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<H2><A NAME="SECTION00031000000000000000">
A Quick Introduction to Views</A>
</H2>
<TT>NSView</TT> is a class whose objects represent a rectangular area
in a window. Each <TT>NSView</TT> has its own internal coordinate
system; <TT>NSView</TT> provides a complete framework for managing the
rectangle represented by the view, its coordinate system, and for
drawing and getting user events inside the view's rectangle.
<P>
Any object which can be displayed (and/or can accept user events)
inside a window (such as a button, a text field, a scroll view, a
slider, etc) is actually an instance of a subclass of <TT>NSView</TT>.
The specific subclass (for example, <TT>NSButton</TT>) implements a
specific way of drawing and managing user events inside the view's
rectangle. An unsubclassed <TT>NSView</TT> by default draws nothing
and reacts to no user events, even if it contains all the machinery
(ready for subclasses to use) to do both.
<P>
When you create a window, the library automatically creates an
<TT>NSView</TT> covering the whole content area of the window.
This view is called the window's <I>content view</I>. You can replace
the default content view (which is transparent and reacts to no user
events) with an <TT>NSView</TT> of your choice (which can be an
instance of a subclass of <TT>NSView</TT>, such as a
<TT>NSButton</TT> or a <TT>NSScrollView</TT>), but you should not change
the rectangle it represents, which must always cover the whole window.
<P>
If you want to have a view which covers only a part of a window, you
need to add it as a <I>subview</I> of the content view. We will
explore subviews in a forth-coming tutorial; in our first example, we
will go for the simplest possible solution avoiding use of subviews:
we will create a single <TT>NSButton</TT>, make a window to fit
exactly our button, and then replace the default window's content view
with our button. In this way we'll get a window containing exactly
our button. In the next tutorial, we'll learn how to enclose views
one over the other in a subview tree, so that we can put more things
in a single window.
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<ADDRESS>
Nicola Pero
2000-07-29
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