Multiple Forms & Procedures
Feb 23, 2016
Multiple Forms & Procedures
Form
• Methods:– Show, Hide, Activate, Close
• Events:– Load, Activated, Closing, Closed
Form Closing Event Example
If MessageBox.Show("Are you sure?", "Warning", MessageBoxButtons.YesNo) = DialogResult.Yes Then
e.Cancel = False
Else
e.Cancel = True
End If
Modeless or Modal
• Modeless form: Other forms can receive input focus while this form remains active.– formName.Show()
• Modal form: No other form can receive focus while this form remains active.– formName.ShowDialog()
Multiple FormsTwo forms: Form1, Form2To Open Form2 from Form1:
Dim f2 As New Form2() f2.Show()
Open Form2 as a Modal form:f2.ShowDialog()
Note: Form is defined as a class. Must create an instance of the form class by using the keyword New to access the form.
SharingVariables Among Forms
• Define these variables with class-level scope using the Public keyword.– Must use formName to qualify variables.– – Dim f2 As New Form2()– TextBox1.Text = f2.g2 *** g2 is declared in form2
SharingVariables Among Forms
• Define these variables with project-level scope in a module using the Public keyword:
– Module Module1– Public testVar As Integer– End Module
– Note: Use Project/Add Windows form to add a module.
Modules
• A file contains code such as:– Variable declarations– Procedures and functions
• Variables and procedures used by more than one form should store in a module.
• Global variables: Public
Starting Application with Sub Main
• The Sub Main procedure must reside in a module.
• Choose Sub Main as startup object. The code in Main will execute.– Write code in Main to open forms.
Sub Main ExamplePublic Sub main()
Dim dateToday As Date
dateToday = Today
If dateToday.DayOfWeek = DayOfWeek.Saturday Or dateToday.DayOfWeek = DayOfWeek.Sunday Then
Dim frmWeekEnd As New Form2()
frmWeekEnd.ShowDialog()
Else
Dim frmWeekDay As New Form1()
frmWeekDay.ShowDialog()
End If
End Sub
Declare Form’s Instance Variable in An Open Form Event Procedure
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim f2 As New Form2()
f2.Show()
End Sub
Problem: This will create a new instance of the form every time the button is clicked.
Declare Form’s Instance Variable in a Module as Public Variable
Module Module1 Public f1 As New Form1() Public f2 As New Form2()End ModuleIt can be accessed in a procedure:Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click f2.Show() End Sub
Problem: Once the form is closed the variable is no longer exists. Attempt to access it again will trigger error.
Close Form Or Hide Form
Use a form’s Closing event to change Close to Hide:
Private Sub Form2_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
e.Cancel() = True
Me.Hide()
End Sub
Main Menu Control
– Add MainMenu control and follow the TypeHere instruction.
– Each submenu and each item on a submenu is represented by a MenuItem control.
– Use an & to specify an access key in the caption. Ex. &File, Sho&s
– Write an event procedure for each menu item.
• Shortcut keys– Select the Shortcut key property in the MenuItem’s
property window.– Select the Shorcut key from list.– Set ShowShortcut property to true.
• Separator bar– Right clock a menu item/Insert Separator
• Inserting, deleting a menu item– Right click and select the option.
• Rearranging menu items– dragging
Context Menu
• A context menu is a menu that displays when an object on the screen is right-clicked.
• Add the ContextMenu control (it is placed in a tray under the form). Right-click the control and choose Edit to create the menu.
• Use the object’s ContextMenu property to bind the object to the context menu.
Simple Text Editor
• Textbox’s properties, methods– Help– Object Browser
• System.Windows.Forms
• Menu: Undo, Copy, Cut, Paste, SelectAll, Search • Clipboard:
– Clipboard.SetDataObject(TextBox1.SelectedText)– Clipboard.GetDataObject()
Procedures
. Sub procedure:Private/Public Sub SubName(Arguments)
…End Sub
Private: Can only be accessed by procedures in the same form.
Public: Can be accessed by procedures in other forms.
• To call a sub procedure SUB1• Call SUB1(Argument1, Argument2, …)
Function
• Private Function tax(salary) As Double• tax = salary * 0.1• End Function
– Or• Private Function tax(salary)• Return salary * 0.1• End Function
Call by Reference Call by Value
• ByRef– Default– The address of the item is passed. Any changes
made to the passing variable are made to the variable itself.
• ByVal– Only the variable’s value is passed.
ByRef, ByVal examplePrivate Sub Command1_Click()
Dim myStr As String
myStr = TextBox1.Text
ChangeTextRef (myStr)
TextBox1.Text = myStr
End Sub
Private Sub ChangeTextRef(ByRef strInput As String)
strInput = "New Text"
End Sub
Input, Output Arguments
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim sal, tax As Single sal = CSng(TextBox1.Text) Call CalTax(sal, tax) TextBox2.Text = tax.ToString End Sub Private Sub CalTax(ByVal Salary As Single, ByRef Tax As Single) Tax = 0.1 * Salary End Sub
Can we pass the Tax ByVal?
Static Variables
• Static VariableName as DataType• Static variables are local variables but are
not destroyed when a procedure terminates. Their value are kept.
Static Variable Example
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Static clickCounter As Integer = 0
clickCounter += 1
If clickCounter > 3 Then
MsgBox("Sorry, you can only click 3 times!")
Button1.Enabled = False
End If
End Sub
Event Procedures
• Example:– Private Sub Button1_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button1.Click
• The Handles keyword– Procedure name may change
• Handling multiple events:– Private Sub AllButtons_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button1.Click, Button2.CLick
Using One Event Handler to Handle Events Generated by Many Controls• Assume we have 3 buttons.• Use the Handles clause in the event
procedure to associate controls with the event procedure.
• We can assign a value for each control’s Tag property, or use control’s TabIndex property to determine which control generates the event.
Private Sub Button_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click, Button2.Click, Button3.Click
If sender.tag = "1" Then
MessageBox.Show("button 1 clicked")
ElseIf sender.tag = "2" Then
MessageBox.Show("button 2 clicked")
Else
MessageBox.Show("button 3 clicked")
End If
End Sub
Note: VB IntelliSense will not show the Tag property after you type sender.
Early Binding/Late Binding
• Early binding: VB compiler knows the object’s data type. It enables the use of IntelliSense.
• Late binding: VB compiler can’t determine the type of object that we are calling. This occurs because the object is declared as Object data type.
Demo: Phone Simulator