QPixmapCache(3qt)

1QPixmapCache(3qt)                                            QPixmapCache(3qt)
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NAME

6       QPixmapCache - Application-global cache for pixmaps
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SYNOPSIS

9       #include <qpixmapcache.h>
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11   Static Public Members
12       <li class=fn>int cacheLimit () <li class=fn>void setCacheLimit ( int n
13       ) <li class=fn>QPixmap * find ( const QString & key ) <li class=fn>bool
14       find ( const QString & key, QPixmap & pm ) <li class=fn>bool insert (
15       const QString & key, QPixmap * pm ) (obsolete) <li class=fn>bool insert
16       ( const QString & key, const QPixmap & pm ) <li class=fn>void remove (
17       const QString & key ) <li class=fn>void clear ()
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DESCRIPTION

20       The QPixmapCache class provides an application-global cache for
21       pixmaps.
22
23       This class is a tool for optimized drawing with QPixmap. You can use it
24       to store temporary pixmaps that are expensive to generate without using
25       more storage space than cacheLimit(). Use insert() to insert pixmaps,
26       find() to find them and clear() to empty the cache.
27
28       For example, QRadioButton has a non-trivial visual representation so we
29       don't want to regenerate a pixmap whenever a radio button is displayed
30       or changes state. In the function QRadioButton::drawButton(), we do not
31       draw the radio button directly. Instead, we first check the global
32       pixmap cache for a pixmap with the key "$qt_radio_nnn_", where nnn is a
33       numerical value that specifies the the radio button state. If a pixmap
34       is found, we bitBlt() it onto the widget and return. Otherwise, we
35       create a new pixmap, draw the radio button in the pixmap, and finally
36       insert the pixmap in the global pixmap cache, using the key above. The
37       bitBlt() is ten times faster than drawing the radio button. All radio
38       buttons in the program share the cached pixmap since QPixmapCache is
39       application-global.
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41       QPixmapCache contains no member data, only static functions to access
42       the global pixmap cache. It creates an internal QCache for caching the
43       pixmaps.
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45       The cache associates a pixmap with a string (key). If two pixmaps are
46       inserted into the cache using equal keys, then the last pixmap will
47       hide the first pixmap. The QDict and QCache classes do exactly the
48       same.
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50       The cache becomes full when the total size of all pixmaps in the cache
51       exceeds cacheLimit(). The initial cache limit is 1024 KByte (1 MByte);
52       it is changed with setCacheLimit(). A pixmap takes roughly
53       width*height*depth/8 bytes of memory.
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55       See the QCache documentation for more details about the cache
56       mechanism.
57
58       See also Environment Classes, Graphics Classes, and Image Processing
59       Classes.
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MEMBER FUNCTION DOCUMENTATION

int QPixmapCache::cacheLimit () [static]

63       Returns the cache limit (in kilobytes).
64
65       The default setting is 1024 kilobytes.
66
67       See also setCacheLimit().
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void QPixmapCache::clear () [static]

70       Removes all pixmaps from the cache.
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QPixmap * QPixmapCache::find ( const QString & key ) [static]

73       Returns the pixmap associated with the key in the cache, or null if
74       there is no such pixmap.
75
76       Warning: If valid, you should copy the pixmap immediately (this is
77       fast). Subsequent insertions into the cache could cause the pointer to
78       become invalid. For this reason, we recommend you use find(const
79       QString&, QPixmap&) instead.
80
81       Example:
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83               QPixmap* pp;
84               QPixmap p;
85               if ( (pp=QPixmapCache::find("my_big_image", pm)) ) {
86                   p = *pp;
87               } else {
88                   p.load("bigimage.png");
89                   QPixmapCache::insert("my_big_image", new QPixmap(p));
90               }
91               painter->drawPixmap(0, 0, p);
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bool QPixmapCache::find ( const QString & key, QPixmap & pm ) [static]

94       This is an overloaded member function, provided for convenience. It
95       behaves essentially like the above function.
96
97       Looks for a cached pixmap associated with the key in the cache. If a
98       pixmap is found, the function sets pm to that pixmap and returns TRUE;
99       otherwise leaves pm alone and returns FALSE.
100
101       Example:
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103               QPixmap p;
104               if ( !QPixmapCache::find("my_big_image", pm) ) {
105                   pm.load("bigimage.png");
106                   QPixmapCache::insert("my_big_image", pm);
107               }
108               painter->drawPixmap(0, 0, p);
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bool QPixmapCache::insert ( const QString & key, const QPixmap & pm ) [static]

111
112       Inserts a copy of the pixmap pm associated with the key into the cache.
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114       All pixmaps inserted by the Qt library have a key starting with" $qt",
115       so your own pixmap keys should never begin "$qt".
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117       When a pixmap is inserted and the cache is about to exceed its limit,
118       it removes pixmaps until there is enough room for the pixmap to be
119       inserted.
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121       The oldest pixmaps (least recently accessed in the cache) are deleted
122       when more space is needed.
123
124       See also setCacheLimit().
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bool QPixmapCache::insert ( const QString & key, QPixmap * pm ) [static]

127       This function is obsolete. It is provided to keep old source working.
128       We strongly advise against using it in new code.
129
130       Inserts the pixmap pm associated with key into the cache. Returns TRUE
131       if successful, or FALSE if the pixmap is too big for the cache.
132
133        Note: pm must be allocated on the heap (using new).
134
135       If this function returns FALSE, you must delete pm yourself.
136
137       If this function returns TRUE, do not use pm afterwards or keep
138       references to it because any other insertions into the cache, whether
139       from anywhere in the application or within Qt itself, could cause the
140       pixmap to be discarded from the cache and the pointer to become
141       invalid.
142
143       Due to these dangers, we strongly recommend that you use insert(const
144       QString&, const QPixmap&) instead.
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void QPixmapCache::remove ( const QString & key ) [static]

147       Removes the pixmap associated with key from the cache.
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void QPixmapCache::setCacheLimit ( int n ) [static]

150       Sets the cache limit to n kilobytes.
151
152       The default setting is 1024 kilobytes.
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154       See also cacheLimit().
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COPYRIGHT

162       Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com.  See the
163       license file included in the distribution for a complete license
164       statement.
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AUTHOR

167       Generated automatically from the source code.
168

BUGS

170       If you find a bug in Qt, please report it as described in
171       http://doc.trolltech.com/bughowto.html.  Good bug reports help us to
172       help you. Thank you.
173
174       The definitive Qt documentation is provided in HTML format; it is
175       located at $QTDIR/doc/html and can be read using Qt Assistant or with a
176       web browser. This man page is provided as a convenience for those users
177       who prefer man pages, although this format is not officially supported
178       by Trolltech.
179
180       If you find errors in this manual page, please report them to qt-
181       bugs@trolltech.com.  Please include the name of the manual page
182       (qpixmapcache.3qt) and the Qt version (3.3.8).
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186Trolltech AS                    2 February 2007              QPixmapCache(3qt)