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TQPixmapCache Class Reference

The TQPixmapCache class provides an application-global cache for pixmaps. More...

#include <ntqpixmapcache.h>

List of all member functions.

Static Public Members

• int cacheLimit ()
• void setCacheLimit ( int n )
• TQPixmap * find ( const TQString & key )
• bool find ( const TQString & key, TQPixmap & pm )
• bool insert ( const TQString & key, TQPixmap * pm )  (obsolete)
• bool insert ( const TQString & key, const TQPixmap & pm )
• void remove ( const TQString & key )
• void clear ()

Detailed Description

The TQPixmapCache class provides an application-global cache for pixmaps.

This class is a tool for optimized drawing with TQPixmap. 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, TQRadioButton 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 TQRadioButton::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 TQPixmapCache is application-global.

TQPixmapCache contains no member data, only static functions to access the global pixmap cache. It creates an internal TQCache 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 TQDict and TQCache 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 TQCache documentation for more details about the cache mechanism.

See also Environment Classes, Graphics Classes, and Image Processing Classes.

Member Function Documentation

int TQPixmapCache::cacheLimit () [static]

The default setting is 1024 kilobytes.

See also setCacheLimit().

void TQPixmapCache::clear () [static]

TQPixmap * TQPixmapCache::find ( const TQString & key ) [static]

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 TQString&, TQPixmap&) instead.

Example:

        TQPixmap* pp;
        TQPixmap p;
        if ( (pp=TQPixmapCache::find("my_big_image", pm)) ) {
            p = *pp;
        } else {
            p.load("bigimage.png");
            TQPixmapCache::insert("my_big_image", new TQPixmap(p));
        }
        painter->drawPixmap(0, 0, p);
    

bool TQPixmapCache::find ( const TQString & key, TQPixmap & pm ) [static]

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:

        TQPixmap p;
        if ( !TQPixmapCache::find("my_big_image", pm) ) {
            pm.load("bigimage.png");
            TQPixmapCache::insert("my_big_image", pm);
        }
        painter->drawPixmap(0, 0, p);
    

bool TQPixmapCache::insert ( const TQString & key, const TQPixmap & pm ) [static]

All pixmaps inserted by the TQt 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 TQPixmapCache::insert ( const TQString & key, TQPixmap * pm ) [static]

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 TQt 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 TQString&, const TQPixmap&) instead.

void TQPixmapCache::remove ( const TQString & key ) [static]

void TQPixmapCache::setCacheLimit ( int n ) [static]

The default setting is 1024 kilobytes.

See also cacheLimit().

This file is part of the TQt toolkit. Copyright © 1995-2007 Trolltech. All Rights Reserved.