Submodels allow FLAME GPU 2 models to contain nested models, this is particularly useful for models which contain iterative algorithms. Submodels were originally developed to enable a biological model to use an iterative cell-cell mechanical model, which would have otherwise have been difficult to implement with layers or a dependency graph.

A submodel acts as a full model nested inside a parent model, the only requirement is that the submodel contains atleast one exit condition. Agent variables can be mapped between the two models, with each model also having private agent variables (or even private agents). The agent ID of bound agents will always be mapped. Unbound agents which are recreated in each time the submodel is executed are not guaranteed to have the same ID between runs. Environment properties can also be shared with submodels, with the ability to mark them as constant within the submodel.

Where an agent variable is mapped between model and submodel, the parent model’s default value will always be used when creating new agents.

A submodel must exist independently within a layer of the parent model, it cannot execute simultaneously with other agent or host functions.

Models may contain multiple submodels, and submodels may contain sub-submodels. The only limitation to nested submodels is that circular dependencies are not permitted (e.g. a submodel adding it’s parent model as a submodel), if one is detected an exception will be raised.

Defining a Submodel

A submodel as defined the same as a regular model, via a ModelDescription. It must then be added to the model which should host the submodel using newSubModel() which returns a SubModelDescription.

Mappings between agent states, agent variables and environment properties can then be created using bindAgent() which returns a SubAgentDescription. When mapping variables and properties, they must be of the same type and length.

When calling bindAgent(), it is also possible to automatically bind compatible variables and states with matching names by passing true to the second and third arguments respectively.

// Define a submodel
flamegpu::ModelDescription sub_m("submodel");
    // Define a simple agent with variable 'x' and state 'foo'
    flamegpu::AgentDescription a = sub_m.newAgent("subagent");
    // Add an agent function to the model
    flamegpu::AgentFunctionDescription af1 = a.newFunction("example_function", ExampleFn);
    // Give the model an exit condition, this is required for all submodels
// Define the parent model
flamegpu::ModelDescription m("model");
    // Define a simple agent with variable 'y' and state 'bar'
    flamegpu::AgentDescription a = m.newAgent("agent");
    // Add the submodel
    flamegpu::SubModelDescription smd = m.newSubModel("sub", sub_m);
    // Map agent's foo and bar
    // We pass false, as we don't wish for matching variables and states to be automatically mapped
    flamegpu::SubAgentDescription agent_map = smd.bindAgent("subagent", "agent", false, false);
    // Map the agent variables and states
    agent_map.mapState("foo", "bar");
    agent_map.mapVariable("x", "y");
    // Add the submodel to a layer

Environment properties can also be mapped in a similar manner by calling mapProperty() on the SubEnvironmentDescription.

It is not possible to map a non-const environment property within a submodel, to a const property in the parent model.

// Define a submodel
flamegpu::ModelDescription sub_m("submodel");
// As these properties will be mapped, their initial values are redundant as they will always be inherited
sub_m.Environment().newProperty<float>("foo", 0);
// This property is const in the sub model, so it can only be updated by the parent model
sub_m.Environment().newProperty<float>("foo2", 0, true);

// Define the parent model
flamegpu::ModelDescription m("model");
m.Environment().newProperty<float>("bar", 12.0f);
m.Environment().newProperty<float>("bar2", 21.0f);

// Setup the mapping
flamegpu::SubModelDescription smd = m.newSubModel("sub", sub_m);
flamegpu::SubEnvironmentDescription senv = smd.SubEnvironment();
senv.mapProperty("foo", "bar");
senv.mapProperty("foo2", "bar2");