3.2 VB.NET Events

• An Event Procedure

• Properties and Event Procedures of the Form

• Tab Order of Controls

• Exercises

An Event Procedure

• An event is an action, such as the user clicking on a button

• Usually, nothing happens until the user does something and generates an event

Demo – Sales Order app with no code

The three steps in creating a VB.NET program:

1. Create the interface; that is, generate, position, and size the objects.

2. Set properties; that is, configure the appearance of the objects.

3. Write the code that executes when events occur.

Changing Properties

• Properties can be changed in code with the following:

controlName.property = setting

• This is an assignment statement

txtBox.ForeColor = Color.Red

Note: Right side is always assigned to left side

Demo: blank sol’n

Object Properties

• Some are values, some are string, some are boolean (true or false)

• Strings always go in quotes!txtBox.text = “Enter a phrase”

• Boolean and values do not have quotes

Control Name Prefixes

Control Prefix Example button btn btnComputeTotal label lbl lblInstructions list box lst lstOutput text box txt txtAddress

Note: Can be named anything but this is a convention

Ensure your naming is intuitive!

Most Common Errors

• Misspelling the name of a control in code

• VB editor will help you!

• Renaming controls does not update in the body of an event procedure

Event ProceduresPrivate Sub objectName_event(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles objectName.event

Shown in the book as:

Private Sub objectName_event(…) Handles objectName.event

Each object has default event procedure – double click ityou are unsure – try a label, textbox, button and listbox

Structure of an Event Procedure

Private Sub objectName_event(...)

Handles objectName.event

statements

End Sub

Sometimes there are multiple actions that can trigger an event – e.g. Textbox.enter event

Program Region

IntelliSense

Automatically pops up to give the programmer help.

Demo: wrong spelling for a control

Code for WalkthroughPrivate Sub txtFirst_TextChanged(...) Handles txtFirst.TextChanged txtFirst.ForeColor = Color.BlueEnd Sub

Private Sub btnRed_Click(...) Handles btnRed.Click txtFirst.ForeColor = Color.RedEnd Sub

Private Sub txtFirst_Leave(...) Handles txtFirst.Leave txtFirst.ForeColor = Color.BlackEnd Sub

Event Demo

Private Sub txtBox_MouseHover ….

txtBox.BackColor = Color.AliceBlue

End Sub

Private Sub txtBox_MouseLeave ….

txtBox.BackColor = Color.White

End Sub

Running Code

• From menu – Debug – Run

• Or F5

• Be careful – code is automatically saved when run

Assigning properties in code

• The following won't work:

Form1.Text = "Demonstration"

• The form is referred to by the keyword Me.

Me.Text = "Demonstration"

The Declaration Statement of an Event Procedure

• A declaration statement for an event procedure:Private Sub btnOne_Click(...) Handles btnOne.Click

• The name can be changed at will. For examplePrivate Sub ButtonPushed(...) Handles btnOne.Click

• Handling more than one event:Private Sub ButtonPushed(...) Handles btnOne.Click, btnTwo.Click

Tab Order of Controls

• Determines which control gets the focus when you hit the Tab key

• Order is initially established by the order controls are placed on the form

• Controlled by tabindex property

• Demo with Sales Order form

Exercises p. 69 to 74

Solutions to odd numbers are in the back of text book – try some on your own!

Do #37 together & #38 aloneHomework - #46 – page 74