Class: QgsVectorLayerCache

class qgis.core.QgsVectorLayerCache

Bases: PyQt5.QtCore.QObject

This class caches features of a given QgsVectorLayer.

The cached features can be indexed by QgsAbstractCacheIndex.

Proper indexing for a given use-case may speed up performance substantially.

Enums

Methods

addCacheIndex Adds a QgsAbstractCacheIndex to this cache.
cacheGeometry Returns True if the cache will fetch and cache feature geometries.
cacheSize Returns the maximum number of features this cache will hold.
cachedFeatureIds Returns the set of feature IDs for features which are cached.
checkInformationCovered Checks if the information required to complete the request is cached.
childEvent
connectNotify
customEvent
disconnectNotify
featureAtId Gets the feature at the given feature id.
featureCount Returns the number of features contained in the source, or -1 if the feature count is unknown.
featureRemoved Gets called, whenever a feature has been removed.
fields Returns the fields associated with features in the cache.
getFeature Query the layer for the feature with the given id.
getFeatures Query this VectorLayerCache for features.
hasFullCache Returns True if the cache is complete, ie it contains all features.
isFidCached Check if a certain feature id is cached.
isSignalConnected
layer Returns the layer to which this cache belongs
receivers
removeCachedFeature Removes the feature identified by fid from the cache if present.
requestCompleted Gets called, whenever the full list of feature ids for a certain request is known.
sender
senderSignalIndex
setCacheAddedAttributes If this is enabled, the subset of cached attributes will automatically be extended to also include newly added attributes.
setCacheGeometry Enable or disable the caching of geometries
setCacheSize Sets the maximum number of features to keep in the cache.
setCacheSubsetOfAttributes Set the subset of attributes to be cached
setFullCache This enables or disables full caching.
sourceCrs Returns the coordinate reference system for features in the cache.
timerEvent
wkbType Returns the geometry type for features in the cache.

Signals

attributeValueChanged Emitted when an attribute is changed.
cachedLayerDeleted Is emitted when the cached layer is deleted.
featureAdded Emitted when a new feature has been added to the layer and this cache.
finished When filling the cache, this signal gets emitted once the cache is fully initialized.
invalidated The cache has been invalidated and cleared.

Attributes

addCacheIndex(self, cacheIndex: QgsAbstractCacheIndex)

Adds a QgsAbstractCacheIndex to this cache. Cache indices know about features present in this cache and decide, if enough information is present in the cache to respond to a QgsFeatureRequest. The layer cache will take ownership of the index.

Parameters:cacheIndex – The cache index to add.
attributeValueChanged

Emitted when an attribute is changed. Is re-emitted after the layer itself emits this signal. You should connect to this signal, to be sure, to not get a cached value if querying the cache. [signal]

cacheGeometry(self) → bool

Returns True if the cache will fetch and cache feature geometries.

New in version 3.0.

cacheSize(self) → int

Returns the maximum number of features this cache will hold. In case full caching is enabled, this number can change, as new features get added.

Returns:int
cachedFeatureIds(self) → object

Returns the set of feature IDs for features which are cached.

See also

isFidCached()

New in version 3.0.

cachedLayerDeleted

Is emitted when the cached layer is deleted. Is emitted when the cached layers layerDelete() signal is being emitted, but before the local reference to it has been set to None. So call to layer() will still return a valid pointer for cleanup purpose. [signal]

checkInformationCovered(self, featureRequest: QgsFeatureRequest) → bool

Checks if the information required to complete the request is cached. i.e. If all attributes required and the geometry is held in the cache. Please note, that this does not check, if the requested features are cached.

Parameters:featureRequest – The QgsFeatureRequest to be answered
Returns:True if the information is being cached, False if not
childEvent()
connectNotify()
customEvent()
disconnectNotify()
featureAdded

Emitted when a new feature has been added to the layer and this cache. You should connect to this signal instead of the layers’, if you want to be sure that this cache has updated information for the new feature

Parameters:fid – The featureid of the changed feature [signal]
featureAtId(self, featureId: int, feature: QgsFeature, skipCache: bool = False) → bool

Gets the feature at the given feature id. Considers the changed, added, deleted and permanent features

Parameters:
  • featureId – The id of the feature to query
  • feature – The result of the operation will be written to this feature
  • skipCache – Will query the layer regardless if the feature is in the cache already
Returns:

True in case of success

featureCount(self) → int

Returns the number of features contained in the source, or -1 if the feature count is unknown.

featureRemoved(self, fid: int)

Gets called, whenever a feature has been removed. Broadcasts this information to indices, so they can invalidate their cache if required.

