5. Chair of Software Engineering tuple_safety_test local t1: TUPLE [STRING, INTEGER] t2: TUPLE [food: STRING; quantity: INTEGER] s : STRING do t1 := [“chicken”, 66] t2 := [“chicken”, 66] s := t1.item(1) s := t2.item(1) s := t2.food ‏‏ end Labeled vs. unlabeled tuples Hands-On

6. Chair of Software Engineering Tuple type inheritance TUPLE [A, B] TUPLE [A] TUPLE

7. Chair of Software Engineering tuple_conformance local t0: TUPLE t2: TUPLE [INTEGER, INTEGER] do create t2 t2 := [10, 20] t0 := t2 print (t0.item (1).out + "%N") ‏ print (t0.item (3).out) ‏ end Tuple conformance Compiles, but precondition violation at runtime Hands-On Not necessary in this case Implicit creation

8. Chair of Software Engineering What are agents in Eiffel?  Objects that represent operations  Can be seen as operation wrappers  Similar to delegates in C#, closures in Java, functors in C++

9. Chair of Software Engineering Agent definition  Every agent has an associated routine, the one that the agent wraps and is able to invoke  To get an agent, use the agent keyword e.g. an_agent := agent my_routine  This is called agent definition  What’s the type of an_agent?

10. Chair of Software Engineering EiffelBase classes representing agents ROUTINE  PROCEDURE FUNCTION call   deferred effective   item PREDICATE 

11. Chair of Software Engineering Open and closed agent arguments  An agent can have both “closed” and “open” arguments:  closed arguments set at agent definition time  open arguments set at agent call time.  To keep an argument open, replace it by a question mark u := agent a0.f (a1, a2, a3) -- All closed w := agent a0.f (a1, a2, ?) ‏ x := agent a0.f (a1, ?, a3) y := agent a0.f (a1, ?, ?) z := agent a0.f (?, ?, ?) -- All open

12. Chair of Software Engineering  An agent invokes its routine using feature “call” f (x1: T1; x2: T2; x3: T3) -- defined in class C with --a0: C; a1: T1; a2: T2; a3: T3 u := agent a0.f (a1, a2, a3) ‏ Agent calls v := agent a0.f (a1, a2, ?) u.call ([]) ‏ v.call ([a3]) ‏ w := agent a0.f (a1, ?, a3) w.call ([a2]) ‏ x := agent a0.f (a1, ?, ?) x.call ([a2, a3]) ‏ y := agent a0.f (?, ?, ?) y.call ([a1, a2, a3]) ‏

13. Chair of Software Engineering Agent declaration p: PROCEDURE [ANY, TUPLE] -- Agent representing a procedure belonging to a -- class that conforms to ANY. At least 0 open -- arguments q: PROCEDURE [C, TUPLE [X, Y, Z]] -- Agent representing a procedure belonging to a --class that conforms to C. At least 3 open arguments f: FUNCTION [ANY, TUPLE [X, Y], RES] -- Agent representing a function belonging to a class -- that conforms to ANY. At least 2 open arguments, -- result of type RES

14. Chair of Software Engineering Example class ANIMAL create make feature {NONE} -- Initialization make (s: STRING) ‏ -- Create a new animal with name `s'. do name := s end feature -- Access name: STRING feature -- Basic operations eat (food: STRING; quantity: INTEGER) ‏ -- Eat `quantity' of `food'. do print (name + " eats " + quantity.out + " " + food + ".%N") ‏ end... See next page end

15. Chair of Software Engineering Eating with an agent Fill in the blanks, so that when eat_wrapper is called on an animal called “Mew”, the text “Mew eats 5 grass” is printed. class ANIMAL... (same as before) ‏ eat_wrapper -- Call `eat' with an agent. local a: do a := a.call (["grass", 5]) ‏ end Hands-On agent eat (?,?) ‏ PROCEDURE [ANIMAL, TUPLE [STRING, INTEGER]]

16. Chair of Software Engineering Eating with an agent (2) Fill in the blanks again, so that the text “Mew eats 5 grass” is printed. class C feature r local a: ANIMAL an_agent: do create a.make (“Mew") ‏ an_agent := an_agent.call (["grass", 5]) ‏ end Hands-On PROCEDURE [ANIMAL, TUPLE [STRING, INTEGER]] agent a.eat

17. Chair of Software Engineering Eating with an agent (3) class C feature r local a: ANIMAL an_agent: PROCEDURE [ANIMAL, TUPLE] do create a.make (“Mew") ‏ an_agent := an_agent. end Hands-On agent a.eat (“grass”, 5) ‏ call ([]) ‏

