Java Reference
In-Depth Information
The
paintBorder()
and
paintChildren()
methods tend not to be overridden. The
paintBorder()
method draws a border around the component, a concept described more
fully in Chapter 7. The
paintChildren()
method draws the components within the Swing
container object, if any are present.
To optimize painting, the
JComponent
class provides three additional painting properties:
opaque
,
optimizedDrawingEnabled
, and
doubleBuffered
. These work as follows:
Opacity:
The
opaque
property for a
JComponent
defines whether a component is trans-
parent. When transparent, the container of the
JComponent
must paint the background
behind the component. To improve performance, you can leave the
JComponent
opaque
and let the
JComponent
draw its own background, instead of relying on the container to
draw the covered background.
•
•
Optimization:
The
optimizedDrawingEnabled
property determines whether immediate
children can overlap. If children cannot overlap, the repaint time is reduced consider-
ably. By default, optimized drawing is enabled for most Swing components, except for
JDesktopPane
,
JLayeredPane
, and
JViewport
.
•
Double buffering:
By default, all Swing components double buffer their drawing operations
into a buffer shared by the complete container hierarchy; that is, all the components
within a window (or subclass). This greatly improves painting performance, because
when double buffering is enabled (with the
doubleBuffered
property), there is only a
single screen update drawn.
■
Note
For synchronous painting, you can call one of the
public void paintImmediately()
methods.
(Arguments are either a
Rectangle
or its parts—position and dimensions.) However, you'll rarely need to
call this directly unless your program has real-time painting requirements.
The public void
revalidate()
method of
JComponent
also offers painting support. When
this method is called, the high-level container of the component validates itself. This is unlike
the AWT approach requiring a direct call to the
revalidate()
method of that high-level
component.
The last aspect of the Swing component painting enhancements is the
RepaintManager
class.
RepaintManager Class
The
RepaintManager
class is responsible for ensuring the efficiency of repaint requests on the
currently displayed Swing components, making sure the smallest “dirty” region of the screen is
updated when a region becomes invalid.
Although rarely customized,
RepaintManager
is public and provides a static installation
routine to use a custom manager:
public static void setCurrentManager(RepaintManager
manager)
. To get the current manager, just ask with
public static void currentManager
(JComponent)
. The argument is usually
null
, unless you've customized the manager to provide
component-level support. Once you have the manager, one thing you can do is get the off-
screen buffer for a component as an
Image
. Because the buffer is what is eventually shown on
