1.NETDelegates & eventsNOEA / PQC Delegates & events Observer pattern Delegates –Semantics –Cil – kode –Anvendelse Events

2.NETDelegates & eventsNOEA / PQC 2009 Observer pattern Also called Observable in some litterature Might also be a interface

3.NETDelegates & eventsNOEA / PQC 2009 Callback interfaces Interfaces describes a common behavior that is shared by different classes Used in Observer pattern to construct a callback mecanism –The key is the Update() method in the Observer interface. –Note that it do not have to be named Update ;-) Callback can be used in languages that supports interfaces Callback can be implemented with function pointers in C (But then it is not by using observer pattern)

4.NETDelegates & eventsNOEA / PQC 2009 Subject (or Observable) implements an add and a remove method to add and remove observers to/from an internal container (typical an arraylist) Has also a notify() method that calls update() on all observer object in the container Observer implements the update() method. That will be the event handler in an event based system

5.NETDelegates & eventsNOEA / PQC 2009 Delegate Delegate is a new language construction in C# Java programmers will compare it to observer pattern C/C++, Pascal/Delphi… programmers will compare it to function pointers A delegate gives the possibility to pass a method of a class as a parameter to objects of another class. The method can be a static or an instance method Delegates are object oriented, typesafe and undependent of method name. Delegates are objects itselves of classes that iniherit from System.Delegate via System.MulticastDelegate

6.NETDelegates & eventsNOEA / PQC 2009

7 4 steps to create and use delegates 1.Declare a delegate with an appropiate signature 2.Define method(s) with the same signature 3.Instansiate a delegate object with a method as parameter The method will be encapsulated in the delegate 4.Call the method by the delegate

8.NETDelegates & eventsNOEA / PQC 2009 Step 1: 1.Declare a delegate with an appropiate signature. That means with the same parameters and return values as the method to be encapsulated delegate void Notifier(string sender);

9.NETDelegates & eventsNOEA / PQC 2009 Step 2: Define method with the same signature as the delegate void SayHello (string sender) { Console.WriteLine("Hello from "+ sender); } void SayGoodBye (string sender) { Console.WriteLine("Good Bye from "+ sender); }

10.NETDelegates & eventsNOEA / PQC 2009 Step 3: Instanstiate the delegate object and add the methods Note: Static methods might be used as well. HelloGoodbye MyObject=new HelloGoodbye(); //Instanstiate delegate and thereby a container Notifier notify = new Notifier(MyObject.SayHello); //Add a new Notifier to the container (multicast); notify += new Notifier(MyObject.SayGoodBye); //For the example: remove Notifier: notify -= new Notifier(SayHello);

11.NETDelegates & eventsNOEA / PQC 2009 Trin 4: Call the methods via the delegate: notify ("Pingo");

12.NETDelegates & eventsNOEA / PQC 2009 Demo Incl. ildasm

13.NETDelegates & eventsNOEA / PQC 2009 Events Delegates are often used for event based systems The Event keyword in C# makes it little easier The mecanism behind is the same. A event is a kind of delegate. Events are often used in object oriented gui programs Events can be used with advantage in other areas such as simulation and realtime systems.

14.NETDelegates & eventsNOEA / PQC 2009 Gui demo To see the use of event handlers

15.NETDelegates & eventsNOEA / PQC 2009 Events (not gui) 1.Declare a delegate with an appropriate signature 2.Declare the events a class can send 3.Define event handlers (methods) 4.Associate event and eventhandler 5.Trigger event by calling event handler by the event name.

16.NETDelegates & eventsNOEA / PQC 2009 Step 1: 1.Declare a delegate with an appropriate signature delegate void GreetingHandler(string sender);

17.NETDelegates & eventsNOEA / PQC 2009 Trin 3: Declare the events a class can send. Events are members in the class they are generated from public static event GreetingHander Hello; public static event GreetingHander GoodBye;

18.NETDelegates & eventsNOEA / PQC 2009 Step 3: Define methods with the same signature as the delegate void OnHello (string sender) { Console.WriteLine("Hello from "+ sender); } void OnGoodBye (string sender) { Console.WriteLine("Good Bye from "+ sender); }

19.NETDelegates & eventsNOEA / PQC 2009 Trin 4: Instanstiate the delegate object and add the methods that shall be called MinKlasse.Hello+=new MinKlasse.GreetingHandler(OnHello); MinKlasse.GoodBye+=new MinKlasse.GreetingHandler(OnGoodBye); Event name in the class Delegate name in the class Event handler (method to be called)

20.NETDelegates & eventsNOEA / PQC 2009 Step 5: Call the methods by the event name: // Somewhere in a class.. if(Hello != null) //Be sure there is an eventhandler { Hello("Pingo"); }

21.NETDelegates & eventsNOEA / PQC 2009 What is the difference between delegate and event? From online.net/CSharp_FAQ:_What_is_the_difference_between_delegate_and _event online.net/CSharp_FAQ:_What_is_the_difference_between_delegate_and _event An event offers more restricted access than a delegate. When an event is public, other classes are only able to add or remove handlers for that event: such classes cannot—necessarily—fire the event, locate all the handlers for it, or remove handlers of which they are unaware..