13. delegates & events
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1 .NET Delegates & events NOEA / PQC 2009
13. Delegates & events
• Observer pattern• Delegates
– Semantics– Cil – kode– Anvendelse
• Events
2 .NET Delegates & events NOEA / PQC 2009
Observer pattern Also called Observable in some litteratureMight also be a
interface
3 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / PQC 2009
St r i ng Ar r ay Val ueType Except i on Del egat e Cl ass1
Mul t i castDel egat e
Cl ass2
Cl ass3
Obj ect
Enum1
St r uct ur e1EnumPr i mi t i ve t ypes
Bool ean
Byt e
I nt 16
I nt 32
I nt 64
Char
Si ngl e
Doubl e
Deci mal
Dat eTi me
System-defined types
User-defined types
Del egat e1
Ti meSpan
Gui d
7 .NET Delegates & events NOEA / PQC 2009
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 parameterThe method will be encapsulated in the delegate
4. Call the method by the delegate
8 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 containerNotifier 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 .NET Delegates & events NOEA / PQC 2009
Trin 4:
• Call the methods via the delegate:
notify ("Pingo");
12 .NET Delegates & events NOEA / PQC 2009
Demo
• Incl. ildasm
13 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / PQC 2009
Gui demo
• To see the use of event handlers
15 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / PQC 2009
Step 1:
1. Declare a delegate with an appropriate signature
delegate void GreetingHandler(string sender);
17 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / 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 .NET Delegates & events NOEA / PQC 2009
What is the difference between delegate and event?
• Fromhttp://en.csharp-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..
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