System objects allow you to access system resources like
Clipboard, and Screen etc. Visual Basic
allows you to access the system resources through properties and methods of the
system objects. The following is the list of system objects and what they do.
Detailed discussion about each system object will follow.
System Object
|
Description
|
Screen
|
Allows you to access properties
of screen such as width, height, fonts available screen etc.
|
Printer
|
Allows you to access the
default printer.
|
Clipboard
|
Allows you to place data on
clipboard and take data from clipboard.
|
App
|
Lets you access important
information regarding the current application.
|
Debug
|
Allows you to send output to
immediate window at runtime.
|
Table 11.1: System objects.
The following section will discuss each of the system
objects in detail.
Screen Object
Allows you to access screen-related properties. Screen
object is accessed using keyword Screen.
The following are the various properties of Screen object.
Property
|
Description |
ActiveControl
|
Returns the control that
currently has focus.
|
ActiveForm
|
Returns the form that
currently has focus.
|
FontCount
|
Returns the number of fonts
available for the display device.
|
Fonts()
|
This is an array that contains
the names of all the available fonts for display device.
|
MousePointer
|
Determines the type of mouse
pointer to be used.
|
MouseIcon
|
Contains the custom mouse
icon. This is taken when MousePointer property is set to vbCustom.
|
Height and Width
|
Returns the size of the screen
in Twips.
|
TwipsperpixelX , TwipsperpixelY
|
Return the number of twips per
pixel on the screen.
|
Table 11.2:
Screen object properties.
The following snippets of code demonstrate how to use screen
object.
To get the list of
fonts supported by the display device place them in a list box, enter:
For I = 0 to screen.fontcount
–1
List1.additem (screen.fonts(I))
Next
To change mouse
pointer to a custom mouse pointer, enter:
Screen.mouseicon =
LoadPicture(“arrow.ico”);
Screen.mousepointer = vbCustom
To center the form on
the screen, enter the following code in FORM_LOAD event:
Private Sub Form_Load()
Dim x as integer, y as integer
x = (Screen.Width - Me.Width) / 2
y = (Screen.Height
- Me.Height) / 2
Me.Move x, y
End Sub
Printer Object
Printer object allows you to print to default printer. The
following are the important properties of Printer
object.
Property
|
Meaning |
Copies
|
Specifies the number of copies
to be printed. Default is 1.
|
ColorMode
|
Determines whether a color
printer prints output in color or monochrome. Valid options are: 1 – monochrome, 2 – color. If the
printer is monochrome then it ignores this property.
|
Devicename
|
Returns the name of the
device.
|
Drivername
|
Returns the name of the driver
|
Orientation
|
Indicates whether documents
are printed in portrait or landscape mode.
1 – portrait, 2 – landscape.
|
Page
|
Returns the current page
number.
|
PaperSize
|
Returns or sets a value
indicating the paper size for the current printer Please see on-line help for
valid options.
|
PaperBin
|
Indicates the bin on the
printer from which paper is fed when printing. Please see on-line help for
valid options.
|
Printquality
|
Returns or sets a value
indicating the printer resolution. Valid options are:
-1 Draft, -2 Low, -3 Medium,
-4 High.
You can also set this property
to dots per inch (DPI), like 300.
|
Zoom
|
Returns or sets the percentage
by which printed output is to be scaled up or down. Default is 0, which means
normal size.
|
Table 11.3:
Properties that are unique to Printer Object
Printer object
also has collection of methods. While properties are used to change the
settings, methods are used to actually print and control printing operations.
Following are the methods that are specific to Printer object. Remember Printer object has some of the common
methods such as Circle, Line etc. But in this section, I will
discuss about only the methods that are specific to Printer object. For complete list, please see on-line help.
Method
|
What it does? |
EndDoc
|
Terminates a print operation
sent to the printer using Printer object and releases the document to the
print device or spooler. Printing will not start unless you execute EndDoc.
|
KillDoc
|
Terminates the current print
job immediately.
|
NewPage
|
Ends the current pages and advances to the next page.
|
Print
|
Prints the given text to
printer.
|
Table 11.4:
Methods that are specific to Printer object.