18. Chair of Software Engineering Eating with an agent (4) class C feature f do r (“grass”, 5) ‏ end r (s: STRING; i: INTEGER) ‏ local an_agent: PROCEDURE [ANIMAL, TUPLE [STRING, INTEGER]] a: ANIMAL do create a.make (“Mew") ‏ an_agent := an_agent. end Hands-On agent a.eat (?, ?) ‏ call ([s, i]) ‏

19. Chair of Software Engineering Open targets  It is also possible to leave the target of an agent call open, to increase flexibility  This is done at agent definition an_agent := {MY_TYPE} agent my_routine  MY_TYPE is the type to which my_routine belongs

20. Chair of Software Engineering Eating with an agent (5) class C feature f local a: ANIMAL do create a.make (“Mew”) ‏ r (a, “grass”, 5) ‏ end r (a: ANIMAL; s: STRING; i: INTEGER) ‏ local an_agent: PROCEDURE [ANIMAL, TUPLE [ANIMAL, STRING, INTEGER]] do an_agent := an_agent. end Hands-On agent {ANIMAL}.eat (?, ?) ‏ call ([a, s, i]) ‏

21. Chair of Software Engineering Why do we need agents anyway?  Suppose that depending on a certain event that may happen, you have to call one routine out of many  Suppose you don’t know exactly which routine to invoke, because you have to (or want to) leave the choice to your clients

22. Chair of Software Engineering Another example  Consider you are designing a GUI.  When a button in the GUI is pressed, several other classes may be interested in this event.  However, you shouldn’t hard-code in the BUTTON class which routines of which classes will be called when the button is pressed.  Rather, you should let any classes interested in this event register one of their routines to be called when the button is pressed.  These routines will be passed to the BUTTON class as agents and will be called when the button is pressed.  And this brings us to the Publish-Subscribe Pattern…

23. Chair of Software Engineering The Publish-Subscribe Pattern PUBLISHER * PUB_1 SUBSCRIBER * SUB_1 update* update + Deferred (abstract) Effective (implemented) * + Inherits from Client (uses) subscribe + unsubscribe + subscribed: LIST […] attach detach + + SUB_2 … PUB_2 … 23 trigger

24. Chair of Software Engineering The Publish-Subscribe pattern Publisher keeps a list of observers: subscribed: LINKED_LIST [SUBSCRIBER] To register itself, an observer may execute subscribe (some_publisher) ‏ where subscribe is defined in SUBSCRIBER: subscribe (p: PUBLISHER) ‏ -- Make current object observe p. require publisher_exists: p /= Void do p. attach (Current) ‏ end

25. Chair of Software Engineering Attaching an subscriber In class PUBLISHER: attach (s: SUBSCRIBER) ‏ -- Register s as subscriber to current publisher. require subscriber_exists: s /= Void do subscribed. extend (s) ‏ end The invariant of PUBLISHER includes the clause subscribed /= Void (List subscribed is created by creation procedures of PUBLISHER) ‏

26. Chair of Software Engineering trigger -- Ask all observers to -- react to current event. do from subscribed.start until subscribed.after loop subscribed.item. subscribed.forth end end Each descendant of SUBSCRIBER defines its own version of update Triggering an event update PUBLISHER * GUI_CLASS attach detach SUBSCRIBER * APP_CLASS update* update+

27. Chair of Software Engineering Limitations of the pattern  Each publisher object knows about its observers  There is only one update procedure in SUBSCRIBER:  At most one operation  Not reusable — must be coded anew for each application 27

28. Chair of Software Engineering Exercise Write a program that handles a GUI with 2 buttons. When the first button is pressed, a counter gets increased, while when the second button is pressed, the same counter gets decreased. The value of the counter is printed in both cases to the console.  Provide 2 solutions, one that does not use the agents and one that uses them.  Discuss the differences Hands-On 28

29. Chair of Software Engineering 29 Backup slides

30. Chair of Software Engineering Exercise: a news agency  Consider you must create an application which allows a news agency to dispatch news to various papers, radio and TV stations, etc.  How would you design and implement this application? Hands-On

31. Chair of Software Engineering Exercise You are given a working program with  1 effective publisher: BUTTON  3 effective subscribers: PRINTER, COUNTER, and RANDOM_NUM  subscribers react on (simulated) button clicks Your task is to  merge 3 subscribers into one class: BUTTON_LISTENER  keep update features separate (rename them)  pass the renamed update features as agents to publisher Hands-On 31