Events, Methods, Functions, and More

1
Events, Methods, Functions, and More

• Advanced GIS
• 10-23-03

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Objects/Methods/Events

• Visual Basic objects have properties, methods,
  and events. Properties can be thought of as an
  object's attributes, methods as its actions, and
  events as things that happen to the object. You
  write code to execute when events occur.
• An event is always associated with an object. You
  make an application work by setting object
  properties, and perform actions using object
  methods.
• In Visual Basic, the easiest type of object to
  conceptualize is a control with which users
  interact. In the graphic below, command buttons,
  labels, and text boxes are all examples of
  control objects. The form itself is also an
  object.
• Other less tangible Visual Basic constructs, like
  collections, are also objects. An object is an
  object if it can be controlled or manipulated
  with code by setting properties, invoking
  methods, or responding to events. In ArcInfo 8,
  you will find objects like Maps, PageLayout, and
  Layers. In MapObjects, objects include Map
  controls, MapLayer, and symbols. Not all objects
  have events for example, in MapObjects, only the
  Map control has events

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Properties

• Properties describe an object's characteristics.
  In Visual Basic, examples of an object's
  properties include Name, Appearance, and
  BorderStyle. The Map object in ArcInfo 8 has
  properties of Scale and MapUnits. A MapLayer
  object in MapObjects has properties including
  Extent and Visible.
• You can set many object properties at design time
  in the Properties window. Below, Form1's
  BackColor and Caption properties are set in the
  Properties window. The BackColor has been set to
  a red color, and the Caption, which appears in
  the form's title bar, has been set to "My Form."
  
• The Properties window is used here to set two
  properties of the form object, Caption and
  BackColor.

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Properties -2

• Some properties, such as Left, Top, Width and
  Height, change automatically when you move or
  resize controls. During run time, you cannot
  change object properties in the Properties
  window. Rather, you must change them using Visual
  Basic code. The syntax for setting a property is
  
• Object.Property Value
• Not all properties can be set (written) at run
  time however, almost all properties can be
  evaluated (read). The Name property for any
  object, for example, can be set only at design
  time. It is said to be read-only at run time.
• The following example shows how to set and read
  Label1's Caption property and assign the result
  to Text1's Text property. The Text property of a
  text box object is the text displayed in the text
  box.
• Label1.Caption "Welcome to VB"Text1.Text
  Label1.Caption

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Methods

• Methods are actions which objects know how to do.
  In Visual Basic, objects come equipped with the
  code to perform predefined methods. The syntax
  for using methods is
• Object.Method (Arguments)
• The example below shows a list box using an
  AddItem method to add items to itself. "Hello" is
  the argument for the AddItem method.
• Some methods, like AddItem, have arguments, while
  others do not. Not all methods need arguments.
  For example, a list box's Clear method does not
  need an argument to remove all the contents from
  the list box.
• List1.Clear
• When a method returns a value that you accept
  (i.e., assign to a variable and use in the code),
  you must surround its arguments with parentheses,
  as in the MapObjects example below. There, the
  AddRelate method requires three arguments. It
  also returns a True or False value, indicating if
  the AddRelate action was successfully performed.
  If you assign the returned value (either True or
  False) to the bItWorked variable, you need to
  surround its arguments with parentheses. If you
  don't want to assign the returned value to a
  variable, you don't need to include the
  parentheses.

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Is Like Object.Request in Avenue

• 'MapObjects example--parentheses required
  bItWorked myLayer.AddRelate("APN", parcelTbl,
  "APN")'MapObjects example--no parentheses
  requiredmyLayer.AddRelate "APN", parcelTbl,
  "APN"
• Some methods don't return a value by default,
  like the AddItem method of the list box control.
  When a method does not return a value, you must
  not surround its arguments with parentheses
• List1.AddItem "Welcome to VB"

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Events

• Events are actions that an object can recognize.
  When a command button object is clicked, the
  command button reacts according to the code
  supplied by the programmer.
• In Visual Basic, forms and controls have their
  own associated events. The possible events for
  forms and controls are listed in the Procedure
  list box in Code Editor window. For example, a
  command button can have events such as Click,
  DragDrop, and DropOver.

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Events 2

• The Visual Basic syntax for an event procedure is
  shown below
• Object_event(arguments)
• Some events, such as the Click event, have no
  arguments
• Command1_Click()
• Others, such as the MouseDown event, have several
  arguments
• Command1_MouseDown(Button As Integer, Shift As
  Integer, X As Single, Y As Single)
• Any argument of an event can be used
  programmatically within the event procedure.
• Although many events happen as a direct result of
  user interaction, other events happen indirectly
  or may be completely removed from any user
  interaction whatsoever. The Timer control, for
  instance, has a Timer event that is triggered at
  regular intervals (e.g., every second), while the
  Form object has a Load event that occurs when the
  Form is first initiated.