Parameters:fid – The feature id of the removed feature.
fields(self) → QgsFields

Returns the fields associated with features in the cache.

finished

When filling the cache, this signal gets emitted once the cache is fully initialized. [signal]

getFeature(self, fid: int) → QgsFeature

Query the layer for the feature with the given id. If there is no such feature, the returned feature will be invalid.

getFeatures(self, featureRequest: QgsFeatureRequest = QgsFeatureRequest()) → QgsFeatureIterator

Query this VectorLayerCache for features. If the VectorLayerCache (and moreover any of its indices) is able to satisfy the request, the returned QgsFeatureIterator will iterate over cached features. If it’s not possible to fully satisfy the request from the cache, part or all of the features will be requested from the data provider.

Parameters:featureRequest – The request specifying filter and required data.
Returns:An iterator over the requested data.

getFeatures(self, expression: str) -> QgsFeatureIterator Query the layer for features matching a given expression.

getFeatures(self, fids: object) -> QgsFeatureIterator Query the layer for the features with the given ids.

getFeatures(self, rectangle: QgsRectangle) -> QgsFeatureIterator Query the layer for the features which intersect the specified rectangle.

hasFullCache(self) → bool

Returns True if the cache is complete, ie it contains all features. This may happen as a result of a call to setFullCache() or by through a feature request which resulted in all available features being cached.

See also

setFullCache()

New in version 3.0.

invalidated

The cache has been invalidated and cleared. Note that when a cache is invalidated the fullCache() setting will be cleared, and a full cache rebuild via setFullCache( True ) will need to be performed. [signal]

isFidCached(self, fid: int) → bool

Check if a certain feature id is cached.

Parameters:fid – The feature id to look for
Returns:True if this id is in the cache
isSignalConnected()
layer(self) → QgsVectorLayer

Returns the layer to which this cache belongs

receivers()
removeCachedFeature(self, fid: int) → bool

Removes the feature identified by fid from the cache if present.

Parameters:fid – The id of the feature to delete
Returns:True if the feature was removed, False if the feature id was not found in the cache
requestCompleted(self, featureRequest: QgsFeatureRequest, fids: object)

Gets called, whenever the full list of feature ids for a certain request is known. Broadcasts this information to indices, so they can update their tables.

Parameters:
  • featureRequest – The feature request that was answered
  • fids – The feature ids that have been returned
sender()
senderSignalIndex()
setCacheAddedAttributes(self, cacheAddedAttributes: bool)

If this is enabled, the subset of cached attributes will automatically be extended to also include newly added attributes.

Parameters:cacheAddedAttributes – Automatically cache new attributes
setCacheGeometry(self, cacheGeometry: bool)

Enable or disable the caching of geometries

Parameters:cacheGeometry – Enable or disable the caching of geometries

See also

cacheGeometry()

setCacheSize(self, cacheSize: int)

Sets the maximum number of features to keep in the cache. Some features will be removed from the cache if the number is smaller than the previous size of the cache.

Parameters:cacheSize – indicates the maximum number of features to keep in the cache
setCacheSubsetOfAttributes(self, attributes: Iterable[int])

Set the subset of attributes to be cached

Parameters:attributes – The attributes to be cached
setFullCache(self, fullCache: bool)

This enables or disables full caching. If enabled, all features will be held in the cache. The cache size will incrementally be increased to offer space for all features. When enabled, all features will be read into cache. As this feature will most likely be used for slow data sources, be aware, that the call to this method might take a long time.

Parameters:fullCacheTrue: enable full caching, False: disable full caching

Note

when a cache is invalidated() (e.g. by adding an attribute to a layer) this setting is reset. A full cache rebuild must be performed by calling setFullCache( True ) again.

See also

hasFullCache()

sourceCrs(self) → QgsCoordinateReferenceSystem

Returns the coordinate reference system for features in the cache.

timerEvent()
wkbType(self) → QgsWkbTypes.Type

Returns the geometry type for features in the cache.