Lesson Event Handling

8/3/2019 Lesson Event Handling

http://slidepdf.com/reader/full/lesson-event-handling 1/33

Visual Basic.NET

Events

http://www.apiit.edu.my/



Visual Basic Slide 2 of 42

Objectives

• this lecture will cover:

– event-driven programming and the event loop

– different types of events

– accessing the Code Window and using its features – implementing simple event handlers

– correcting simple syntax errors

– the program development cycle

– pseudo-code solutions for simple problems





Event-driven programming

• program waits until an event occurs• User controls the program• An event can occur in 3 ways:

–The user doing something (e.g. Click a button) – Something happening within the computer (e.g.System shutdown)

– Something happening within the program (e.g.Program crash)

• Determine which "things" respond to whichevents• Write code to be executed when event occurs to

perform an action e.g. Calculate the average of aseries of numbers





Events in VB

• Events occur to objects on a form or the form

– Activate events: user selects item on screen

– Changed events: control property modified

– Focus events: control gets or loses focus – Key events: user interacts with keyboard

– Mouse events: user interacts using mouse

•One user event can result in many programevents





Event loop

• if event handler exists, it is executed

continuously monitors events





Event handlers

Event handlers have:header identifies event handlerbody contains statements to be executed

header

body

Private SubPrivate Sub btnPress_Click( _ btnPress_Click( _ ByValByVal sendersender AsAs System.Object, _ System.Object, _ ByValByVal ee AsAs System.EventArgs) _ System.EventArgs) _ HandlesHandles btnPress.ClickbtnPress.Click

'statements that form the body of event handler'statements that form the body of event handler'statement'statement

End SubEnd Sub





Method header

• Name of the event handler – defaults to control name, underscore and event

– can be modified but normally left unchanged

• Control and event involved in brackets – sender : details of control affected

• used if one handler is used for several events

– e: details of event such as character typed or mouselocation

• Events that will be handled by the method – can add events to be handled





Accessing Code Window

• Code is viewed using the Code Window – Project code block automatically generated

– All project code must be between Public Class andEnd Class

• Code Window has 2 drop-down combo boxes: – Class Name: list of all objects in the Solution

– Method Name: list of all events associated withcurrent object

Event Type





Editor features

• useful features:

– standard clipboard operations automatic indentation of

code

– spacing of statements – capitalisation of keywords

– ensuring consistent capitalisation of names

– auto code completion

– collapsible methods

– colour coding



http://slidepdf.com/reader/full/lesson-event-handling 10/33Visual Basic Slide 10 of 42

Running Applications

• Program is run by either:

– Clicking the Start Debugging

icon

– Pressing F5 or

• IDE:

– Compiles or builds program

• Traps any syntax errors

• Displays message box if errors occur

– Creates an executable file in the bin | Debug folder




Run-time vs. Design-time

• At design time, the programmer can:

– Add controls

– Set control properties

– Add code to controls – Edit code

• In run-time mode the form is locked

– can only respond to user input




Coding Event Handlers

• Code in event handler executed each time the

event occurs

• Programmer:

– Adds required code to handlers

– Runs program to check that correct actions are taken

when the events occur




Adding Comments to Code

• Comments improve readability of code

• To add a comment use the ‘ (single quote)character or the keyword REM

• Can also use the toolbar to comment statements

Private Sub btnPress_Click(...)Handles ...

'comment

'statements to be executed

End Sub

Comment / Uncomment Toolbar Icons




Types of properties

• Appearance

• Behaviour

• Size & position

properties can

be displayedalphabetically orcategorized




Changing properties at runtime

• Code can modify properties at runtime

• Assignment ( = ) statement specifies:

– control name followed by a dot

– property to be modified

– new value

Private Sub btnPress_Click(...) Handles ...

'disable btnPress

btnPress.Enabled = False

End Sub




Changing Color Properties

• Color class contains approximately 140

predefined colors

• Accessed by specifying:

– class name Color followed by a dot

• E.g. Color.Red

– required colour Private Sub btnPink_Click(...) Handles ...

'change background colour

lblPink.BackColor = Color.HotPink

End Sub




Changing Text property

• Strings are enclosed in double quotes

• Any number of strings can be concatenated using the & operator

• vbNewline moves to start of next line

Private Sub btnConcatenate_Click (...) Handles ...