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Procedure

• Procedures are the building blocks of a Visual
  Basic application. Procedures allow you to break
  the program into smaller logical units and to
  debug (find errors) easily. Procedures are also
  useful for repeated or shared tasks.
• A simple event procedure is shown below
• Private Sub cmdApply_Click()
• MsgBox "Hello"
• End Sub
• Each procedure has template code as the first and
  last lines of a procedure. Within the template
  code (sometime called wrapper code) is the actual
  executable code. In the above example, a message
  box displays "Hello" when the command button
  cmdApply is clicked

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Statements

• Statements are Visual Basic commands which
  provide much of the syntax and structure for the
  Visual Basic language. Statements are the
  building blocks of a procedure.
• Statements initiate an action. Some statements
  take one or more arguments (sometimes referred to
  as parameters) that specify how the action is to
  be performed.
• In the example below, the Unload statement needs
  an argument so it knows which form to unload,
  while the Beep statement does not need any
  argument.
• 'Statement with argumentUnload Form1'Statement
  without argumentBeep

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Functions

• Functions are statements which return a result,
  or value. There are two types of functions
  Visual Basic intrinsic functions and functions
  created using a function procedure.
• Intrinsic functions are provided by the language
  itself. Examples of intrinsic functions are
• Mathematic function Sqr, Sin
• InputBox function
• Time/Date function Now
• The syntax for a function is
• ReturnedValue Function()
• The example below uses the intrinsic function
  Sin()
• theAngle Sin(txtAngle.Text)
• Functions can be nested.
• theAngle Sin(InputBox("Enter an Angle"))

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Functions - 2

• All functions return a value however, you don't
  always need to retain the value. If the returned
  value needs to be saved, parentheses must be
  used.
• In the following example, the MsgBox function
  returns which button (Yes or No) is clicked by
  the user. If the value is not saved, you do not
  have to use parentheses. If you need to track
  which button is clicked, however, use
  parentheses.
• 'Value not retainedMsgBox "Save changes to the
  file?", vbYesNo 'Value retainedtheButton
  MsgBox("Save changes to the file?", vbYesNo)

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Variables

• Variables are used to temporarily store values
  during the execution of a program. In Visual
  Basic, a variable has a name and a data type.
• You declare a variable with the Dim statement.
  "Dim" is short for Dimension--Dimension means
  size. In Visual Basic, the equal sign "" is
  often used as the symbol of assignment. The
  statement X Y means to take value of variable Y
  and assign it to variable X. Variable X now
  stores the value of variable Y.
• In the following example, an integer variable is
  used to store the user input information. The
  information is incremented and reported back to
  the user.
• 'Declare the variableDim intAge As Integer
  'Assign the variableintAgeInputBox("Enter your
  age")'Increment the variableintAge intAge 1
  'Retrieve the variableMsgbox "Next year you are
  " intAge

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Constant

• A constant is a meaningful name used to replace a
  fixed number or string that is difficult to
  understand and remember. For example, colors are
  stored in Visual Basic using integer numbers. The
  color red in Visual Basic is 65280, to which a
  constant vbRed is assigned. To change the
  background color of a form to red, you can write
  either of the following two lines of code.
• frmColor.BackColor 65280'orfrmColor.BackColor
  vbRed
• Obviously, the second example is easier to write
  and understand.
• Hundreds of constants come with Visual Basic, and
  you can also create your own.
• Const conPi 3.14159265358979Const conVersion
  "8.1"
• Code created with Visual Basic constants is
  forward-compatible. According to Microsoft, while
  the numeric codes might change, the constants
  will be updated with each new release to point at
  the correct new number.

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Naming convention

• Understanding and practicing Visual Basic
  programming conventions will help you write
  well-structured and documented code. Following
  defined conventions makes code easier to
  understand and maintain.

