QPixmapCache(3qt)QPixmapCache(3qt)NAMEQPixmapCache - Application-global cache for pixmaps
SYNOPSIS
#include <qpixmapcache.h>
Static Public Members
<li class=fn>int cacheLimit () <li class=fn>void setCacheLimit ( int n
) <li class=fn>QPixmap * find ( const QString & key ) <li class=fn>bool
find ( const QString & key, QPixmap & pm ) <li class=fn>bool insert (
const QString & key, QPixmap * pm ) (obsolete) <li class=fn>bool insert
( const QString & key, const QPixmap & pm ) <li class=fn>void remove (
const QString & key ) <li class=fn>void clear ()
DESCRIPTION
The QPixmapCache class provides an application-global cache for
pixmaps.
This class is a tool for optimized drawing with QPixmap. You can use it
to store temporary pixmaps that are expensive to generate without using
more storage space than cacheLimit(). Use insert() to insert pixmaps,
find() to find them and clear() to empty the cache.
For example, QRadioButton has a non-trivial visual representation so we
don't want to regenerate a pixmap whenever a radio button is displayed
or changes state. In the function QRadioButton::drawButton(), we do not
draw the radio button directly. Instead, we first check the global
pixmap cache for a pixmap with the key "$qt_radio_nnn_", where nnn is a
numerical value that specifies the the radio button state. If a pixmap
is found, we bitBlt() it onto the widget and return. Otherwise, we
create a new pixmap, draw the radio button in the pixmap, and finally
insert the pixmap in the global pixmap cache, using the key above. The
bitBlt() is ten times faster than drawing the radio button. All radio
buttons in the program share the cached pixmap since QPixmapCache is
application-global.
QPixmapCache contains no member data, only static functions to access
the global pixmap cache. It creates an internal QCache for caching the
pixmaps.
The cache associates a pixmap with a string (key). If two pixmaps are
inserted into the cache using equal keys, then the last pixmap will
hide the first pixmap. The QDict and QCache classes do exactly the
same.
The cache becomes full when the total size of all pixmaps in the cache
exceeds cacheLimit(). The initial cache limit is 1024 KByte (1 MByte);
it is changed with setCacheLimit(). A pixmap takes roughly
width*height*depth/8 bytes of memory.
See the QCache documentation for more details about the cache
mechanism.
See also Environment Classes, Graphics Classes, and Image Processing
Classes.
MEMBER FUNCTION DOCUMENTATIONint QPixmapCache::cacheLimit () [static]
Returns the cache limit (in kilobytes).
The default setting is 1024 kilobytes.
See also setCacheLimit().
void QPixmapCache::clear () [static]
Removes all pixmaps from the cache.
QPixmap * QPixmapCache::find ( const QString & key ) [static]
Returns the pixmap associated with the key in the cache, or null if
there is no such pixmap.
Warning: If valid, you should copy the pixmap immediately (this is
fast). Subsequent insertions into the cache could cause the pointer to
become invalid. For this reason, we recommend you use find(const
QString&, QPixmap&) instead.
Example:
QPixmap* pp;
QPixmap p;
if ( (pp=QPixmapCache::find("my_big_image", pm)) ) {
p = *pp;
} else {
p.load("bigimage.png");
QPixmapCache::insert("my_big_image", new QPixmap(p));
}
painter->drawPixmap(0, 0, p);
bool QPixmapCache::find ( const QString & key, QPixmap & pm ) [static]
This is an overloaded member function, provided for convenience. It
behaves essentially like the above function.
Looks for a cached pixmap associated with the key in the cache. If a
pixmap is found, the function sets pm to that pixmap and returns TRUE;
otherwise leaves pm alone and returns FALSE.
Example:
QPixmap p;
if ( !QPixmapCache::find("my_big_image", pm) ) {
pm.load("bigimage.png");
QPixmapCache::insert("my_big_image", pm);
}
painter->drawPixmap(0, 0, p);
bool QPixmapCache::insert ( const QString & key, const QPixmap & pm ) [static]
Inserts a copy of the pixmap pm associated with the key into the cache.
All pixmaps inserted by the Qt library have a key starting with" $qt",
so your own pixmap keys should never begin "$qt".
When a pixmap is inserted and the cache is about to exceed its limit,
it removes pixmaps until there is enough room for the pixmap to be
inserted.
The oldest pixmaps (least recently accessed in the cache) are deleted
when more space is needed.
See also setCacheLimit().
bool QPixmapCache::insert ( const QString & key, QPixmap * pm ) [static]
This function is obsolete. It is provided to keep old source working.
We strongly advise against using it in new code.
Inserts the pixmap pm associated with key into the cache. Returns TRUE
if successful, or FALSE if the pixmap is too big for the cache.
Note: pm must be allocated on the heap (using new).
If this function returns FALSE, you must delete pm yourself.
If this function returns TRUE, do not use pm afterwards or keep
references to it because any other insertions into the cache, whether
from anywhere in the application or within Qt itself, could cause the
pixmap to be discarded from the cache and the pointer to become
invalid.
Due to these dangers, we strongly recommend that you use insert(const
QString&, const QPixmap&) instead.
void QPixmapCache::remove ( const QString & key ) [static]
Removes the pixmap associated with key from the cache.
void QPixmapCache::setCacheLimit ( int n ) [static]
Sets the cache limit to n kilobytes.
The default setting is 1024 kilobytes.
See also cacheLimit().
SEE ALSO
http://doc.trolltech.com/qpixmapcache.html
http://www.trolltech.com/faq/tech.html
COPYRIGHT
Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com. See the
license file included in the distribution for a complete license
statement.
AUTHOR
Generated automatically from the source code.
BUGS
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Trolltech AS 2 February 2007 QPixmapCache(3qt)