CSC106 Intermed PC Appls and Programming - Objects, Events, Event Procedures, and Properties

Objects, Events, Event Procedures, and Properties

Objects

An object in a VB program is a component of the user interface, such as a form (which represents the window in which the program executes) or a control (a button, list, checkbox, textbox, or some other component that is contained in the form). Every object has properties which define the characteristics of the object. One property that every object has is (Name) -- the name of the object. Every object is given a default name when you create it, but we always change the names, using a style of naming that helps us remember the type of the object. We start the name with an abbreviation of the object type:

Type of Object	Name Prefix	Example
button	btn	btnClear
Label	lbl	lblTitle
text box	txt	txtName

Events

While a program is running, the user can do many things, such as move the mouse, click the mouse button, enter text, or move to a different window. Each of these actions is called an event. Each event has two components: what the user did (click, type, etc.) and on which object did they perform the event (which button did they click on, which textbox did they type into). In Visual Basic you can write code that will perform actions in response to events. Some examples of events are:

Action	Event
click mouse button	Click
double click mouse button	DblClick
object is selected ("gets focus")	Enter
object is no longer selected ("loses focus")	Leave
contents of text box changes	TextChanged

Event Procedures

The code that performs actions in response to events is written in event procedures. Each event procedure contains the statements that execute when a particular event occurs on a particular object. Event procedure names have the form objectName_event, where objectName is the name of the object on which the event occurred and event is the type of the event. For example, an event procedure named cmdClear_Click will be executed when the user clicks on the button named cmdClear.

The event procedures are written in the code window. To open the code window, double click on the control for which you want to write the event procedure. The code window will open and it will contain the shell of the event procedure for that object:

     Private Sub btnClear_Click(...) Handles btnClear.Click
        
     End Sub

Sample Properties of Label

(Name) name of the Label control
(Name).Text what is displayed for the Label
(Name).Font set the font face, size, bold, etc.
(Name).ForeColor the color used to displayed contents of the Label
(Name).BackColor the background color of the Label
(Name).TextAlign left justify/center/right justify the contents

Sample Properties of TextBox

(Name) name of the TextBox control
(Name).Text what is displayed in the TextBox
(Name).Font set the font face, size, bold, etc.
(Name).ForeColor the color used to displayed contents of the TextBox
(Name).BackColor the background color of the TextBox
(Name).TextAlign left justify/center/right justify the contents

Sample Properties of Button

(Name) name of the TextBox control
(Name).Text what is displayed on the button
(Name).Font set the font face, size, bold, etc. for the caption
(Name).ForeColor the color used to displayed content on the Button
(Name).BackColor the background color of the Button

A Sample Event Procedure

     Private Sub btnMessage_Click(...) Handles btnMessage.Click
        ' this event procedure will write a message into the lblBanner label
        ' the message will be written in red
        ' it will be executed when the user clicks on the button named
        ' btnMessage
        lblBanner.ForeColor = Color.Red
        lblBanner.Text = "VB Sample Event Procedure!"
     End Sub