QgsWithVariableExpressionFunction class

Handles the with_variable(name, value, node) expression function.

It temporarily appends a new scope to the expression context for all nested nodes.

Base classes

class QgsExpressionFunction: A abstract base class for defining QgsExpression functions.

auto func(const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent, const QgsExpressionNodeFunction* node) -> QVariant override: Returns result of evaluating the function.
auto isStatic(const QgsExpressionNodeFunction* node, QgsExpression* parent, const QgsExpressionContext* context) const -> bool override: Will be called during prepare to determine if the function is static.
auto prepare(const QgsExpressionNodeFunction* node, QgsExpression* parent, const QgsExpressionContext* context) const -> bool override: This will be called during the prepare step() of an expression if it is not static.
auto run(QgsExpressionNode::NodeList* args, const QgsExpressionContext* context, QgsExpression* parent, const QgsExpressionNodeFunction* node) -> QVariant override: Evaluates the function, first evaluating all required arguments before passing them to the function's func() method.

Returns result of evaluating the function.

Parameters
values	list of values passed to the function
context	context expression is being evaluated against
parent	parent expression
node	expression node
Returns	result of function

Will be called during prepare to determine if the function is static.

A function is static if it will return the same value for every feature with different attributes and/or geometry.

By default this will return true, if all arguments that have been passed to the function are also static.

This will be called during the prepare step() of an expression if it is not static.

This can be used by functions to do any preparation steps that might help to speedup the upcoming evaluation.