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KSharedDataCache Class Reference

from PyKDE5.kcoreaddons import *

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@kde.org>
See also:
KImageCache
Since:
4.5


Enumerations

EvictionPolicy { NoEvictionPreference, EvictLeastRecentlyUsed, EvictLeastOftenUsed, EvictOldest }

Methods

 __init__ (self, QString cacheName, unsigned defaultCacheSize, unsigned expectedItemSize=0)
 clear (self)
bool contains (self, QString key)
KSharedDataCache::EvictionPolicy evictionPolicy (self)
bool find (self, QString key, QByteArray destination)
unsigned freeSize (self)
bool insert (self, QString key, QByteArray data)
 setEvictionPolicy (self, KSharedDataCache::EvictionPolicy newPolicy)
 setTimestamp (self, unsigned newTimestamp)
unsigned timestamp (self)
unsigned totalSize (self)

Static Methods

 deleteCache (QString cacheName)

Method Documentation

__init__ (  self,
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:
cacheName  Name of the cache to use/share.
defaultCacheSize  Amount 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 <em>will not</em> be resized. For this reason you should specify some reasonable size.
expectedItemSize  The 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.

clear (   self )

Removes all entries from the cache.

bool contains (  self,
QString  key
)

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().

KSharedDataCache::EvictionPolicy evictionPolicy (   self )

Returns:
The removal policy in use by the shared cache.
See also:
EvictionPolicy

bool find (  self,
QString  key,
QByteArray  destination
)

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:
key  The key to find in the cache.
destination  Is 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 (@p destination will also be updated), false if key was not present (@p destination will be unchanged).

unsigned freeSize (   self )

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.

bool insert (  self,
QString  key,
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.

setEvictionPolicy (  self,
KSharedDataCache::EvictionPolicy  newPolicy
)

Sets the entry removal policy for the shared cache to newPolicy. The default is EvictionPolicy.NoEvictionPreference.

See also:
EvictionPolicy

setTimestamp (  self,
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 <em>all</em> 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:
newTimestamp  The new timestamp to mark the shared cache with.

See also:
timestamp()
Since:
4.6

unsigned timestamp (   self )

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

unsigned totalSize (   self )

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.


Static Method Documentation

deleteCache ( QString  cacheName
)

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().


Enumeration Documentation

EvictionPolicy
Enumerator:
NoEvictionPreference = 0
EvictLeastRecentlyUsed 
EvictLeastOftenUsed 
EvictOldest 

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This documentation is maintained by Simon Edwards.
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