QgsSizeScaleTransformer class

QgsPropertyTransformer subclass for scaling a value into a size according to various scaling methods.

Reference

Base classes

class QgsPropertyTransformer: Abstract base class for objects which transform the calculated value of a property.

Public types

enum ScaleType { Linear, Area, Flannery, Exponential }: Size scaling methods.

Public static functions

static auto fromExpression(const QString& expression, QString& baseExpression, QString& fieldName) -> QgsSizeScaleTransformer*: Attempts to parse an expression into a corresponding QgsSizeScaleTransformer.

Constructors, destructors, conversion operators

QgsSizeScaleTransformer(ScaleType type = Linear, double minValue = 0.0, double maxValue = 1.0, double minSize = 0.0, double maxSize = 1.0, double nullSize = 0.0, double exponent = 1.0): Constructor for QgsSizeScaleTransformer.

Public functions

auto clone() const -> QgsSizeScaleTransformer* override: Returns a clone of the transformer.
auto exponent() const -> double: Returns the exponent for an exponential expression.
auto loadVariant(const QVariant& transformer) -> bool override: Loads this transformer from a QVariantMap, wrapped in a QVariant.
auto maxSize() const -> double: Returns the maximum calculated size.
auto minSize() const -> double: Returns the minimum calculated size.
auto nullSize() const -> double: Returns the size value when an expression evaluates to NULL.
void setExponent(double exponent): Sets the exponent for an exponential expression.
void setMaxSize(double size): Sets the maximum calculated size.
void setMinSize(double size): Sets the minimum calculated size.
void setNullSize(double size): Sets the size value for when an expression evaluates to NULL.
void setType(ScaleType type): Sets the size transformer's scaling type (the method used to calculate the size from a value).
auto size(double value) const -> double: Calculates the size corresponding to a specific value.
auto toExpression(const QString& baseExpression) const -> QString override: Converts the transformer to a QGIS expression string.
auto toVariant() const -> QVariant override: Saves this transformer to a QVariantMap, wrapped in a QVariant.
auto transform(const QgsExpressionContext& context, const QVariant& value) const -> QVariant override: Calculates the transform of a value.
auto transformerType() const -> Type override: Returns the transformer type.
auto type() const -> ScaleType: Returns the size transformer's scaling type (the method used to calculate the size from a value).

Enum documentation

enum QgsSizeScaleTransformer::ScaleType

Size scaling methods.

Enumerators
Linear	Linear scaling.
Area	Area based scaling.
Flannery	Flannery scaling method.
Exponential	Scale using set exponent.

Function documentation

static QgsSizeScaleTransformer* QgsSizeScaleTransformer::fromExpression(const QString& expression, QString& baseExpression, QString& fieldName)

Attempts to parse an expression into a corresponding QgsSizeScaleTransformer.

Parameters
expression	expression to parse
baseExpression	will be set to the component of the source expression which is used to calculate the input to the property transformer. This will be set to an empty string if a field reference is the transformer input.
fieldName	will be set to a field name which is used to calculate the input to the property transformer. This will be set to an empty string if an expression is the transformer input.
Returns	corresponding QgsSizeScaleTransformer, or nullptr if expression could not be parsed to a size scale transformer.

QgsSizeScaleTransformer::QgsSizeScaleTransformer(ScaleType type = Linear, double minValue = 0.0, double maxValue = 1.0, double minSize = 0.0, double maxSize = 1.0, double nullSize = 0.0, double exponent = 1.0)

Constructor for QgsSizeScaleTransformer.

Parameters
type	scaling type
minValue	minimum expected value
maxValue	maximum expected value
minSize	minimum size to return
maxSize	maximum size to return
nullSize	size to return for null values
exponent	exponent for Exponential scaling method

double QgsSizeScaleTransformer::exponent() const

Returns the exponent for an exponential expression.

bool QgsSizeScaleTransformer::loadVariant(const QVariant& transformer) override

Loads this transformer from a QVariantMap, wrapped in a QVariant.

You can use QgsXmlUtils::readVariant to read it from an XML document.

double QgsSizeScaleTransformer::maxSize() const

Returns the maximum calculated size.

double QgsSizeScaleTransformer::minSize() const

Returns the minimum calculated size.

double QgsSizeScaleTransformer::nullSize() const

Returns the size value when an expression evaluates to NULL.

void QgsSizeScaleTransformer::setExponent(double exponent)

Sets the exponent for an exponential expression.

Parameters
exponent	exponent

void QgsSizeScaleTransformer::setMaxSize(double size)

Sets the maximum calculated size.

Parameters
size	maximum size

void QgsSizeScaleTransformer::setMinSize(double size)

Sets the minimum calculated size.

Parameters
size	minimum size

void QgsSizeScaleTransformer::setNullSize(double size)

Sets the size value for when an expression evaluates to NULL.

Parameters
size	null size

void QgsSizeScaleTransformer::setType(ScaleType type)

Sets the size transformer's scaling type (the method used to calculate the size from a value).

Parameters
type	scale type

double QgsSizeScaleTransformer::size(double value) const

Calculates the size corresponding to a specific value.

Parameters
value	value to calculate size for
Returns	calculated size using size scale transformer's parameters and type

QString QgsSizeScaleTransformer::toExpression(const QString& baseExpression) const override

Converts the transformer to a QGIS expression string.

The baseExpression string consists of a sub-expression reflecting the parent property's state.

QVariant QgsSizeScaleTransformer::toVariant() const override

Saves this transformer to a QVariantMap, wrapped in a QVariant.

You can use QgsXmlUtils::writeVariant to save it to an XML document.

QVariant QgsSizeScaleTransformer::transform(const QgsExpressionContext& context, const QVariant& value) const override

Calculates the transform of a value.

Parameters
context	expression context
value	input value to transform

Derived classes must implement this to perform their transformations on input values

ScaleType QgsSizeScaleTransformer::type() const

Returns the size transformer's scaling type (the method used to calculate the size from a value).