Programming Based on Events

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Chapter 9

• Programming Based on Events

Microsoft Visual C .NET From Problem Analysis
to Program Design
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Chapter Objectives

• Define, create, and use delegates and examine
  their relationship to events
• Explore event-handling procedures in C by
  writing and registering event-handler methods
• Create applications that use the ListBox control
  object to enable multiple selections from a
  single control
• Contrast ComboBox to ListBox objects by adding
  both types of controls to an application

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Chapter Objectives (continued)

• Add Menu control options to Window forms and
  program their event-handler methods
• Wire multiple RadioButton and CheckBox object
  events to a single event-handler method
• Work through a programming example that
  illustrates the chapters concepts

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Delegates

• Delegates store references (addresses) to
  methodsas opposed to storing actual data
• Delegates form the foundation for events in C
• Declaration for a delegate looks more like a
  method declaration than a class definition
• Exceptdelegate declaration has no body
• Declaration begins with the keyword delegate
• Declaration ends with a parenthesized list of
  parameters
• Unlike a method, the return type of a delegate
  becomes part of its identifying signature

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Delegates (continued)

• Delegate declaration example
• delegate string ReturnsSimpleString( )
• Delegate signature
• Identifies what types of methods the delegate
  represents
• Above Example represents methods that return a
  string and require no argument
• static string EndStatement( )
• static string ToString( )
• static string ReturnSaying( )

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Delegates (continued)

• Associate delegate with method(s) by creating
  delegate instance(s)
• Example
• ReturnsSimpleString saying3 new
• ReturnsSimpleString(EndStatement)
• Constructor for delegate of the delegate class
  always takes just parameter
• Name of a method for the constructor to
  reference

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Delegates (continued)

• Delegate identifier references the method sent as
  argument to constructor
• Any use of delegate identifier now calls the
  method
• Methods are said to be wrapped by the delegate
• Delegate can wrap more than one methodcalled a
  multicast delegate
• and - operators are used to add/remove
  methods to/ from the delegate chain or invocation
  list
• Multicast delegates must have a return type of
  void

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Relationship of Delegates to Events

• Delegates are used for event-driven application
• Delegate acts as intermediary between objects
  that are raising or triggering an event
• During compilation, the method or methods that
  will be called are not determined
• Events as special forms of delegates
• Place a reference to event-handler methods inside
  a delegate
• Once reference is made, or event is registered,
  delegate is used to call event-handler method
  when an event like a button click is fired

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Event Handling in C

• Form Designer in Visual Studio did much of the
  work for you
• Double-clicked on a Button control object during
  design
• 1) Click event is registered as being of interest
• 2) An event-handler method heading is generated
• Two steps form event wiring process
• Wire an eventassociate (identify) a method to
  handle its event

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Event Handling in C (continued)

• Code associates the methods with a delegate
• this.button1.Click new System.EventHandler(this
  .button1_Click)
• this.button2.Click new System.EventHandler(this
  .button2_Click)
• System.EventHandler is a delegate type
• button1.Click and button2.Click are methods
• Keyword this is added to all code generated by
  Visual Studio .NET to indicate the current
  instance of a class

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ListBox Control Objects

• Displays list of items for single or multiple
  selections
• Scroll bar is automatically added when total
  number of items exceeds the number that can be
  displayed
• Can add or remove items at design time or
  dynamically at runtime
• Includes number of properties and events
• Items property used to set initial values
• Click on (Collections) to add items

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Adding a ListBox Control Object
Add ListBox control, then click on Items property
(Collection) to type entries
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ListBox Control Objects (continued)

• Name property
• Useful to set for program statements
• Sorted property
• Set to true to avoid having to type values in
  sorted order
• Register an event for the ListBox
• Might want to know when the item selection
  changes
• Double-clicking on any control registers its
  default event for the control
• SelectedIndexChangeddefault event for ListBox

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ListBox Control Objects (continued)

• Register its event with the System.EventHandler
  delegate
• this.lstBoxEvents.SelectedIndexChanged new
  System.EventHandler
  (this.listBox1_SelectedIndexChanged)
• Visual Studio .NET adds event handler method
• private void listBox1_SelectedIndexChanged
• (object
  sender, System.EventArgs e)

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ListBox Control Objects (continued)

• To retrieve string data from ListBox use Text
  property
• this.txtBoxResult.Text this.lstBoxEvents.Text
• Place in method body
• When event fires, selection retrieved and stored
  in TextBox object

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ListBox Control Objects (continued)
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Multiple Selections with a ListBox

• SelectionMode Property has values of MultiSimple,
  MultiExtended, None and One
• MultiSimple use the spacebar and click of mouse
• MultiExtended can also use Ctrl key, Shift key,
  and arrow keys
• foreach(string activity in
  lstBoxEvents.SelectedItems)
• result activity " "
• this.txtBoxResult.Text result

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ListBox Control Objects (continued)
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ListBox Control Objects (continued)

• SelectedItem and SelectedItems returns objects
• Store numbers in the ListBox, once retrieved as
  objects, cast the object into an int or double
  for processing
• Adding items to a ListBox at runtime by using
  Add( ) method with the Items property
• lstBoxEvents.Items.Add("string value to add")
• private void btnNew_Click(object sender,
  System.EventArgs e)
• lstBoxEvents.Items.Add(txtBoxNewAct.Text)