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Controls

• The following table shows common prefixes and
  name examples for some common controls.
• Object  Prefix  Example
• Form frm frmSelect
• Label lbl lblSearchString
• Combo boxcbo cboEditFeature
• Command button cmd cmdCancel
• Directory listbox dir dirPath
• Drive listbox drv drvDrive
• Image img imgBitmap
• Listbox lst lstSelectedFile
• Menu mnu mnuFileSaveAs
• Option button opt optTopology
• Timer tmr tmrAlarm
• Common dialog dlg dlgSaveAs

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Comments

• Comments are not just for your coworkers and your
  employer, they can be invaluable to you. If you
  don't include comments in your code, you might
  find yourself completely lost, trying to figure
  out a program that seemed perfectly logical and
  understandable when you created it a few months
  ago. Comments are especially important in Visual
  Basic, because code does not usually flow from
  top to bottom, which can make it difficult to
  track.
• The single quote is the character that denotes a
  comment in Visual Basic. By default, commented
  lines turn green. Comments can follow a statement
  on the same line or they can occupy an entire
  line, as shown in the example below
• Name InputBox("Enter your name") 'Get the
  name'Get the number of characters in
  nameGetNameLength Len(Name)

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Comments - 2

• You can use the Edit toolbar to quickly comment
  and uncomment highlighted blocks of code.
• The Edit toolbar provides two tools you can use
  to comment or uncomment highlighted blocks of
  code at once.

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Indenting

• You can press Tab and ShiftTab to indent and
  outdent code, respectively. The number of spaces
  to indent is your choice, but a minimum of two
  spaces is recommended for clarity. To set the
  Code Editor tab width, choose Options from the
  Tools menu

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Line continuation in code

• Often, an individual line of code can become
  quite long. Some developers feel that code is
  more readable if long lines are split into
  several shorter lines. If you split a line of
  code, you must place an underscore character (_)
  at the end of the line and after a space, to
  indicate that the line continues on the next line
  below.
• 'Get the number of characters in user's name
  GetNameLength Len(InputBox _("Please enter
  your full name."))
• The continuation character cannot be placed
  inside quotes or be followed by a comment on the
  same line.
• Other developers feel that line continuation can
  lead to confusion--what is really one line of
  code appears as several. Whether you use line
  continuation or not, just remember that readable
  code is the goal.

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Homework 7, 30 points, due on 10/30/03 midnight

• You are asked to create a VB form that converts
  geographic degrees in (degree,minutes,seconds) to
  decimal degrees. Add label,commands and text box
  to this form and add code to calculate the
  conversion. (DD degrees minutes/60
  seconds/3600)
• Write code in cmdConvert_Click() event
• txtDD txtDegrees txtMinutes/60
  txtSeconds/3600
• Remember to have two parts of the code for
  differentiate into positive and negative degrees.
• Add Clear and Exit commands to the form
• Save your project and forms as DMSConvert
• Use comments to describe each step

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Homework 7 continued

• Control Name Caption
• Form frmDD Homework 7 DMS to DD
• Command1 cmdConvert Convert
• Command2 cmdClear Clear
• Command3 cmdExit Exit (you need End)
• Label1 lblDegrees Degrees
• Label2 lblMinutes Minutes
• Label3 lblSeconds Seconds
• Label4 lblDecimalDegrees Decimal Degrees
• Text1 txtDegrees
• Text2 txtMinutes
• Text3 txtSeconds
• Text4 txtDD

Notes Be careful with the negative degrees (such
as in western hemisphere). Use
Format(variable, 00,000000) to set the dd
format.
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Homework 8Browse files, 20 points, due on
11/1/03, midnight

• Name Caption
• Form frmMain Slide Show
• DriveListBox drvFile
• DirListBox dirFile
• FileListBox filFile
• Label1 lblSlideInShow Slides in show
• List1 lstSlides
• Command1 cmdAdd (font)Add 10 pt Bold
• Command2 cmdClear (font)Clear 10 pt Bold

• Once user click on the drvFile, the dirFile
  will refresh the change and then the filFile will
  refresh its contents accordingly.
• The highlighted file in the filFile will be
  added to the lstSlides if users click on cmdAdd.
  If cmdClear is clicked, the contents of the
  lstSlides will be cleared.

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Hints

• Once user click drvFile, dirFile should have an
  updated list (use dirFile drvFile.Drive in the
  event drvFile_Change())
• Once user click dirFile, filFile should have an
  updated list (use filFile dirFile.Path in the
  event dirFile_Change()).
• Once user double-click on the file list, the file
  will be added to the List (in the event of
  filFile_DblClick() add the following code-
  lstSlides.AddItem (filFile.FileName)), or use
  cmdAdd to add the highlighted file to the list
  (in the event of cmdAdd_Click())
• In the event of cmdClear_Click() add following
  tasks
• If List1.ListCount 0 Then
• MsgBox "No file listed here"
• ElseIf List1.SelCount 0 Then
• MsgBox "Please select file from the list"
• Else
• List1.RemoveItem (List1.ListIndex)
• End If