The following are various examples regarding how to use Printer object properties and methods.
To display two lines
in draft resolution mode, enter:
Printer.PrintQuality = -1
'draft resolution
Printer.Print "Who is the author of this book?"
Printer.Print "P.Srikanth"
Printer.EndDoc
To display two lines
in two different pages with various other setting, enter:
Printer.CurrentX = 0
Printer.CurrentY = 0
Printer.colromode = 2 '
put in color mode
Printer.Orientation = 2 ' landscape
Printer.Zoom = 100 '
double the size
Printer.Print "This is at the uppper-left corner"
Printer.NewPage ' go
to next page
Printer.Print "This is in the second page"
Printer.EndDoc
App object
App object contains important information regarding the
current application such as help file to
be used, whether previous instance of this application is running etc.
The following are the important properties of App object. For the complete list of
properties, please see on-line help for
App object.
Property
|
Meaning |
ExeName
|
Returns the root name of the
executable file (without the extension) that is currently running. If running
in the development environment, returns the name of the project
|
Taskvisible
|
Returns or sets a value that
determines if the application appears in the Windows task list.
|
HInstance
|
Returns a handle to the
instance of the application. If project is run in IDE, returns the handle of
the IDE.
|
LogMode
|
Specifies how logging is to be carried out.
|
LogPath
|
Specifies the path and file
name to which logging information should be written.
|
UnAttendedApp
|
Returns a value that
determines if an application will run without any user interface.
|
Helpfile
|
Specifies the path and
filename of a Help file used by your application to display Help or online
documentation.
|
PrevInstance
|
Returns a value indicating
whether a previous instance of the application is already running.
|
Startmode
|
Returns or sets a value that
determines whether an application starts as a stand-alone project or as an
ActiveX component.
0 – Standalone, 1 - ActiveX component.
|
Table 11.5: Properties of App object.
The following are the available methods of App object.
Method
|
What it does? |
Logevent logbuffer, eventtype
|
Logs an event in the
application's log target, which is specified using LogPath property.
Logbuffer is the message to be
logged.
EventType is: 1-error, 2-warning, 4 – information.
|
StartLogging
logTarget, logMode
|
Sets the log target and log
mode of an operation.
|
Table 11.6:Methods
of App object.
To test whether
application has started as a standalone or as an ActiveX component:
If App.StartMode = 0 then
‘do what you want
for standalone application
Else
'do what you want for
ActiveX component
End if
To prevent second instance
from starting:
If not IsNull (App.PrevInstance) then
Unload me
End if
Debug Object
Allows you to write to immediate window. This object is
meaningful only when project is run in development environment. At run time Debug object is ignored.
The following are the methods available in Debug object.
Method
|
What it does? |
Print
|
Prints the given text to Immediate window.
|
Assert
|
Suspends the execution at the
line where method appears if the given condition is false. If the condition
is true, it has no effect on execution.
|
Table 11.7:
Methods of Debug object.
To suspend the execution of program if the value of
variable amount is more than 10000, enter:
Debug.Assert
amount > 10000
To print the value of variable amount to immediate window,
enter:
Debug.Print amount
Note:
you can suspend execution of the program using stop method by pressing Ctrl +
Break in VB IDE
Clipboard Object
Allows you to access system clipboard. Visual Basic
applications can place data on to clipboard so that other applications can
receive the data. In the same way it is also possible to receive data that is
currently placed on clipboard. The
following are the available methods in clipboard.
Method
|
What it does? |
Settext
|
Places the given text on the
clipboard.
|
Setdata
|
Places the given binary data,
such as pictures, on the clipboard.
|
Gettext
|
Returns the text that is
currently on clipboard.
|
Getdata
|
Returns the data that is
currently on clipboard. This is used to retrieve binary data such as
pictures.
|
Clear
|
Clears data that is on
clipboard.
|
Getformat
|
Returns true if clipboard
contains the data of the specified format. Please see on-line help for the
list of valid formats.
|
Table 11.8:
methods of Clipboard object.
