Evaluators are an addition to SMTK’s Attribute system that are a mechanism to process an Attribute that is an expression. ValueItem already supported linkage of Attributes to a ValueItem by means of the <ExpressionType> SBT tag, however Evaluators provide a means to transform the Items in those expression Attributes into meaningful results inside SMTK, as opposed to consumers of an Attribute Resource. These results can be shown to the user in Qt components and serialized by consuming application code.

This document outlines changes and additions to support this feature.


smtk::attribute::Evaluator is the abstract base class of Evaluators. Subclasses must implement the method evaluate(), which processes the Attribute.

Implementations of evaluate() which depend on trees of expressions must manage the state of dependent symbols (i.e. Attributes) by using the API provided in smtk::attribute::SymbolDependencyStorage. SymbolDependencyStorage provides methods to maintain and check for expressions which depend on the result of one another, preventing infinite recursion in evaluate().


Evaluator are created per Attribute through EvaluatorFactory. Evaluators types are registered via template with an std::string alias, and multiple definition can be used for a single Evaluator, as long as they conform to the Items needed by the Evaluator. A Definition can only be used with one type of Evaluator at a time, else EvaluatorFactory could create multiple Evaluators for Attributes of that Definition type.

EvaluatorFactory::createEvaluator(Attribute) returns a std::unique_ptr to an Evaluator. Any consumer with access to an EvaluatorFactory can create an Evaluator as long as it is evaluatable.

Evaluator Registration

Plugins can register evaluator via a Registrar as is done by default for smtk::attribute::InfixExpressionEvaluator in smtk::attribute::Registrar with smtk::attribute::EvaluatorManager. This means an smtk::attribute::Resource created with an smtk::resource::Manager will have their EvaluatorFactory seeded with the Evaluator type and std::string alias specified in the Registrar.


The designer of an SBT can specify the Evaluator to be used for a Definition in the following way, as in the case of InfixExpressionEvaluator.

  <AttDef Type="infixExpression" Label="Expression">
      <String Name="expression" Extensible="true"/>

  <Evaluator Name="InfixExpressionEvaluator">
    <Definition Type="infixExpression">

This means that an Evaluator registered with alias InfixExpressionEvaluator will be created by EvaluatorFactory for Definitions of type infixExpression. The Definition infixExpression has a StringItem called expression because InfixExpressionEvaluator expects an infix math expression in string form.

Changes to smtk::attribute::Resource and smtk::attribute::ValueItemTemplate

Attribute Resources are now constructed with an EvaluatorFactory to obtain Evaluators for Attributes in the Resource.

ValueItemTemplate has been modified to attempt to evaluate() expressions set on the Item in value(std::size_t element), and serialize their result to std::string in valueAsString().

Infix Expression Parsing

A tao PEGTL grammar and structures for computng infix expressions have been added to smtk::common in InfixExpressionGrammarImpl.h and InfixExpressionEvalution.h, respectively. These are wrapped together by InfixExpressionGrammar.

UI Additions Specific to Infix Expressions

A new qtItem has been added to the smtk::extension namespace. qtInfixExpressionEditor shows the result of an infix expression as the user changed a QLineEdit. This is linked to the underlying StringItem of the infix expression. It is created by specifying an ItemView with Type InfixExpression.

qtInputsItems has been modified to show the result of an evalutable expression displayed in a read-only QLineEdit when the Item is in expression mode and the expression Attribute is evaluatable.