C#  Event  Handling  

Jussi  Pohjolainen  Tampere  University  of  Applied  Sciences  

Method  call  Directly   using System; public class MyClass { public void Method() { Console.WriteLine("Method() begin"); Console.WriteLine("Method() end"); } } public class Test { static void Main(string[] args) { MyClass myObject = new MyClass(); myObject.Method(); } }

About  Delegates  

•  A  delegate  lets  you  pass  a  funcCon  as  a  parameter!  

•  “Anonymous”  invocaCon  •  You  can  call  a  method  X()  and  define  the  X  to  be  whatever  during  runCme!  

Basic  idea  behind  Delegate:  A  delegate  will  allow  us  to  specify  what  the  

func4on  we'll  be  calling  looks  like  without  having  to  specify  which  func4on  to  call!  

Examples  

•  public delegate void MyDelegate(); – MyDelegate  encapsulates  any  method  with  no  parameters  and  no  return  

•  public delegate int ButtonClicked(object o1, object o2);

– BuPonClicked  encapsulates  any  method  with  return  type  int  and  two  object  parameters  

•  Just  like  a  declaraCon  of  a  method!  

using System; // Declaration of the delegate public delegate void MyDelegate(); class TestDelegate { public static void MyMethod() { Console.WriteLine("Delegate calling!"); } public static void Main() { // Instantiation. MyDelegate encapsulates the method! MyDelegate myDelegate = new MyDelegate(MyMethod); // Invocation.. calls MyMethod! myDelegate(); } }

using System; // Declaration of the delegate public delegate void MyDelegate(); class TestDelegate { public static void MyMethod() { Console.WriteLine("Delegate calling!"); } public static void Main() { // Instantiation. MyDelegate encapsulates the method! MyDelegate myDelegate = new MyDelegate(MyMethod); // Invocation.. calls MyMethod! myDelegate();

// MyDelegate extends class System.Delegate! Console.WriteLine( myDelegate.ToString() ); Console.WriteLine( myDelegate.GetInvocationList().Length ); } }

// Declaration of the delegate public delegate void MyDelegate(); class TestDelegate { public static void A() { Console.WriteLine("A()"); } public static void B() { Console.WriteLine("B()"); } public static void Main() { MyDelegate myDelegate1 = new MyDelegate(A); MyDelegate myDelegate2 = new MyDelegate(B); myDelegate1(); // A() myDelegate2(); // B() } }

// Declaration of the delegate public delegate string MyDelegate(string someParameter); class TestDelegate { public static string A(string s) { Console.WriteLine("A()"); return s; } public static void Main() { MyDelegate myDelegate = new MyDelegate(A); Console.Write( myDelegate("hello") ); // A() hello } }

// Declaration of the delegate public delegate string MyDelegate(string someParameter); class TestDelegate { public static string A(string s) { Console.WriteLine("A()"); return s; } public static void Main() { MyDelegate myDelegate = new MyDelegate(A); someMethod(myDelegate); } public static void someMethod( MyDelegate md ) { Console.Write( md("hello") ); // A() hello } }

Passing  object  

// MULTICASTING // A multicast delegate maintains a list of functions that will // all be called when the delegate is invoked. // Declaration of the delegate public delegate void MyDelegate(); class TestDelegate { public static void A() { Console.WriteLine("A()"); } public static void B() { Console.WriteLine("B()"); } public static void Main() { MyDelegate myDelegate = new MyDelegate(A); myDelegate += new MyDelegate(B); myDelegate(); // A() B() } }

public delegate void MyDelegate(); class TestDelegate { public static void A() { Console.WriteLine("A()"); } public static void B() { Console.WriteLine("B()"); } public static void Main() { MyDelegate myDelegate = A; myDelegate += B; myDelegate(); // A() B() } }

System.Delegate  

•  Delegate  types  are  derived  from  System.Delegate  or  MulCcastDelegate  (subclass  of  Delegate)  

  public static void Main()

{

MyDelegate myDelegate = A;

myDelegate += B;

myDelegate(); // A(), B()

int invocationCount = myDelegate.GetInvocationList().GetLength(0);

Console.Write(invocationCount); // 2

}

EVENTS  

About  Events  

•  NoCficaCon  system  •  Usually  in  GUI:  buPon  clicked  -­‐>  something  happens  

•  Events  are  declared  using  delegates  

In  Java  

•  In  Java:  – buPon.addAcConListener(  someObjectThatImplementsAcConListener  );  

