Chapter 13' Graphical User Interface Concepts: Part 1 Continue from previouse lecture

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Chapter 13. Graphical User Interface Concepts
Part 1Continue from previouse lecture
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13.3 Event-Handling Model

• GUIs are event driven
• Event
• A class member that enables an object or class to
  provide notifications.
• Event handlers
• Methods that process events.
• Delegate
• Type safe method pointer
• Objects that reference methods
• Contain lists of method references
• Must have same signature

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Events and Event-Handler
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Events and Event-Handler
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SimpleForm9.cs

• using System
• using System.Windows.Forms
• class SimpleForm
• public static void Main()
• MyForm f1 new MyForm()
• Application.Run(f1)
• class MyForm Form
• private Button button1
• public MyForm()
• this.button1 new System.Windows.Forms.Bu
  tton()

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Events and Event-Handler
this
button1
button1_Click
Click
MyClass
button1_Click2
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Events and Event-Handler
this
button1
button1_Click
Click
MyClass
button1_Click2
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What have we learned so far?

• Understand what an event is.
• Understand what an event-handler is.
• Write an event handler
• Attach event-handler(s) to an event
• What if I want to define my own event in my own
  class?

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Defining Customized Event

• Identify where the event will be fired
• Define the event argument class (what kind of
  information the event need to send)
• Define a delegate type that describes the event
  handlers that will be used. (signature of the
  event handler)
• Define the event. (Event name)
• Write the code that fires the event where it was
  identified in step1

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Where the event will be fired in my
classFireAlarm.cs

• using System
• public class FireAlarm
• public void ActivateFireAlarm(string room,
  int ferocity)
• //FireEvent
• //will be called here
• // end of class FireAlarm

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What kind of information the even will send?

• using System
• // FireEventArgs a custom event inherited from
  EventArgs.
• public class FireEventArgs EventArgs
• public FireEventArgs(string room, int
  ferocity)
• this.room room
• this.ferocity ferocity
• // The fire event will have two pieces of
  information--
• // 1) Where the fire is, and 2) how bad it
  is.
• public string room
• public int ferocity
• //end of class FireEventArgs

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Define a delegate type that describes the event
handlers that will be used. (signature of the
event handler)

• using System
• public class FireAlarm
• // Events are handled with delegates, so we
  must establish a FireEventHandler
• // as a delegate
• public delegate void FireEventHandler(object
  sender, FireEventArgs fe)
• public void ActivateFireAlarm(string room,
  int ferocity)
• //FireEvent
• //will be called here
• // end of class FireAlarm

Delegate Name
Signature
keyword
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Define the event. (Event name)

• using System
• public class FireAlarm
• // Events are handled with delegates, so we
  must establish a FireEventHandler
• // as a delegate
• public delegate void FireEventHandler(object
  sender, FireEventArgs fe)
• // Now, create a public event "FireEvent"
  whose type is our FireEventHandler delegate.
• public event FireEventHandler FireEvent
• public void ActivateFireAlarm(string room,
  int ferocity)
• //FireEvent
• //will be called here
• // end of class FireAlarm

Event name
Keyword
Delegate used in the Event
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Write the code that fires the event

• using System
• public class FireAlarm
• // Events are handled with delegates, so we
  must establish a FireEventHandler
• // as a delegate
• public delegate void FireEventHandler(object
  sender, FireEventArgs fe)
• // Now, create a public event "FireEvent"
  whose type is our FireEventHandler delegate.
• public event FireEventHandler FireEvent
• public void ActivateFireAlarm(string room,
  int ferocity)
• // Create an instance of the Event
  Arguement (the second argument)
• FireEventArgs fireArgs new FireEventArgs(room,
  ferocity)
• // Now, raise the event by invoking the
  delegate. Pass in
• // the object that initated the event (this) as
  well as FireEventArgs.
• // The call must match the signature of
  FireEventHandler.

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FireHandlerClass

• class FireHandlerClass
• // Create a FireAlarm to handle and raise the
  fire events.
• public FireHandlerClass(FireAlarm fireAlarm)
• // Add a delegate containing the
  ExtinguishFire function to the class'
• // event so that when FireAlarm is
  raised, it will subsequently execute
• // ExtinguishFire.
• fireAlarm.FireEvent new
  FireAlarm.FireEventHandler(ExtinguishFire)
• // This is the function to be executed when a
  fire event is raised.
• void ExtinguishFire(object sender,
  FireEventArgs fe)
• Console.WriteLine("\nThe ExtinguishFire
  function was called by 0.", sender.ToString())
• // Now, act in response to the event.
• if (fe.ferocity lt 2)
• Console.WriteLine("This fire in the
  0 is no problem. I'm going to pour some water
  on it.", fe.room)
• else if (fe.ferocity lt 5)

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FireEventTest

• public class FireEventTest
• public static void Main()
• // Create an instance of the class that
  will be firing an event.
• FireAlarm myFireAlarm new FireAlarm()
• // Create an instance of the class that
  will be handling the event. Note that
• // it receives the class that will fire
  the event as a parameter.
• FireHandlerClass myFireHandler new
  FireHandlerClass(myFireAlarm)
• //use our class to raise a few events and
  watch them get handled
• myFireAlarm.ActivateFireAlarm("Kitchen",
  3)
• myFireAlarm.ActivateFireAlarm("Study",
  1)
• myFireAlarm.ActivateFireAlarm("Porch",
  5)
• return
• //end of main
• // end of FireEventTest

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What if no one has registered for the event..

• using System
• public class FireAlarm
• // Events are handled with delegates, so we
  must establish a FireEventHandler
• // as a delegate
• public delegate void FireEventHandler(object
  sender, FireEventArgs fe)
• // Now, create a public event "FireEvent"
  whose type is our FireEventHandler delegate.
• public event FireEventHandler FireEvent
• public void ActivateFireAlarm(string room,
  int ferocity)
• if (FireEvent ! null)
• // Create an instance of the Event
  Arguement (the second argument)
• FireEventArgs fireArgs new
  FireEventArgs(room, ferocity)
• // Now, raise the event by invoking
  the delegate. Pass in
• // the object that initated the event
  (this) as well as FireEventArgs.
• // The call must match the signature
  of FireEventHandler.