'display Visual Basic on separate lines in label

lblConc.Text = "Visual" & vbNewline & "Basic"

End Sub




Changing Form properties

• Form properties are modified using the keyword

Me

• The keyword Me refers to the current form

Private Sub btnFormColour_ Click (...) Handles ...

'change form background

Me.BackColor = Color.BlanchedAlmondEnd Sub




Handling multiple events

• same method can be executed when a number

of events occur

• all events to be handled are listed after Handles

keyword

Private Sub btnEither_Click( _

ByVal sender As System.Object, _

ByVal e As System.EventArgs) _

Handles btnOne.Click, btnTwo.Click'report button pressed

lblPressed.Text = "Button pressed"

End Sub




Determining object

• Object that triggered event is held in sender

• This can determine which object was used

Private Sub btnEither_Click( _

ByVal sender As System.Object, _

ByVal e As System.EventArgs) _

Handles btnOne.Click, btnTwo.Click

'report button pressedlblPressed.Text = sender.Name & " pressed"

End Sub




Errors in code

• Some errors are recognized by the IDE i.e.

Syntax error

• Some may not become apparent until the

program is executed i.e. Logic error andCompiler error

• Correcting errors can be difficult:

– explanation of error may be difficult to understand due

to the technical terminologies used

– Location of error may not be exactly as indicated by

the compiler




Syntax errors

• Misspelled words and missing punctuation

• Normally trapped by IDE as they are typed

– explanation shown when cursor held over error

– should be corrected before running program

• Compiler traps uncorrected syntax errors

– message box prompts user to continue – should select

'No'

– Error List window displays compilation errors




Run-time errors

• Program can not run as required

– invalid calculation

– attempt to open a non-existent file

• Only apparent when program runs• IDE displays a message box:

– nature of error

– Approximate location of error (line number)




Logic (Semantic) Errors

• Errors program logic

• Not trapped as IDE only checks structure

• Can cause the program to:

– crash – hang

– continue but produce unexpected results

• Only discovered during program execution when

input data is entered• Testing can show that code has errors

• Debugging is finding and correcting errors




Using the debugger

• Facilities to:

– Set breakpoints in code

– Halt execution at statement before breakpoints

– Examine values in the code – Step through and execute individual lines




Program development

• Stage 1: Design screens

– ensure screens are logical and easy to follow

• Stage 2: Design pseudo-code

– decide what happens when

• Stage 3: Implement screens

– place controls on form

–change initial appearance

– attach code to events

• Stage 4: Test and debug program




Worked example - Greetings

• Display or clear a "Hello" message in a labelwhen the relevant command is clicked

• User should not be able to:

– use "Display" command when message is visible – use "Clear" command when no message displayed

• "Exit“ method is required to terminate theprogram




Design screens

• Single screen with 3 buttons and 1 label

Display Clear Exit

HellolblGreetings

btnExitbtnClearbtnDisplay





Design pseudo-code

• When btnDisplay button is clicked: – Set Text of lblGreetings to "Hello" – Enable btnClear button – Disable btnDisplay button

• When btnClear button is clicked: – Set Text of lblGreetings to "" – Enable btnDisplay button – Disable btnClear button

• When btnExit button is clicked: – End the program





Implement screens and code

• Invoke Visual Basic

• Click New Project button on Start Page

• Enter details of new project

• Create interface• Set properties

• Attach program code





Private Sub btnDisplay_Click(...) Handles btnDisplay.Click

‘display greetinglblGreetings.Text = "Hello"btnClear.Enabled = TruebtnDisplay.Enabled = False

End Sub

Private Sub btnClear_Click(…) Handles btnClear.Click‘clear greeting

lblGreetings.Clear()btnDisplay.Enabled = True

btnClear.Enabled = False

End Sub Private Sub btnExit_Click(…)Handles btnExit.Click

‘end program

End

End Sub






Test program

• Run program and ensure: – Clear button initially disabled – Greetings label initially blank

• Click Display button and ensure: – Hello displayed in Greetings label – Display button becomes disabled – Clear button becomes enabled

• Click Clear button and ensure: – Hello cleared from Greetings label – Display button becomes enabled – Clear button becomes disabled

• Click Exit button and ensure: – program ends




Summary

• This lecture has covered:

– the principles of event-driven programming

– writing code to handle events that occur within a

program – finding and correcting syntax errors

– the stages involved in developing a program


Lesson Event Handling

Documents

Transcript of Lesson Event Handling