The QPixmapCache class provides an application-wide cache for pixmaps. More...

#include <qpixmapcache.h>

Classes
class	Key
	The QPixmapCache::Key class can be used for efficient access to the QPixmapCache. \inmodule QtGui. More...

class	KeyData

Static Public Member Functions
static int	cacheLimit ()

static void	setCacheLimit (int)

static bool	find (const QString &key, QPixmap *pixmap)

static bool	find (const Key &key, QPixmap *pixmap)

static bool	insert (const QString &key, const QPixmap &pixmap)

static Key	insert (const QPixmap &pixmap)

static bool	replace (const Key &key, const QPixmap &pixmap)

static void	remove (const QString &key)

static void	remove (const Key &key)

static void	clear ()

Detailed Description

The QPixmapCache class provides an application-wide cache for pixmaps.

\inmodule QtGui

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.

QPixmapCache contains no member data, only static functions to access the global pixmap cache. It creates an internal QCache object for caching the pixmaps.

The cache associates a pixmap with a user-provided string as a key, or with a QPixmapCache::Key that the cache generates. Using QPixmapCache::Key for keys is faster than using strings. The string API is very convenient for complex keys but the QPixmapCache::Key API will be very efficient and convenient for a one-to-one object-to-pixmap mapping - in this case, you can store the keys as members of an object.

If two pixmaps are inserted into the cache using equal keys then the last pixmap will replace the first pixmap in the cache. This follows the behavior of the QHash and QCache classes.

The cache becomes full when the total size of all pixmaps in the cache exceeds cacheLimit(). The initial cache limit is 10240 KB (10 MB); you can change this by calling setCacheLimit() with the required value. A pixmap takes roughly ({width} * {height} * {depth})/8 bytes of memory.

The {Qt Quarterly} article \l{http://doc.qt.io/archives/qq/qq12-qpixmapcache.html}{Optimizing with QPixmapCache} explains how to use QPixmapCache to speed up applications by caching the results of painting.

Note: QPixmapCache is only usable from the application's main thread. Access from other threads will be ignored and return failure.

See also: QCache, QPixmap

Definition at line 49 of file qpixmapcache.h.

Member Function Documentation

◆ cacheLimit()

int QPixmapCache::cacheLimit ( )

static

Returns the cache limit (in kilobytes).

The default cache limit is 10240 KB.

See also: setCacheLimit()

Definition at line 616 of file qpixmapcache.cpp.

◆ clear()

void QPixmapCache::clear ( )

static

Removes all pixmaps from the cache.

Definition at line 668 of file qpixmapcache.cpp.

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◆ find() [1/2]

bool QPixmapCache::find	(	const Key &	key,
		QPixmap *	pixmap
	)

static

Looks for a cached pixmap associated with the given key in the cache. If the pixmap is found, the function sets pixmap to that pixmap and returns true; otherwise it leaves pixmap alone and returns false. If the pixmap is not found, it means that the key is no longer valid, so it will be released for the next insertion.

Since: 4.6

Definition at line 527 of file qpixmapcache.cpp.

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◆ find() [2/2]

bool QPixmapCache::find	(	const QString &	key,
		QPixmap *	pixmap
	)

static

Looks for a cached pixmap associated with the given key in the cache. If the pixmap is found, the function sets pixmap to that pixmap and returns true; otherwise it leaves pixmap alone and returns false.

Since: 4.6

Example:

Definition at line 508 of file qpixmapcache.cpp.

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◆ insert() [1/2]

QPixmapCache::Key QPixmapCache::insert ( const QPixmap & pixmap )

static

Inserts a copy of the given pixmap into the cache and returns a key that can be used to retrieve it.

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(), replace()

Since: 4.6

Definition at line 582 of file qpixmapcache.cpp.

◆ insert() [2/2]

bool QPixmapCache::insert	(	const QString &	key,
		const QPixmap &	pixmap
	)

static

Inserts a copy of the pixmap pixmap 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.

The function returns true if the object was inserted into the cache; otherwise it returns false.

See also: setCacheLimit()

Definition at line 560 of file qpixmapcache.cpp.

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◆ remove() [1/2]

void QPixmapCache::remove ( const Key & key )

static

Removes the pixmap associated with key from the cache and releases the key for a future insertion.

Since: 4.6

Definition at line 654 of file qpixmapcache.cpp.

◆ remove() [2/2]

void QPixmapCache::remove ( const QString & key )

static

Removes the pixmap associated with key from the cache.

Definition at line 641 of file qpixmapcache.cpp.

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◆ replace()

bool QPixmapCache::replace	(	const Key &	key,
		const QPixmap &	pixmap
	)

static

Replaces the pixmap associated with the given key with the pixmap specified. Returns true if the pixmap has been correctly inserted into the cache; otherwise returns false.

See also: setCacheLimit(), insert()

Since: 4.6

Definition at line 598 of file qpixmapcache.cpp.

◆ setCacheLimit()

void QPixmapCache::setCacheLimit ( int n )

static

Sets the cache limit to n kilobytes.

The default setting is 10240 KB.

See also: cacheLimit()

Definition at line 631 of file qpixmapcache.cpp.

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The documentation for this class was generated from the following files:

src/gui/image/qpixmapcache.h
src/gui/image/qpixmapcache.cpp

Classes

Static Public Member Functions

Detailed Description

Member Function Documentation

◆ cacheLimit()

◆ clear()

◆ find() [1/2]

◆ find() [2/2]

◆ insert() [1/2]

◆ insert() [2/2]

◆ remove() [1/2]

◆ remove() [2/2]

◆ replace()

◆ setCacheLimit()