1 Chapter Eleven Handling Events. 2 Objectives Learn about delegates How to create composed...

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Chapter Eleven

Handling Events

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Objectives

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

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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 Control’s tab order

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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#

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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

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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

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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

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Creating Composed Delegates

• Delegate3 Program

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Creating Composed Delegates

• Output of Delegate3 program

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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 arguments—the sender

and an EventArgs object

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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

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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

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Handling Control Component Events

• Some Control Class Public Instance Events (continued)

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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

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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

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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

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Setting Controls’ Tab Order

• ManyButtons Form with three Buttons

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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

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Using the sender Object in an Event

• ManyButtons Form including Label

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Using the sender Object in an Event

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

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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

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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

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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

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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

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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