•  In  C#  – buPon.ClickedEvent  +=  MyEventHandler;  

Listener  class Listener { public void MyMethod1() { System.Console.WriteLine("MyMethod1"); } public void MyMethod2() { System.Console.WriteLine("MyMethod2"); } public void MyMethod3() { System.Console.WriteLine("MyMethod3"); } }

public delegate void MyEventHandler(); class GUIWindow { public event MyEventHandler ClickedEvent; // Declare a method that triggers the event: public void ClickHasHappened() { ClickedEvent(); // Calls now MyMethod1, MyMethod2 and MyMethod3 } static void Main() { GUIWindow window = new GUIWindow(); Listener listener = new Listener(); // Subscribe to the event by associating the handlers with the event: window.ClickedEvent += new MyEventHandler(listener.MyMethod1); window.ClickedEvent += new MyEventHandler(listener.MyMethod2); window.ClickedEvent += new MyEventHandler(listener.MyMethod3); // Trigger the event: window.ClickHasHappened(); } }

About  event  

•  What  happens  here?  –  public event MyEventHandler ClickedEvent;  

•  Events  are  a  special  kind  of  mulCcast  delegate  that  can  only  be  invoked  from  within  the  class  or  struct  where  they  are  declared  (the  publisher  class  

class GUIWindow { // Declare the event of type MyEventHandler. Event handlers // for TriggerIt must have the same signature as MyEventHandler. public event MyEventHandler ClickedEvent; // Declare a method that triggers the event: public void ClickHasHappened() { ClickedEvent(); // Calls now MyMethod1, MyMethod2 and MyMethod3 } static void Main() { GUIWindow window = new GUIWindow(); Listener listener = new Listener(); // Subscribe to the event by associating the handlers with the event: window.ClickedEvent += new MyEventHandler(listener.MyMethod1); window.ClickedEvent += new MyEventHandler(listener.MyMethod2); window.ClickedEvent += new MyEventHandler(listener.MyMethod3); // Trigger the event: window.ClickHasHappened(); window.ClickedEvent(); // Works! Inside GUIWindow class!! } }

class GUIWindow { // Declare the event of type MyEventHandler. Event handlers // for TriggerIt must have the same signature as MyEventHandler. public event MyEventHandler ClickedEvent; // Declare a method that triggers the event: public void ClickHasHappened() { ClickedEvent(); // Calls now MyMethod1, MyMethod2 and MyMethod3 } } class App { static void Main() { GUIWindow window = new GUIWindow(); Listener listener = new Listener(); // Subscribe to the event by associating the handlers with the event: window.ClickedEvent += new MyEventHandler(listener.MyMethod1); window.ClickedEvent += new MyEventHandler(listener.MyMethod2); window.ClickedEvent += new MyEventHandler(listener.MyMethod3); // Trigger the event: window.ClickHasHappened(); window.ClickedEvent(); // DOES NOT WORK! Outside of class GUIWindow! } }

CreaCng  the  Listener  Method  

•  Usually  listener  method:  – public delegate void MyEventHandler(object sender, EventArgs e);

•  Listener  receives  informaCon:  – Who  is  the  publisher?  (Sender)  – What  kind  of  arguments  did  the  sender  pass?  

App  class App { static void Main() { GUIWindow window = new GUIWindow(); Listener listener = new Listener();

// Subscribe to the event by associating the handlers with the event: window.ClickedEvent += new MyEventHandler(listener.MyMethod1); window.ClickedEvent += new MyEventHandler(listener.MyMethod2); window.ClickedEvent += new MyEventHandler(listener.MyMethod3); // Trigger the event: window.ClickHasHappened(); } }

Listener  class Listener

{

public void MyMethod1(object sender, EventArgs e)

{

System.Console.WriteLine("Sender {0}, MyMethod1", sender.ToString());

}

public void MyMethod2(object sender, EventArgs e)

{

System.Console.WriteLine("Sender {0}, MyMethod2", sender.ToString());

}

public void MyMethod3(object sender, EventArgs e)

{

System.Console.WriteLine("Sender {0}, MyMethod3", sender.ToString());

}

}

Publisher  public delegate void MyEventHandler(object sender, EventArgs e);

class GUIWindow

{

public event MyEventHandler ClickedEvent;

public void ClickHasHappened()

{

if(ClickedEvent != null)

ClickedEvent(this, null);

}

}

Sender  GUIWindow,  MyMethod1  Sender  GUIWindow,  MyMethod2  Sender  GUIWindow,  MyMethod3  

Event  Model  •  The  following  example  followed  the  guidelines  for  .NET  event  model.  

