KSharedDataCache

#include <KSharedDataCache>

Public Types

enum  EvictionPolicy { NoEvictionPreference = 0, EvictLeastRecentlyUsed, EvictLeastOftenUsed, EvictOldest }
 

Public Member Functions

 KSharedDataCache (const QString &cacheName, unsigned defaultCacheSize, unsigned expectedItemSize=0)
 
 KSharedDataCache (const KSharedDataCache &)=delete
 
void clear ()
 
bool contains (const QString &key) const
 
EvictionPolicy evictionPolicy () const
 
bool find (const QString &key, QByteArray *destination) const
 
unsigned freeSize () const
 
bool insert (const QString &key, const QByteArray &data)
 
KSharedDataCacheoperator= (const KSharedDataCache &)=delete
 
void setEvictionPolicy (EvictionPolicy newPolicy)
 
void setTimestamp (unsigned newTimestamp)
 
unsigned timestamp () const
 
unsigned totalSize () const
 

Static Public Member Functions

static void deleteCache (const QString &cacheName)
 

Detailed Description

A simple data cache which uses shared memory to quickly access data stored on disk.

This class is meant to be used with KImageCache and similar classes but can be used directly if used with care.

Example usage:

QString loadTranslatedDocument(KSharedDataCache *cache) {
// Find the data
QByteArray document;
if (!cache->find("translated-doc-template", &document)) {
// Entry is not cached, manually generate and then add to cache.
document = translateDocument(globalTemplate());
cache->insert(document);
}
// Don't forget to encode/decode properly
return QString::fromUtf8(document);
}
Author
Michael Pyne mpyne[email protected]@kde[email protected].org
See also
KImageCache
Since
4.5

Definition at line 49 of file kshareddatacache.h.

Constructor & Destructor Documentation

KSharedDataCache::KSharedDataCache ( const QString cacheName,
unsigned  defaultCacheSize,
unsigned  expectedItemSize = 0 
)

Attaches to a shared cache, creating it if necessary.

If supported, this data cache will be shared across all processes using this cache (with subsequent memory savings). If shared memory is unsupported or a failure occurs, caching will still be supported, but only in the same process, and only using the same KSharedDataCache object.

Parameters
cacheNameName of the cache to use/share.
defaultCacheSizeAmount of data to be able to store, in bytes. The actual size will be slightly larger on disk due to accounting overhead. If the cache already existed then it will not be resized. For this reason you should specify some reasonable size.
expectedItemSizeThe average size of an item that would be stored in the cache, in bytes. Choosing an average size of zero bytes causes KSharedDataCache to use whatever it feels is the best default for the system.

Definition at line 1388 of file kshareddatacache.cpp.

Member Function Documentation

void KSharedDataCache::clear ( )

Removes all entries from the cache.

Definition at line 1628 of file kshareddatacache.cpp.

bool KSharedDataCache::contains ( const QString key) const

Returns true if the cache currently contains the image for the given filename.

NOTE: Calling this function is threadsafe, but it is in general not possible to guarantee the image stays cached immediately afterwards, so if you need the result use find().

Definition at line 1641 of file kshareddatacache.cpp.

void KSharedDataCache::deleteCache ( const QString cacheName)
static

Removes the underlying file from the cache.

Note that this is all that this function does. The shared memory segment is still attached and will still contain all the data until all processes currently attached remove the mapping.

In order to remove the data see clear().

Definition at line 1656 of file kshareddatacache.cpp.

KSharedDataCache::EvictionPolicy KSharedDataCache::evictionPolicy ( ) const
Returns
The removal policy in use by the shared cache.
See also
EvictionPolicy

Definition at line 1697 of file kshareddatacache.cpp.

bool KSharedDataCache::find ( const QString key,
QByteArray destination 
) const

Returns the data in the cache named by key (even if it's some other process's data named with the same key!), stored in destination.

If there is no entry named by key then destination is left unchanged. The return value is used to tell what happened.

If you simply want to verify whether an entry is present in the cache then see contains().

Parameters
keyThe key to find in the cache.
destinationIs set to the value of key in the cache if key is present, left unchanged otherwise.
Returns
true if key was present in the cache (destination will also be updated), false if key was not present (destination will be unchanged).

Definition at line 1585 of file kshareddatacache.cpp.

unsigned KSharedDataCache::freeSize ( ) const

Returns the amount of free space in the cache, in bytes.

Due to implementation details it is possible to still not be able to fit an entry in the cache at any given time even if it is smaller than the amount of space remaining.

Definition at line 1682 of file kshareddatacache.cpp.

bool KSharedDataCache::insert ( const QString key,
const QByteArray data 
)

Attempts to insert the entry data into the shared cache, named by key, and returns true only if successful.

Note that even if the insert was successful, that the newly added entry may be evicted by other processes contending for the cache.

Definition at line 1432 of file kshareddatacache.cpp.

void KSharedDataCache::setEvictionPolicy ( KSharedDataCache::EvictionPolicy  newPolicy)

Sets the entry removal policy for the shared cache to newPolicy.

The default is EvictionPolicy::NoEvictionPreference.

See also
EvictionPolicy

Definition at line 1706 of file kshareddatacache.cpp.

void KSharedDataCache::setTimestamp ( unsigned  newTimestamp)

Sets the shared timestamp of the cache.

Timestamping is supported to allow applications to more effectively "version" the data stored in the cache. However, the timestamp is shared with all applications using the cache so you should always be prepared for invalid timestamps.

When the cache is first created (note that this is different from attaching to an existing shared cache on disk), the cache timestamp is initialized to the time returned by time(2). KSharedDataCache will not update the timestamp again, it is only updated through this method.

Example:

QImage loadCachedImage(const QString &key)
{
// Check timestamp
if (m_sharedCache->timestamp() < m_currentThemeTimestamp) {
// Cache is stale, clean it out.
m_sharedCache->clear();
m_sharedCache->setTimestamp(m_currentThemeTimestamp);
}
// Check cache and load image as usual...
}
Parameters
newTimestampThe new timestamp to mark the shared cache with.
See also
timestamp()
Since
4.6

Definition at line 1722 of file kshareddatacache.cpp.

unsigned KSharedDataCache::timestamp ( ) const
Returns
The shared timestamp of the cache. The interpretation of the timestamp returned is up to the application. KSharedDataCache will initialize the timestamp to the time returned by time(2) on cache creation, but KSharedDataCache will not touch the timestamp again.
See also
setTimestamp()
Since
4.6

Definition at line 1713 of file kshareddatacache.cpp.

unsigned KSharedDataCache::totalSize ( ) const

Returns the usable cache size in bytes.

The actual amount of memory used will be slightly larger than this to account for required accounting overhead.

Definition at line 1667 of file kshareddatacache.cpp.


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