Teach Yourself Borland C++ in 14 Days - portal

146 Day 5
Events
Windows is said to be an event-driven environment. Event-driven means that a
program is driven by events that occur within the Windows environment. Events
include mouse movements, mouse clicks, and keypresses.
Programmers moving from DOS or mainframe programming environments may have some
difficulty with the concept of something being event driven. A Windows program continually
polls Windows for events. Events in Windows include a menu being activated, a button
being clicked, a window being moved, a window needing repainting, a window being
activated, and so forth. Windows notifies a program of an event by sending a Windows
message. There are somewhere in the neighborhood of 175 possible messages that Windows
can send to an application. That’s a lot of messages! Fortunately, you don’t have to know
about each and every one of them to program in C++Builder; there are only a couple dozen
that are used frequently.
In VCL, an event is anything that occurs in the component that the user might need to know
about. Each component is designed to respond to certain events. Usually this means a
Windows event, but it can mean other things as well. For example, a button component is
designed to respond to a mouse click, as you would expect. But a nonvisual control such as
a database component might respond to non-Windows events such as the user reaching the
end of the table.
When you respond to a component’s event, you are said to handle the event.
Events are handled through functions called event handlers.
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VCL makes it incredibly easy to handle events. The events that a component has been
designed to handle are listed under the Events tab in the Object Inspector window. An event
name is descriptive of the event to which it responds. For instance, the event to handle a
mouse click is called OnClick.
You don’t have to handle every event that a component defines. In fact,
you rarely do. If you do not respond to a particular event, the event
message is either discarded or handled in a default manner, as described
by either VCL or the component itself. You can handle any events you
have an interest in and ignore the rest.
This will make more sense if you can put it into practice. To begin, let’s start a new application
from scratch. Choose File | New Application from the main menu. If you are prompted to
save the current project, click No. Now you should again have a blank form. First, let’s set
up the main form: