In SMTK, operations performed on resources are represented by instances of the Operation class. Each operation is a subclass that contains the code to perform the operation by overriding the operate() and ableToOperate() methods. These subclasses register themselves (via SMTK’s auto-init macros) with a Manager.

The next sections describe in detail: first, how operators specify the inputs they require and outputs they produce; and second, how operators register themselves for introspection by applications and scripts.

Inputs and Outputs

The inputs that an operator requires in order to run and the outputs that an operator may produce are each modeled as attribute definitions. These definitions reside in an attribute resource.

Each operator’s inputs are specified by a Definition that inherits a base attribute-definition named “operator” while the its outputs are specified by a Definition that inherits a base attribute-definition named “result”. Furthermore, if an operator’s input specification is named “foo”, its output specification must be named “result(foo)”.

SMTK uses this naming convention to construct results automatically. Given an instance of an Operator subclass, you can call its specification() method to obtain the Attribute which models its inputs. When the operator is executed, it returns an Attribute instance that models its result.

Recall that an attribute resource holds definitions which contain item definitions. When an attribute is instantiated from a definition, all its active item definitions are instantiated as items. The definitions themselves specify what types of persistent objects may be associated with their attributes using a list of pairs of specifier strings; the first string in the pair specifies the type-name of resource while the second string specifies constraints on what types of components are allowed. Associations between an operation’s parameters and persistent objects such as model entities denote what objects the operator will act upon. The operation’s parameters may also contain attribute items that specify other configuration information for the operation such as point locations, geometric distances, etc. that determine how the operation will proceed.

The fact that inputs and outputs are specified using SMTK’s own attribute resource means that one need not construct an instance of the operator’s C++ class in order to obtain information about it; instead, simply call operatorSystem() on the session and ask for all the definitions which inherit “operator”.

Sideband information

Applications may need to pass information to or retrieve information from operations. While technically applications could always modify their operations’ input parameters and output results, this is often inconvenient or aesthetically displeasing – especially if an operation may be used in different ways by different applications. Thus, SMTK provides a way to pass “side-band” or “out-of-band (OOB)” information to and from operations.

For example, an operation may be used inside a script where user interaction is not possible as well as inside an interactive client. When run interactively, the operation may wish to indicate to the application how to adjust camera parameters to focus on a modified component. While the operation’s definition could be modified to include camera information in its result data, this is undesirable because it clutters the output with information that is specific to the application’s behavior rather than the operation’s behavior. Also, there may be many operations that all need to provide this information keeping all of their result definitions consistent with one another is not easy to maintain.

In these cases, you may place application-specific information into an smtk::common::Managers object that you provide to the operation manager via a shared pointer. Any operation constructed by the operation manager will have access to the Managers object and can fetch arbitrary data from it in a type-safe manner. Be aware that when using asynchronous operation launchers, multiple operations may simultaneously access data in the Managers object; methods that may be accessed in this way must be re-entrant.


Operations specific to a modeling session often implement functionality present in many different sessions using a consistent set of parameters. SMTK provides a grouping mechanism so that applications can find sets of operations that provide “like” functionality.

For example, a CAD modeling session and a discrete modeling session will generally provide operations to import, export, read, and write data. Each of these tasks has its own group of operations that constrain their member operations in different ways. Consider the task of importing data; import operations all require the location of the data to be imported. The smtk::operation::ImporterGroup holds a list of operations that can import files. It constrains operations added to the group to have a filename item.


Operators are registered with an operation manager via a Registrar class that implements registerTo() and unregisterFrom() methods. Besides informing the operation manager of classes implementing operations, your registerTo() method should also add operations to groups as needed.

Finally, each operation class may have an icon. To register an icon for your operation, your Registrar class should provide a second registerTo() method that accepts a view manager. The view manager provides an operationIcons() method that you can use to register an icon to your operation. Icons are functors that take in a secondary (background) color and return a string holding SVG for an icon that contrasts well with the background.

Launching operations

You can always call the operate() method of an operation instance to run it immediately and synchronously. However, operations may take a long time to complete and should not interfere with applications tending to user input in the meantime. Because of this, operations may also run in a separate thread from user interface components.

The job of running operations asynchronously is delegate to operation launchers. SMTK provides a default launcher and one specific to the Qt library (which is discussed in the Qt section). If you use a launcher to run an operation, instead of getting an operation result object, you will receive a std::shared_future<Operation::Result>. Shared futures provide a way for applications to check whether an operation is complete as well as block until an operation completes (if needed).

In order to avoid multiple operations from making simultaneous changes to the same objects at once, the input parameters are scanned and each involved resource is read-locked or write-locked (depending on how the operation’s parameters are marked) as needed before the operation runs. Thus, operations that run on separate resources may run simultaneously, as may operations that only require read access to the same resource. However, operations that require write access to the same resource will be run sequentially.

Observing operations

Regardless of whether an operation is run synchronously or asynchronously, if it was created by an operation manager, the manager will invoke any observers that have been registered with it at two points:

  • When an operation is about to run, observers are invoked that may inspect the operation and its parameters. Any of the observers may cancel the operation at this point, so there is no guarantee that observing an operation before it is run will generate a second event.

  • When an operation has run, observers are invoked that may inspect the operation, its parameters, and its result (which indicates success or failure and which may also contain additional information).

While in general these observations may occur on any thread, most applications will force the observations to occur on the main thread because user-interface toolkits are rarely re-entrant.