•  To  raise  a  event,  two  elements  are  included  –  Sender  – Data  

•  .NET  Framework  has  defined  following  delegate  in  class  library!  public delegate void EventHandler( Object sender, EventArgs e

)

App,  now  Using  EventHandler  class App { static void Main() { GUIWindow window = new GUIWindow(); Listener listener = new Listener();

// Subscribe to the event by associating the handlers with the event: window.ClickedEvent += new EventHandler(listener.MyMethod1); window.ClickedEvent += new EventHandler(listener.MyMethod2); window.ClickedEvent += new EventHandler(listener.MyMethod3); // Trigger the event: window.ClickHasHappened(); } }

EventHandler  is  a  delegate  from  class  library!  

Listener,  exactly  the  same  class Listener

{

public void MyMethod1(object sender, EventArgs e)

{

System.Console.WriteLine("Sender {0}, MyMethod1", sender.ToString());

}

public void MyMethod2(object sender, EventArgs e)

{

System.Console.WriteLine("Sender {0}, MyMethod2", sender.ToString());

}

public void MyMethod3(object sender, EventArgs e)

{

System.Console.WriteLine("Sender {0}, MyMethod3", sender.ToString());

}

}

Publisher  // We don’t need this!

// public delegate void MyEventHandler(object sender, EventArgs e);

class GUIWindow

{

public event EventHandler ClickedEvent;

public void ClickHasHappened()

{

if(ClickedEvent != null)

ClickedEvent(this, null);

}

}

Sender  GUIWindow,  MyMethod1  Sender  GUIWindow,  MyMethod2  Sender  GUIWindow,  MyMethod3  

Passing  Data  

•  The  listener  receives  two  things  – Sender  – Data  

•  Data  is  passed  as  a  subclass  of  EventArgs  

Data  public class CustomEventArgs : EventArgs { public CustomEventArgs(string s) { msg = s; } private string msg; public string Message { get { return msg; } } }

Pass  InformaCon  class GUIWindow

{

public event EventHandler ClickedEvent;

public void ClickHasHappened()

{

if(ClickedEvent != null)

ClickedEvent(this, new CustomEventArgs("Click!"));

}

}

Listener  class Listener

{

public void MyMethod1(object sender, EventArgs e)

{

System.Console.WriteLine("Event: {0}", e.ToString());

}

…

}

Problem!  We  can’t  call  any  CustomEvent  method,  since  the  parameter  is  a  type  of  EventArgs.  So  this  does  not  work:     System.Console.WriteLine("Event: {0}", e.Message);    

Listener  class Listener

{

public void MyMethod1(object sender, CustomEventArgs e)

{

System.Console.WriteLine("Event: {0}", e.Message);

}

…

}

Let’s  change  the  listener.  But  now  the  problem  is  that  the  used  delegate  from  class  library  was  in  the  form  of    public  delegate  void  EventHandler(object  sender,  EventArgs  e);    

EventHandler  with  Generics  class GUIWindow

{

public event EventHandler<CustomEventArgs> ClickedEvent;

public void ClickHasHappened()

{

if(ClickedEvent != null)

ClickedEvent(this, new CustomEventArgs("Click!"));

}

}

We  can  define  the  second  parameter!  

EventHandler  with  Generics  class App

{

static void Main()

{

GUIWindow window = new GUIWindow();

Listener listener = new Listener();

// Subscribe to the event by associating the handlers with the event:

window.ClickedEvent += new EventHandler<CustomEventArgs>(listener.MyMethod1);

window.ClickedEvent += new EventHandler<CustomEventArgs>(listener.MyMethod2);

window.ClickedEvent += new EventHandler<CustomEventArgs>(listener.MyMethod3);

// Trigger the event:

window.ClickHasHappened();

}

}

And  this  is  also  possible..  class App

{

static void Main()

{

GUIWindow window = new GUIWindow();

Listener listener = new Listener();

// Subscribe to the event by associating the handlers with the event:

window.ClickedEvent += listener.MyMethod1;

window.ClickedEvent += listener.MyMethod2;

window.ClickedEvent += listener.MyMethod3;

// Trigger the event:

window.ClickHasHappened();

}

}