Model development

Model development means implementing the SimObject interface and composing those SimObjects into complete simulation models.

A SimObject can be stateful (ODE-based) or stateless, and typically defines:

• parameters,
• input ports,
• output ports,
• optional state variables,
• optional visualization callbacks.

Choose your path

Goal	Start here
Set up your development environment	Setup developer environment
Understand SimObject design concepts	SimObject modeling concept
Create your first SimObject	Create a new SimObject
Package objects into a library	Create a new SimObject library
Use implicit integration (stiff systems)	Implicit Integration with Analytical Jacobians
Understand how the engine assembles the Jacobian	How analytical Jacobians are assembled
Choose a Jacobian format, linear solver or method order	SUNDIALS and linear-solver configuration
Find out why a Jacobian is wrong	Jacobian diagnostics and troubleshooting
Share computations between objects	Common computations
Write automated tests	Testing

Recommended reading path

If you are new to FhSim model development, follow these pages in order:

1. Setup developer environment — Compiler, Conan, and CMake setup.
2. SimObject modeling concept — Core design concepts and SimObject anatomy.
3. Create a new SimObject — Step-by-step: create, register, build.
4. Create a new SimObject library — Package objects for distribution.
5. Running a compiled SimObject — Load and run your library.
6. Testing — Unit and integration testing.
7. Test Tools Library — Test utilities reference.
8. Common computations — Shared computations between objects.
9. Implicit Integration with Analytical Jacobians — Analytical Jacobians for stiff systems. Its own subpages cover How analytical Jacobians are assembled, SUNDIALS and linear-solver configuration and Jacobian diagnostics and troubleshooting.
10. SimObject methods — Complete method reference.
11. SimObject construction methods — Construction-time API reference.