CIS 338: Events

1
CIS 338 Events

• Dr. Ralph D. Westfall
• April, 2011

2
What Is an Event?

• an event is when something happens
• common user events
• start the program
• press a key
• click the mouse
• move the mouse
• close/end the program

3
Other Events

• events don't have to be from local user
• time (update virus checker every Wednesday at 10
  AM)
• e-mail message arrives
• E\ drive not ready
• event happens when program tries to read or write
  the missing CD

4
Event Driven Programming

• interactive programs respond to events
• generated by users, by the operating system,
  possibly by sensors, or by the application itself
• object oriented languages are well suited for
  writing interactive programs
• event "listeners" can be added to Java objects,
  are built into many VB.NET ones

5
Common Controls Events

• Click with left mouse button
• MouseDown click with either button
• MouseMove cursor moved over object
• Enter object becomes "active"
• user tabs to it, or clicks on it (GotFocus)
• Leave loses active status (LostFocus)
• KeyDown key pressed when object has focus
  (instruction "press any key")
• VB6 (still works, but don't use!)

6
Form Events

• Resize first thing that happens when a form is
  loaded
• also happens when user or code changes its size
• Move next thing that happens
• also happens when user drags the form
• Load goes into memory
• not visible yet (run MsgBox out of Load)
• Activated when form is shown or gets focus if
  already visible
• useful when have multiple forms

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Form Events - 2

• Deactivate when hidden or loses focus
• Closing when being removed from memory
• Closed when destroyed completely
• by code, or when a Visual Basic.NET application
  ends

8
VB.NET Event Procedures

• Windows (operating system) detects an event and
  sends a message to VB.NET
• VB.NET looks for code to handle it
• Private Sub btnExit_Click(etc.) Handles
  btnExit.Click
• 'code runs on a click event
• End
• End Sub
• btnExit current button name
• .Click Event

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Handling Multiple Controls

• a procedure can handle multiple events
• Private Sub ProcessEnter(ByVal sender As
  Object, ByVal e As System.EventArgs) Handles
  txtName.Enter, txtAddress.Enter
• can test Name of control or Tag property
  defined by programmer to handle some aspects
  differently
• End Sub 'more information

10
Creating an Event Procedure

• start VB and draw a control(s) on form
• open Code window
• ViewgtCode, click Solution Explorer's View Code
  Icon, or right click formgtView Code
• select control in left ComboBox
• select event in right ComboBox
• write in code to handle this event in the Sub
  that VB.NET creates for you

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Event Procedure Example

• start a new Windows Application
• in code window, select (frmName Events) and
  MouseMove event in top ComboBoxes
• add following code to the Sub, and then move
  mouse over Form when running
• Me.Text "X" e.X " Y" e.Y
• 'Me means this form Notes

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Code to Cause Event to Happen

• create an event procedure (previous slides)
• code a subroutine, or use an existing one
• put in a line of code to generate the event
• Private Sub RespondSomeEvent( etc. )
• Call Button1_Click(sender, e) 'args required
• End Sub 'Call is optional
• useful when something needs to happen just after
  something else happens
• in above code, program clicks a button when some
  event specified by user happens
• 'previous page notes

13
Exercise

• add a ListBox to a form
• add some Items in the Properties
• double click inside the ListBox to create a Sub
  to handle the event when a different item is
  selected
• add a MsgBox to the Sub to show the new item
  selection (see code below if can't find necessary
  code through Intellisense)