Command-Based Programming

WPILib's framework for organizing robot behavior with injection support.

Core Concepts

Concept	Description
Subsystem	A physical mechanism on the robot (drive, shooter, elevator)
Command	An action that runs on one or more subsystems
Requirement	A command claims the subsystems it needs

Subsystem with Injection

public class ShooterSubsystem extends BaseSubsystem {
    private final XCANMotorController motor;

    @Inject
    public ShooterSubsystem(
        XCANMotorControllerFactory factory,
        ElectricalContract contract) {

        if (contract.isShooterReady()) {
            this.motor = factory.create(contract.getShooterMotor(), ...);
        }
    }

    public void setTargetVelocity(double rpm) {
        motor.setRawVelocityTarget(RPM.of(rpm), MotorPidMode.DutyCycle, 0);
    }

    @Override
    public void periodic() {
        // Called every ~20ms -- log data, update dashboard
    }
}

Command with Injection

public class ShooterFireCommand extends BaseCommand {
    private final ShooterSubsystem shooter;

    @Inject
    public ShooterFireCommand(ShooterSubsystem shooter) {
        this.shooter = shooter;
        addRequirements(shooter);
    }

    @Override
    public void initialize() {
        shooter.setTargetVelocity(5000);
    }

    @Override
    public boolean isFinished() {
        return shooter.isAtTargetVelocity();
    }

    @Override
    public void end(boolean interrupted) {
        // Clean up: stop motors, log
    }
}

Command Lifecycle

initialize()  →  execute()  →  isFinished()?  →  end()
                   (every loop)    No: repeat
                                   Yes: end()

Method	When	Purpose
initialize()	Once at start	Set targets, reset sensors
execute()	Every loop	Continuous actions
isFinished()	Every loop	Return true to end
end(boolean interrupted)	Once at end	Clean up

Binding to Buttons

@Inject
public OperatorCommandMap(
    OperatorInterface oi,
    ShooterFireCommand fire,
    ShooterStopCommand stop) {

    oi.driverGamepad.a().onTrue(fire);
    oi.driverGamepad.a().onFalse(stop);
}

Command Groups

Sequential (one after another)

new SequentialCommandGroup(
    new IntakeDeployExtendCommand(),
    new WaitCommand(0.5),
    new ShooterFireCommand()
);

Parallel (at the same time)

new ParallelCommandGroup(
    new ElevatorToHeightCommand(),
    new HoodToAngleCommand()
);

Deadline (run until one finishes)

new ParallelDeadlineGroup(
    new ShooterFireCommand(),       // Main event
    new IntakeFeederCommand()       // Cancelled when shooter is ready
);

Default Commands

A default command runs when no other command requires the subsystem:

shooter.setDefaultCommand(new ShooterMaintainerCommand(shooter));

This gives the subsystem a "resting behavior" (like a car idling).

Source Code

• TeamXbot2026 ShooterSubsystem
• TeamXbot2026 ShooterFireCommand
• SeriouslyCommonLib BaseCommand
• TeamXbot2026 OperatorCommandMap

---

Quiz

Q1: What does addRequirements(shooter) do?

• [ ] A) Makes the command run faster
• [ ] B) Claims the subsystem so no other command can use it at the same time
• [ ] C) Deletes the subsystem
• [ ] D) Adds a new motor to the robot

Answer

B) Claims the subsystem so no other command can use it at the same time

addRequirements() prevents two commands from controlling the same subsystem simultaneously, which would cause unpredictable behavior.

Q2: What is the difference between SequentialCommandGroup and ParallelCommandGroup?

• [ ] A) Sequential runs commands one after another; Parallel runs them at the same time
• [ ] B) Sequential is faster
• [ ] C) Parallel only works in autonomous
• [ ] D) There is no difference

Answer

A) Sequential runs commands one after another; Parallel runs them at the same time

Sequential waits for each command to finish before starting the next. Parallel starts all commands at once and waits for all to finish.

Q3: When does a command's initialize() method run?

• [ ] A) Every 20ms
• [ ] B) Once when the command starts
• [ ] C) When the robot is disabled
• [ ] D) When the match ends

Answer

B) Once when the command starts

initialize() runs exactly once when the command is first scheduled. It is where you set targets, reset sensors, and start motors.