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The command-based pattern is built around two core ideas: commands and subsystems.

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Commands

Commands represent actions the robot can take. They are scheduled by the command scheduler and run until either:
  • They finish naturally (their end condition is met), or
  • They are interrupted by another command.
Commands can also be composed to build more complex robot behaviors from smaller, reusable pieces. For example, you might chain together a “drive forward” command, then a “turn,” then an “intake” command.

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Subsystems

Subsystems represent independent collections of hardware that work together — for example, a drivetrain, an arm, or an intake mechanism. Subsystems serve two main purposes:
  1. Resource management Only one command can control a subsystem at a time, which prevents conflicting hardware instructions.
  2. Abstraction Subsystems hide hardware details behind a clean interface. This keeps your robot code simple and lets you change the internal hardware implementation without touching the rest of your code.

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How Commands Are Run

Commands and subsystems are tied together by the scheduler.
  • The scheduler keeps track of which commands are running.
  • It ensures that only one command at a time can use a given subsystem.
  • Each iteration of the robot loop, the scheduler calls the appropriate methods on commands and subsystems to update behavior.
This structure makes robot code modular, reusable, and easy to extend — perfect for building complex autonomous routines or cleanly organizing teleop controls.