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

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Chapter 13. Graphical User Interface Concepts
Part 1Continue from previous 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 class

• Objects typically maintain state
• state changes over time

class Student string name double gpa
int units public void RecordClass(int
grade) gpa (gpa units grade) /
(units 1) units ...
store state
change state
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Notification

• May want to notify interested parties of state
  change
• notification widely used throughout .NET
  framework
• user interface event handling most common example

Parent
Student
RecordClass
Registrar
new grade causes gpa to change
notify
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Pattern

• Notification typically involves registration and
  callback
• target registers with caller
• caller calls back target when state changes
• pattern also called publish/subscribe

Parent (target)
Student (caller)
register
callback
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Delegates

• Delegates are type safe function pointer
• .NET Framework uses delegates to implement
  callbacks
• intermediary between caller and target
• declaration defines callback method signature
• instance stores object reference and method token

target object
caller
target method
delegate
callback
callback
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Information flow

• Caller and target need to agree on information
  flow
• data passed through delegate

Parent object
Student
Parent method
double
double
delegate
void
void
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Delegate definition

• Define delegate with delegate keyword
• syntax similar to method declaration without body
• delegate name placed where method name usually
  goes

name of delegate
delegate keyword
delegate void StudentCallback(double gpa)
target method return type
target method parameter
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Delegate as type

• Delegate name is type name
• can declare references
• can create objects

delegate void StudentCallback(double gpa)
define delegate
StudentCallback a new StudentCallback(...)
Method name with the same signature And
return type
reference
object
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Define the event. (Event name)

• Events give private data/public accessor pattern
  for delegates
• created by applying event keyword to delegate
• external code can uses and -
• no external assignment or invocation

class Student public event StudentCallback
GpaChanged ...
event
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Write the code that fires the event

• Delegate is reference type
• defaults to null when used as field
• typical to guard invocation

class Student public StudentCallback
GpaChanged public void RecordClass(int
grade) // update gpa ...
GpaChanged(gpa)
Fire event
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NULL Reference if no one registered for the event

• Delegate is reference type
• defaults to null when used as field
• typical to guard invocation

class Student public StudentCallback
GpaChanged public void RecordClass(int
grade) // update gpa ... if
(GpaChanged ! null) GpaChanged(gpa)
Test before call
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Target use of delegate

• Target defines method with signature specified by
  delegate
• parameters and return type must match
• method name not constrained

delegate defines required signature
delegate void StudentCallback(double gpa)
class Parent public void Report(double gpa)
...
target
method signature matches delegate
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Registration

• Create delegate object and store in caller to
  register
• pass target object and method to delegate
  constructor

void Run() Student ann new
Student("Ann") Parent mom new Parent()
StudentCallback a new StudentCallback(mom.Report
) ann.GpaChanged a ...
caller
create
target
store
target object
target method
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Summary of delegate use in events
delegate void StudentCallback(double gpa) class
Parent public void Report(double gpa) ...
class Student public StudentCallback
GpaChanged public void RecordClass(int
grade) // update gpa ...
GpaChanged(gpa)
define delegate
target method
caller stores delegate
caller invokes delegate
Student ann new Student("Ann") Parent mom
new Parent() ann.GpaChanged new
StudentCallback(mom.Report) ann.RecordClass(4)
// 4 'A'
create and install delegate