Transcript Delegates & events

Delegates & events
• Observer pattern
• Delegates
– Semantics
– Cil – kode
– Anvendelse
• Events
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Delegates & events
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An example (from java!)
public class FrameMedKnap extends javax.swing.JFrame {
private javax.swing.JButton jButton1;
public FrameMedKnap() {
jButton1 = new javax.swing.JButton();
jButton1.setText("jButton1");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt)
{
jButton1ActionPerformed(evt);
}
});
this.add(jButton1, java.awt.BorderLayout.CENTER);
}
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt)
{
jButton1.setText("Der er trykket på knap");
}
}
.NET
Delegates & events
NOEA / PQC 2007
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Observer pattern
Also called Observable
in some litterature
Might also be a
interface
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Delegates & events
NOEA / PQC 2007
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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)
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• 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
What is what in the
java example?
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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
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System-defined types
Object
User-defined types
String
Array
Primitive types
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Boolean
Single
Byte
Double
Int16
Decimal
Int32
DateTime
Int64
TimeSpan
Char
Guid
ValueType
Exception
Delegate
Class1
Enum
Structure1
Multicast
Delegate
Class2
Delegate1
Class3
Enum1
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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
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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);
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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);
}
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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);
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Trin 4:
• Call the methods via the delegate:
notify ("Pingo");
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Demo
• Incl. ildasm
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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.
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Gui demo
• To see the use of event handlers
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Events (not gui)
1.
2.
3.
4.
5.
Declare a delegate with an appropriate signature
Declare the events a class can send
Define event handlers (methods)
Associate event and eventhandler
Trigger event by calling event handler by the event name.
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Step 1:
1. Declare a delegate with an appropriate signature
delegate void GreetingHandler(string sender);
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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;
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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);
}
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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
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Delegate
name in the
class
Delegates & events
Event handler
(method to be
called)
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Step 5:
• Call the methods by the event name:
// Somewhere in a class..
if(Hello != null) //Be sure there is an eventhandler
{
Hello("Pingo");
}
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What is the difference between delegate and event?
• From
http://en.csharponline.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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