QgsAbstractLabelProvider class

The QgsAbstractLabelProvider class is an interface class.

Reference

Implementations return list of labels and their associated geometries - these are used by QgsLabelingEngine to compute the final layout of labels.

Implementations also take care of drawing the returned final label positions.

Derived classes

class QgsVectorLayerDiagramProvider: The QgsVectorLayerDiagramProvider class implements support for diagrams within the labeling engine.
class QgsVectorLayerLabelProvider: The QgsVectorLayerLabelProvider class implements a label provider for vector layers.

Public types

enum Flag { DrawLabels = 1 << 1, DrawAllLabels = 1 << 2, MergeConnectedLines = 1 << 3, CentroidMustBeInside = 1 << 4, LabelPerFeaturePart = 1 << 6 }

Constructors, destructors, conversion operators

QgsAbstractLabelProvider(QgsMapLayer* layer, const QString& providerId = QString()): Construct the provider with default values.

Public functions

void drawLabel(QgsRenderContext& context, pal::LabelPosition* label) const pure virtual: draw this label at the position determined by the labeling engine
auto flags() const -> Flags: Flags associated with the provider.
auto labelFeatures(QgsRenderContext& context) -> QList<QgsLabelFeature*> pure virtual: Returns list of label features (they are owned by the provider and thus deleted on its destruction)
auto layer() const -> QgsMapLayer*: Returns the associated layer, or nullptr if no layer is associated with the provider.
auto layerId() const -> QString: Returns ID of associated layer, or empty string if no layer is associated with the provider.
auto linePlacementFlags() const -> unsigned int: For layers with linestring geometries - extra placement flags (or-ed combination of QgsPalLayerSettings::LinePlacementFlags)
auto name() const -> QString: Name of the layer (for statistics, debugging etc.) - does not need to be unique.
auto obstacleType() const -> QgsPalLayerSettings::ObstacleType: How the feature geometries will work as obstacles.
auto placement() const -> QgsPalLayerSettings::Placement: What placement strategy to use for the labels.
auto priority() const -> double: Default priority of labels (may be overridden by individual labels)
auto providerId() const -> QString: Returns provider ID - useful in case there is more than one label provider within a layer (e.g.
void setEngine(const QgsLabelingEngine* engine): Associate provider with a labeling engine (should be only called internally from QgsLabelingEngine)
auto subProviders() -> QList<QgsAbstractLabelProvider*> virtual: Returns list of child providers - useful if the provider needs to put labels into more layers with different configuration.
auto upsidedownLabels() const -> QgsPalLayerSettings::UpsideDownLabels: How to handle labels that would be upside down.

Protected variables

const QgsLabelingEngine* mEngine: Associated labeling engine.
Flags mFlags: Flags altering drawing and registration of features.
QgsWeakMapLayerPointer mLayer: Weak pointer to source layer.
QString mLayerId: Associated layer's ID, if applicable.
unsigned int mLinePlacementFlags: Extra placement flags for linestring geometries.
QString mName: Name of the layer.
QgsPalLayerSettings::ObstacleType mObstacleType: Type of the obstacle of feature geometries.
QgsPalLayerSettings::Placement mPlacement: Placement strategy.
double mPriority: Default priority of labels.
QString mProviderId: Associated provider ID (one layer may have multiple providers, e.g. in rule-based labeling)
QgsPalLayerSettings::UpsideDownLabels mUpsidedownLabels: How to handle labels that would be upside down.

Enum documentation

enum QgsAbstractLabelProvider::Flag

Enumerators
DrawLabels	Whether the labels should be rendered.
DrawAllLabels	Whether all features will be labelled even though overlaps occur.
MergeConnectedLines	Whether adjacent lines (with the same label text) should be merged.
CentroidMustBeInside	Whether location of centroid must be inside of polygons.
LabelPerFeaturePart	Whether to label each part of multi-part features separately.

Function documentation

QString QgsAbstractLabelProvider::providerId() const

Returns provider ID - useful in case there is more than one label provider within a layer (e.g.

in case of rule-based labeling - provider ID = rule's key). May be empty string if layer ID is sufficient for identification of provider's configuration.