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.

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.

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 (QgsAbstractCacheIndex) – 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]

Parameters
  • fid (int) –

  • field (int) –

  • value (Any) –

cacheGeometry(self) → bool

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

New in version 3.0.

Return type

bool

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.

Return type

int

Returns

int

cachedFeatureIds(self) → object

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

See also

isFidCached()

New in version 3.0.

Return type

object

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 (QgsFeatureRequest) – The QgsFeatureRequest to be answered

Return type

bool

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 (int) – 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 (int) – The id of the feature to query

  • feature (QgsFeature) – The result of the operation will be written to this feature

  • skipCache (bool = False) – Will query the layer regardless if the feature is in the cache already

Return type

bool

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.

Return type

int

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 (int) – The feature id of the removed feature.

fields(self) → QgsFields

Returns the fields associated with features in the cache.

Return type

QgsFields

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.

Parameters

fid (int) –

Return type

QgsFeature

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 (QgsFeatureRequest = QgsFeatureRequest()) – The request specifying filter and required data.

Return type

QgsFeatureIterator

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.

Return type

bool

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 (int) – The feature id to look for

Return type

bool

Returns

True if this id is in the cache

isSignalConnected()
layer(self) → QgsVectorLayer

Returns the layer to which this cache belongs

Return type

QgsVectorLayer

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

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

Parameters

fid (int) – The id of the feature to delete

Return type

bool

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 (QgsFeatureRequest) – The feature request that was answered

  • fids (object) – 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 (bool) – Automatically cache new attributes

setCacheGeometry(self, cacheGeometry: bool)

Enable or disable the caching of geometries

Parameters

cacheGeometry (bool) – 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 (int) – 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 (Iterable[int]) – 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

fullCache (bool) – True: 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.

Return type

QgsCoordinateReferenceSystem

timerEvent()
wkbType(self) → QgsWkbTypes.Type

Returns the geometry type for features in the cache.

Return type

QgsWkbTypes.Type