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ListBoxExample
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ListBoxExampleProperties Set
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ListBoxExampleProperties Set (continued)
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Properties of the ListBox Class
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Methods of the ListBox Class
Remember ListBox object also inherits members
from Control class
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ComboBox Controls
Extra TextBox object with ComboBoxUser selects
from list or types new value
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ComboBox Controls (continued)
Top line left blank in ComboBox when
DropDownStyle property is set to DropDown
(default setting)
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Handling ComboBox Events

• ComboBox only allows a single selection to be
  made
• Default event-handler methodSelectedIndexChanged(
  )
• Same as ListBox control object
• Could register KeyPress( ) event-handler method
• BUT, event is fired with each and EVERY keystroke

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Programming Event Handlers

• Since ListBox object allows multiple selections,
  Text property cannot be used
• Text ONLY gets the first one selected
• Use the SelectedItems, SelectedIndices, or Items
  to retrieve a collection of items selected
• Zero-based structures
• Access them as you would access an element from
  an array
• SelectedIndices is a collection of indexes

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Programming Event Handlers (continued)
KeyPress( ) event handler method fired with each
keystroke
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Menu Control Objects
Drag MainMenu control to form, then click here to
display Menu structure
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Menu Control Objects (continued)

• Preceding the character with an ampersand ()
  places an underline under the next character
  typed
• Formatcreates Format and Alto becomes the
  shortcut
• Also have Shortcut property
• Can associate any key combination to Menu options

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Menu Control Objects (continued)

• To create separators, right-click on the text
  label (below the needed separator)
• Select Insert Separator

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Menu Control Objects (continued)
Set the Menu property on the form to the name of
the MainMenu object
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Wire Methods to Menu Option Event

• Set the Name property for each menu option
• Do this first, then wire the event
• Click events are registered by double-clicking on
  the Menu option
• When the menu option is clicked, the event
  triggers, happens, or is fired

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Adding Predefined Standard Windows Dialog Boxes

• Included as part of .NET
• Dialog boxes that look like standard Windows
  dialog boxes
• File Open, File Save, File Print, and File Print
  Preview
• Format Font
• Format Color dialogs

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Adding Predefined Standard Windows Dialog
BoxesColor
Retrieves the current ForeColor property setting
for the Label object

• private void menuColor_Click(object sender,
• System.EventArgs e)
• colorDialog1.Color lblOutput.ForeColor
• if (colorDialog1.ShowDialog( ) !
  DialogResult.Cancel )
• lblOutput.ForeColor colorDialog1.Color

Checks to see if Cancel button clicked
Set to selection made
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Adding Predefined Standard Windows Dialog
BoxesColor (continued)
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Adding Predefined Standard Windows Dialog
BoxesFont

• private void menuFont_Click (object sender,
  System.EventArgs e)
• fontDialog1.Font
• lblOutput.Font
• if (fontDialog1.ShowDialog( )
• ! DialogResult.Cancel )
• lblOutput.Font
• fontDialog1.Font

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GardeningForm Application
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GardeningForm Application (continued)
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GardeningForm Application (continued)
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CheckBox Objects

• Appear as small boxes
• Allow users to make a yes/no or true/false
  selection
• Checked property set to either true or false
  depending on whether a check mark appears or not
• Default false value
• CheckChanged( )default event-handler method
• Fired when CheckBox object states change
• Can wire one event handler to multiple objects

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Wiring One Event Handler to Multiple Objects

• Using Properties window, click on the Events Icon
  
• Click the down arrow associated with that event
• Select method to handle the event
• Follow the same steps for other objects

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CheckBox Object
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GroupBox Objects (continued)

• CheckBox objects may be grouped together for
  visual appearance
• Can move or set properties that impact the entire
  group
• A GroupBox control should be placed on the form
  before you add objects
• GroupBox control adds functionality to
  RadioButton objects
• Allow only one selection

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RadioButton Objects

• Appear as small circles
• Give users a choice between two or more options
• Not appropriate to select more than one CheckBox
  object with RadioButton objects
• Group RadioButton objects by placing them on a
  Panel or GroupBox control
• Setting the Text property for the GroupBox adds a
  labeled heading over the group

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RadioButton Objects (continued)
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RadioButton Objects (continued)

• Turn selection on
• this.radInterm.Checked true
• Raise a number of events, including Click( ) and
  CheckedChanged( ) events
• Wire the event handler methods for RadioButton
  objectsjust like CheckBox

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RadioButton Objects (continued)

• if(this.radBeginner.Checked)
• cost 10
• this.lblMsg.Text
• "Beginner
• -- Extra 10 charge"
• else
• // more statements

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ComputeCost_CheckChanged( ) and Click( ) events
raised
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DinerGui Application Example
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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PropertiesDinerGui Application
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PropertiesDinerGui Application (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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DinerGui Application Example (continued)
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Chapter Summary

• Delegates
• Event-handling procedures
• Registering an event
• ListBox control for multiple selections
• ComboBox versus ListBox objects
• Menu controls
• RadioButton versus CheckBox objects