Understanding Events

The left hand dropdown box provides a list of all controls used by the current form, the form itself, and a special section called General Declarations. The corresponding dropdown box on the right displays a list of all events applicable to the current control (as specified by the left hand dropdown box). Events displayed in bold signify that code has already been written for them, unbold events are unused. To demonstrate that different events can play a significant role in determining the feel of an application, a small example program will be written to add two numbers together and display the answer. The first solution to this problem will use the click event of a command button, while the second will the change event of two text boxes.

Click Event

To do this first remove the equals command button and replace it with a label with the caption set to ‘=’. Now, bring up a code window and copy to the Windows clipboard the line lblAnswer = Str$(Val(txtNumber1.Text) + Val(txtNumber2.Text)). Using the left hand dropdown box select the first text box and then select the Change event from the right dropdown box. Then paste the code from the clipboard into the empty subroutine. Select the second text box and do the same. The same line is required twice because the two click events belong to two separate controls. The final code should look like:

  Private Sub txtNumber1_Change()
    label2.Caption = Str$(Val(text1.Text) + Val(text.Text))
  End Sub

  Private Sub txtNumber2_Change()
    label2.Caption = Str$(Val(text1.Text) + Val(text2.Text))
  End Sub

Run the program again, enter the two numbers and observe what happens. Each time a digit changes the answer is recalculated.

Note: There may be times when recalculating more advanced problems takes too long on each change and so requiring the user to enter all the data first and then click on an answer button might more appropriate.

So far only one event has been used per control, however this does not have to be the case! Add a StatusBar control to the bottom of the form, bring up the code window using , select the first text box (txtNumber1) from the left hand dropdown box, and then select the GotFocus event from the right hand dropdown box. Now some basic instructions can be written in the status bar so that when the cursor is in the text box (the text box has focus) the status bar reads “Enter the first number”. After completing this change to the second text box and using the same GotFocus event change the statusbar text to “Enter a second number”. The code to set the status bar can be seen below.

Private Sub chkBold_Click()
If chkBold.Value = 1 Then ' If checked.
txtDisplay.FontBold = True
Else ' If not checked.
txtDisplay.FontBold = False
End If
End Sub

Private Sub chkItalic_Click()
If chkItalic.Value = 1 Then ' If checked.
txtDisplay.FontItalic = True
Else ' If not checked.
txtDisplay.FontItalic = False
End If
End Sub

This example can be found at "smaples/PGuide/controls/Controls.vbp" or downloaded free from the download page.
The checkboxes can be turned on at runtime by simply typing

name.value = 1 ' 1 On , 0 off

Note: If you create the frame first and then add the option buttons by single clicking on the toolbox and dragging the cross hair cursor on the frame to create the controls, they will be attached to the frame and will move with it if you decide to re-position the frame. Notice, however, that if you create the frame first and double click the screen controls, then drag them from the centre of the form on to the frame, they will not be attached to it and will be left behind when you try to move the frame. Try this out.

Notice that when you run your application the same icon is loaded first (probably the clipboard, if you created that option button first). You can alter the option that has the focus first, by selecting one of the other option buttons and setting its property tabindex to 1.

Option Buttons

Private Sub Form_Load()
BackColor = QBColor(Rnd * 15)
ForeColor = QBColor(Rnd * 10)
Picture1.BackColor = QBColor(Rnd * 15)
Picture1.ForeColor = QBColor(Rnd * 10)
End Sub

List boxes

List boxes and combo boxes are used to supply a list of options to the user. The toolbox icons representing these two controls are for list box and for combo box.

A list box is used when the user is to be presented with a fixed set of selections (i.e. a choice must be made only from the items displayed, there is no possibility of the user typing in an alternative).

Examples might be offering a list of the days in a week, the available modules in an elective catalogue, the holiday destinations available from a particular airport or the types of treatment offered by a beauty salon.

To create a list box, double click on the toolbox icon . Drag the resulting box into place on the form and size it according to the data it will hold. The left hand picture below shows a list box that has been created and sized on Form1. In the middle is the code that is required to load a selection of cars into the list. The data has been included in the procedure Sub Form_Load so that it appears when Form1 is loaded. Finally, the picture on the right shows what appears on the form when the application is run. Notice that vertical scroll bars are added automatically if the list box is not deep enough to display all the choices.

Add to a Lisbox

The source code should look somthing like this :

Removing & Advanced Options

list1.RemoveItem 1

And to add to the list in a paticular order just add

list1.additem "My Day", 5

This will be added after saturday.

And finally to clear the lisbox type

List1.clear This will completly clear the contence of the listbox.

Note: The property ListCount stores the number of items in a list, so list1.ListCount can be used to determine the number of items in list box list1.

The property ListIndex gives the index of the currently selected list item. So the statement list1.RemoveItem List1.ListIndex removes the currently highlighted list item.

Adding an item can be accomplished very neatly using an input dialog box. Try this:

This will open a message box and prompt you for a new day to enter this will then be added to the list index at the bottem unless you specify were it should go.

Combo Boxes

Picture of combo box style 0

The user can either enter text in the edit field or select from the list of items by clicking on the (detached) down arrow to the right. The drop-down Combo box, list is viewed by clicking on the down arrow. This type of combo box does not have a down arrow because the list is displayed at all times. If there are more items than can be shown in the size of box you have drawn, vertical scroll bars are automatically added. As with previous type, users can enter text in the edit field.

Drop-down List box (Style=2)

It is slightly confusing to find this control under combo box. This control behaves like a regular list box except that the choices are not revealed until the down arrow is clicked. The user can select only from the choices given, there is no text entry facility.

Note: Combo boxes of style 0 and 2 cannot respond to double click events.

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