All the controls in the ToolBox except the Pointer are objects in
Visual
Basic. These objects have associated properties, methods and events.
Real world objects are loaded with properties. For example, a flower is loaded
certain color, shape and fragrance. Similarly programming
objects are loaded with properties. A property is a named attribute of a programming
object. Properties define the characteristics of an object such as Size, Color
etc. or sometimes the way in which it behaves. For example, a TextBox accepts
properties such as Enabled, Font, MultiLine, Text, Visible, Width, etc.
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Enables property allows the TextBox to be enabled or disabled at run time depending
on the condition set to True or False.
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Font property sets a particular font in the TextBox.
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MultiLine property allows the TextBox to accept and display multiple lines
at run time.
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Text property of the TextBox control sets a particular text in the control.
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Visible property is used to hide the object at run time.
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Width property sets the TextBox to the desired width at design time.
The properties that are discussed above are design-time properties that can
be set at the design tme by selecting the Properties Window. But certain properties
cannot be set at desgn time. For example, the CurrentX and CurrentY properties
of a Form cannot be set at the design time.
A method is an action that can be performed on objects. For example, a cat
is an object. Its properties might include long white hair, blue eyes, 3 pounds
weight etc. A complete definition of cat must only encompass on its looks, but
should also include a complete itemization of its activities. Therefore, a cat's
methods might be move, jump, play, breath etc.
Siimilarly in object-orinted
programming, a method is a connected or built-in procedure, a block of code
that can be invoked to impart some action on a particular object. A method requires
an object to provide them with a context. For example, the word Move has no meaning
in Visual
Basic, but the statement,
Text1.Move 700, 400
performs a very precise action. The TextBox control has other associated methods
such as Refresh, SetFocus, etc.
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The Refresh method enforces a complete repaint of the control or a Form. For
example, Text1.Refresh refreshes the TextBox.
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The Setfocus method moves the focus on the control. For Example Text1.SetFocus
sets the focus to TextBox control Text1.
Event Driven Programming
Visual
Basic programs are built around events. Events are various things that can
happen in a program. this will become clearer when studied in contrast to procedural
programming.
In procedural languages, an application is written is executed by checking for
the program logically through the program statements, one after another. For a
temporary phase, the control may be transferred to some other point in a program.
While in an event driven application, the program statements are executed only
when a particular event calls a specific part of the code that is assigned to
the event.
Let us consider a TextBox control and a few of its associated events to understand
the concept of event driven programming. The TextBox control supports various
events such as Change, Click, MouseMove and many more that will be listed in the
Properties dropdown list in the code window for the TextBox control. We will look
into a few of them as given below.
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The code entered in the Change event fires when there is a change in the contents
of the TextBox
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The Click event fires when the TextBox control is clicked.
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The MouseMove event fires when the mouse is moved over the TextBox
As explained above, several events are associated with different controls and
forms, some of the events being common to most of them and few being specific
to each control.
