Chapter Eleven

1
Chapter Eleven

• Handling Events

2
Objectives

• Learn about delegates
• How to create composed delegates
• How to handle events
• How to use the built-in EventHandler

3
Objectives

• How to handle Control component events
• How to add more events to an application
• How to use the Visual Studio IDE to generate
  event-handling code
• How to set Controls tab order

4
Objectives

• How to use the sender object in an event
• How to add a main menu to a Form
• How to continue your exploration of C

5
Understanding Delegates

• A delegate is an object that contains a reference
  to a method
• A delegate provides a way to pass a reference to
  a method as an argument to another method
• C provides a compromise between the dangers of
  C pointers and the Java ban on passing
  functions
• You declare a delegate using the keyword
  delegate, followed by an ordinary method
  declaration including a return type, method name,
  and argument list

6
Understanding Delegates

• A delegate can encapsulate any method as long as
  the method has the same return type and the same
  number and types of arguments

7
Creating Composed Delegates

• You can assign one delegate to another using the
  operator
• You can also use the and operators to
  combine delegates into a composed delegate
• Only delegates with the same argument list can be
  composed, and the delegates used must have a void
  return value
• You can use the and - operators to remove a
  delegate from a composed delegate

8
Creating Composed Delegates

• Delegate3 Program

9
Creating Composed Delegates

• Output of Delegate3 program

10
Handling Events

• In C, an event occurs when something interesting
  happens to an object
• You use an event to notify a client program when
  something happens to a class object the program
  is using
• To declare an event, you use a delegate
• An event handler requires two argumentsthe
  sender and an EventArgs object

11
Using the Built-in EventHandler

• The C language allows you to create events using
  any delegate type
• The .NET Framework provides guidelines you should
  follow if you are developing a class that will be
  used by others
• For events that do not use any additional
  information, the .NET Framework has already
  defined an appropriate type named EventHandler

12
Handling Control Component Events

• You can use the same techniques to handle events
  generated or raised by GUI Controls as you do to
  handle events raised by non-Control-generated
  events

13
Handling Control Component Events

• Some Control Class Public Instance Events
  (continued)

14
Adding More Events to an Application

• A Form can contain any number of Controls that
  might have events associated with them
• A single control might be able to raise any
  number of events

15
Using the IDE to Generate Event-Handling Code

• It is possible (and easier) to create
  event-handling code using the IDE
• The only difference between event-handling code
  created by hand and the one you create using the
  IDE is that the IDE automatically creates a lot
  of code for you
• The advantage of using the IDE is the time you
  save typing and correcting typing errors
• The advantage of creating methods by hand is that
  you understand what each statement accomplishes
  and are able to customize methods to perform
  exactly the task you want

16
Setting Controls Tab Order

• A Control is said to have focus if it raises an
  event when the user presses Enter
• TabStop is a Boolean property of a Control that
  identifies whether the Control will serve as a
  stopping place in a sequence of Tab key presses
• TabIndex is a numeric property that indicates the
  order in which the Control will receive focus
  when the user presses the Tab key
• When a Control has a TabIndex of 0, it receives
  focus when the Form is initialized

17
Setting Controls Tab Order

• ManyButtons Form with three Buttons

18
Using the sender Object in an Event

• When a Form contains multiple widgets that you
  can manipulate, you can write event handling
  methods for each one
• When a Form contains multiple widgets, you can
  also write a single event-handling method that
  can take appropriate action based on the Control
  that generated the event
• The Control that causes an event is represented
  as a generic object in the object sender argument
  to an event method

19
Using the sender Object in an Event

• ManyButtons Form including Label

20
Using the sender Object in an Event

• Every object has an Equals() method that can be
  used to determine the sender object

21
Adding a Main Menu to a Form

• Most programs you use in a Windows environment
  contain a main menu
• You can add a main menu to a Form by using the
  MainMenu Control

22
Continuing to Explore C

• The Visual Studio IDE contains numerous Controls,
  each containing hundreds of properties and events
• There are many resources that are available to
  use to learn more about C, including
• Help facility in the Visual Studio IDE
• Tutorials in the Help facility
• C discussion groups on the Internet

23
Chapter Summary

• A delegate is an object that contains a reference
  to, or encapsulates, a method
• You can assign one delegate to another using the
  operator. You also can use the and
  operators to combine delegates into a composed
  delegate that calls the delegates from which it
  is built.
• In C, an event occurs when something
  interesting happens to an object
• For events that do not use any additional
  information, the .NET Framework has defined an
  appropriate delegate type named EventHandler

24
Chapter Summary

• When you use Controls like Buttons and ListBoxes,
  they already contain events with names like
  Click, DragOver, MouseEnter, and MouseLeave
• A Form can contain any number of Controls that
  might have events associated with them
• When designing a Form with events, you can use
  the Visual Studio IDE to automatically create a
  lot of code for you
• When users encounter multiple GUI Controls on a
  Form, usually one Control has Focus

25
Chapter Summary

• When a Form contains multiple widgets that you
  can manipulate, you can write event-handling
  methods for each one
• Most programs you use in the Windows environment
  contain a main menu, which is a horizontal list
  of general options that appears under the title
  bar of a Form
• If you understand good programming principles
  and, more specifically, the syntax and structure
  of C programs, you will find learning about each
  new C feature easier than learning about the
  last one