Sample Application
It is time to consolidate. Let us develop an application
that can place data on the clipboard or send the data to printer. The sample
application displays the list of available fonts so that user can format the
text before printing. The application
also takes care of enabling and disabling options such as cut and paste
automatically.
Figure 11.1:
Application to demonstrate system objects.
See figure 11.1 for the user-interface of the application.
And the following is the menu structure and what each menu item does.
Option
|
What it does? |
Cut
|
Places the text that is
currently selected in text box to clipboard. And removes the selected text
from textbox.
|
Copy
|
Places the text that is
currently selected in textbox to clipboard.
|
Paste
|
Copies the text that is
currently available in clipboard at the cursor position in textbox. If any
text is selected in text box then the text from clipboard replaces the
selected text of the textbox.
|
Table 11.9:
Meaning of options in Edit menu.
Let us now understand what these controls on the form do?
Text Box
Allows user to enter the text that can be sent either to
clipboard or to printer. It supports multiple lines and also contains
scrollbars.
First Combobox
Display the list of fonts available from Screen object. Initially it is set to
font “Arial”. Whenever user selects a new font, the text in textbox will be
displayed in the new font.
Second Combobox
Allows you to select a size for font. It displays number in
the range 6 to 50 with an increment of 2. User is allowed to either select a
number from the list. User is also allowed to enter a new value. The text in
the textbox will be displayed in the new size.
Print command button
Prints the text that is in textbox in the selected font and
with the selected size to default printer.
It uses Printer object to
send text to printer.
Creating the sample application
The following are the steps to be taken to develop the
sample application.
1.
Create a new project using File-> New Project and select Standard Exe as the type of the project.
2.
Place the required control as shown in figure 11.1.
3.
Change the properties as shown in the table below.
Object
|
Property
|
Value |
Form
|
Caption
|
System Objects Demo
|
Label1
|
Caption
|
Print Text
|
Text1
|
Name
|
Txtsample
|
Multiline
|
True
|
|
Scrollbars
|
3 –both
|
|
Text
|
“ “ ( Null string)
|
|
Combo1
|
Name
|
Cmbfonts
|
Style
|
2-dropdown list
|
|
Combo2
|
Name
|
Cmbsize
|
Command1
|
Name
|
Cmdprint
|
Caption
|
&Print
|
|
Command2
|
Name
|
CmdQuit
|
Caption
|
&Quit
|
Write code for Form_Load
event to add the names of the available fonts using Screen object. Also add numbers in the range 6 to 50 with an
increment of 2.
Private Sub Form_Load()
'Load fonts from Screen
objects
For i = 0 To
Screen.FontCount - 1
Cmbfonts.AddItem
Screen.Fonts(i)
Next
Cmbfonts.Text =
"Arial" ‘ select Arial as
the default font
'add numbers with
increment of 2 to cmbsize
For i = 6 To 50 Step 2
cmbsize.AddItem i
Next
‘select 6 as the
default size
cmbsize.ListIndex = 0
End Sub
Listing 11.1:
Code for Load event of the form object.
Now let us write code to change the name of the font
whenever user selects a new font name in ComboBox.
Private Sub
Cmbfonts_Click()
txtsample.Font.Name =
Cmbfonts.Text
End Sub
Listing 11.2: Code
for Click event of cmbFonts combo box.
Text property
contains the name of the font currently selected. Use Name attribute of Font
object to change the name of the font for textbox.
In the same way let us write code to change size of the font
whenever user selects a different size using size Combobox. But as this is a
dropdown combo box, user can also directly enter text into it. So after user
enters the size, the new size should be used to change the size of text in
textbox. For this we use LostFocus event
of the Combobox. LostFocus event occurs when user is
moving out of the Combobox.
So we have to write code for two events of size Combobox. Listing 11.3 shows the code
for size combo box (cmbsize).
Private Sub
cmbsize_Click()
txtsample.Font.Size =
Val(cmbsize.Text)
End Sub
Private Sub
cmbsize_LostFocus()
txtsample.Font.Size =
Val(cmbsize.Text)
End Sub
Listing 11.3:
Code for Size ComboBox.
Whenever user clicks on Print
command button, the text that is entered by the user in the textbox should be
printed using the selected font name and font size. The code in listing 11.4,
accomplishes that task.
