Key Concepts

SMTK resources are much like documents in word processing or presentation software; they are self-contained files that hold user data. Documents in SMTK tend to contain information related to simulation preparation such as geometric models, discretizations of these models, or information that annotates geometric data. Some documents may also contain user preferences and application state, the same way a word processor saves the place you were last editing along with the text of your correspondence.

There are three base classes holding provenance metadata:


serves as a base class for any object that will be serialized to/from file storage (and is thus persistent across runs of your SMTK application). Any such object must have a UUID and provide a name (the default implementation simply stringifies the UUID to produce a name, but this is not user friendly). Inheritance is used to specialize the type of information held by a persistent object (i.e., is it geometric data? does it conform to a schema? what methods provide access?).


instances correspond to files and thus have a URL. Each resource also has a UUID (since it inherits PersistentObject) and a type. Resources may also provide access to a resource of components (see below) present in the file, indexed by UUID.


instances are meaningful subsets of data present in a resource. An attribute resource presents its attributes as resource components. Similarly, geometric model entities (vertices, edges, faces, etc.) are components in the model subsystem. Each component has a UUID (since it inherits PersistentObject) and holds a weak reference to the resource which owns it.

In addition to these useful base classes,


instances hold a collection of resources. How the resources are related to each other is determined by your application, but typically there will be a single resource manager for the entire application.


Resource links connect persistent objects via a unidirectional relationship that exists for a specific purpose, or role. Links are stored separately from the objects they connect, but each resource instance owns the link that holds all outgoing connections from the resource and its components. The smtk::resource::Link class is an abstract API provided on both resources and components; on the resource, the link subclass provides actual storage, while on components, the subclass asks its parent resource for the links object in order to search for connections. Roles are simply integers. Roles less than zero are reserved for internal SMTK use while positive roles are available for user-specified (i.e., application-specific) roles.

SMTK system roles for Links




Link objects to an instance of an attribute to indicate they possess, exhibit, or manifest the attribute.


Link objects to an item of an attribute to indicate the item contains or references the object.


Link a mesh-set object to a source component whose geometry the mesh-set discretizes.


Link objects to one another to indicate the source (left-hand) object (which does not have renderable geometry) should be provided with visibility controls for the target (right-hand) object(s) (which should have renderable geometry).


Resources and components contain a dictionary-like data structure that can hold any copyable type. If the property type has JSON bindings, it will be serialized to file along with the resource; otherwise, the property can still be stored, but will not be persistent. Currently enabled types include long, double, std::string, and std::vectors of these three types. As an example of how to extend the property system to handle non-POD (non-Plain Old Data) types, see the CoordinateFrame property and its JSON serializer.

Properties are stored separately from the objects they annotate; each resource instance owns the properties instance that holds all properties for both itself and its components. The smtk::resource::Properties class is an abstract API provided on both resources and components; on the resource, the properties subclass provides actual storage, while on components, the subclass asks its parent resource for the properties object to search for values. Property data is a map from a string holding the data type-name (e.g., std::vector<int>) to a subclass of TypeMapEntry named PropertiesOfType which owns a std::map<std::string, std::unordered_map<smtk::common::UUID, T> (in our example, T is a std::vector<int>, but in general T is any type allowed as a property value). Thus, properties map value-type-name → key-name → UUID → value.


Resources hold a container of relevant Query objects. Queries prevent the resource or component classes from growing large APIs and internal state by splitting methods that perform queries — such as identifying the spatial bounds of a mesh or model object, finding the closest point on a mesh or model component to some location in space, etc. — into separate classes that are easy to create and invoke.

Furthermore, query objects can inherit their API from other queries, so it is possible to provide a uniform API with different implementations for each resource type.

Queries may also need to store state in order to be performed efficiently. A good example is closest-point searches; usually many queries of this type are performed in a batch and a point locator structure is built to accelerate the query. The cache should outlive the query object, which is usually constructed on the fly by an algorithm, but also be marked dirty when the resource’s components are modified. The Queries object owned by each resource provides a container for cache objects that individual Query objects may use. Multiple query classes can share the same cache object (e.g., ClosestPoint and ClosestCell might both use a PointLocator cache object).