Private Sub
cmdprint_Click()
‘ Change font name
and font size of the printer object so that
‘ text that is
printed is printed with those attributes
Printer.Font.Name =
Cmbfonts.Text
Printer.Font.Size =
Val(cmbsize.Text)
Printer.Print
txtsample ‘ send text to printer
Printer.EndDoc ‘ indicate end of printing job
End Sub
Listing 11.4:
Code for Print command button
Let us also complete code for quit buttons.
Private Sub
cmdquit_Click()
Unload Me
End Sub
Listing 11.5:
Code for Quit button.
At this stage run the project and test whether changing font
name in Combobox is changing the font of the text in textbox. Also test whether
changing size is changing the size of character in textbox. Also test whether
typing size in size Combobox and moving out of it is changing the size of the
character.
If all tests are successful then proceed to next section,
otherwise find out the bug and debug it. Remember, nobody is a born programmer.
Programming is learnt by writing programs. Initially your program may not be
successful. But finding out errors and correcting them is all what programming
is all about. So do it.
Writing code for Edit menu
Now let us turn our attention to Edit menu. Options in Edit menu are used to place text in clipboard
and retrieve text from clipboard.
First let us concentrate on Cut option. When user selects Cut
option, the following steps are to be taken.
¨
Place the selected text of the textbox on
Clipboard
¨
Delete the text that is selected.
We use SelText
property of the textbox. SelText property of the textbox
contains the text that is currently selected (highlighted).
Note: If no text is
selected then SelText refers to
current position in the textbox.
Here is the code for Cut option of edit menu.
Private Sub
mnucut_Click()
'copy the selected text on to clipboard
Clipboard.SetText txtsample.SelText
'cut the selected text
txtsample.SelText = ""
End Sub
Listing 11.6: Code for Cut option.
Copy option also
does the same as Cut option except
that it doesn’t delete the selected text. Here is the code for copy option.
Private Sub
mnucopy_Click()
'copy the selected text
on to clipboard
Clipboard.SetText
txtsample.SelText
End Sub
Listing 11.7:
Code for Copy option.
Paste option
takes the text that is in the clipboard and places it either at the current
position, if nothing is selected, or replaces the selected text. Both the
purposes can be served using SelText property
of textbox. Listing 11.8, shows the code for Paste option.
Private Sub
mnupaste_Click()
‘ get text from
clipboard and replace selected text
‘ or place it at the
cursor position if not text is selected
txtsample.SelText =
Clipboard.GetText
End Sub
Listing 11.8:
Code for Paste option.
The next and last step is automatically enabling and
disabling options in Edit menu
depending upon the context. That means if no text is selected then Cut and Copy options are to be disabled. In the same way if no text is
existing in clipboard then Paste
option is to be disabled.
That means Cut
and Copy are enabled only when text
is selected in textbox.
Paste option is
enabled only when clipboard contains text.
The following is the code for click event of Edit menu. Click event of Edit menu is the place where you have to enable or
disable because, click event occurs before menu is displayed to user.
Note: But you cannot select click event
of Edit menu in design mode like you can select for options. Because whenever
you click on edit menu the options will be displayed but code window is not
invoked. So get into code window and then select mnuEdit as the object and
Click as the event and write the following code.
Private Sub mnuedit_Click()
' disable all options
mnucut.Enabled = False
mnucopy.Enabled = False
mnupaste.Enabled = False
If txtsample.SelText <> "" Then
' enable cut and
copy options
mnucut.Enabled =
True
mnucopy.Enabled =
True
End If
‘ check whether clipboard has any text
If
Clipboard.GetFormat(vbCFText) Then
mnupaste.Enabled =
True
End If
End Sub
Listing 11.9: Code for Edit menu.
GetFormat method
of Clipboard object returns true if
data in the clipboard is of the specified type. Here we are checking whether
clipboard has any text, using vbCFText
constant. For the list of available constants and corresponding numeric values,
please see on-line help for GetFormat method
of Clipboard object.
System objects are useful in Visual Basic. They are used to
access system devices. App does not refer to any device but it contains
information regarding application.
