REVLib - SparkSim does not work

[DylanB5402]

I submitted this to REV support via email last week, also filing an issue here as I was asked to:

I've noticed this on REVLib Java 2027.0.0-alpha-2 and REVLib Java 2027.0.0-alpha-3

I've been experimenting with REVLib in sim using the 2027 WPILib Alpha, and I think I've noticed a small bug that's breaking the SparkSim class:

Spark Maxes are constructed as Sim Devices with a name like SPARK MAX [bus id, device id]. This is how I see them on my SimGUI:

However, the SparkSim constructor constructs SimDeviceSim with only the device id.

m_deviceName = deviceType + " [" + spark.getDeviceId() + "]";
SimDeviceSim sparkSim = new SimDeviceSim(m_deviceName);
m_appliedOutput = sparkSim.getDouble("Applied Output");

As such, attempting to access any data from the SparkSim object (ex: sparkSim.getAppliedOutput()), returns a null pointer exception due to all the SimDoubles (m_appliedOutput) being null.

Also adding the code I was trying to use for reference:

public class DrivetrainSim {

  private SparkMaxSim leftSim, rightSim;
  private final SparkMax leftSpark, rightSpark;

  private final double kGearRatio = 10.71;
  private final double kWheelRadiusMeters = 0.0762; // 3 inches
  private static final double kBusVoltage = 12.0;

  private final DifferentialDrivetrainSim m_driveSim =
      new DifferentialDrivetrainSim(
          DCMotor.getNEO(2), // 2 NEO motors on each side of the drivetrain.
          kGearRatio,
          2.1, // MOI of 2.1 kg m^2 (from CAD model).
          26.5, // Mass of the robot is 26.5 kg.
          kWheelRadiusMeters, // Robot uses 3" radius (6" diameter) wheels.
          0.546, // Distance between wheels in meters.
          null);

  private final StructPublisher<Pose2d> simPosePublisher =
      NetworkTableInstance.getDefault()
          .getStructTopic("SimPose", Pose2d.struct)
          .publish();

  /**
   * Creates a drivetrain simulation that bridges the REV SparkMax motor controllers and the
   * Systemcore onboard IMU with WPILib's {@link DifferentialDrivetrainSim} physics model.
   *
   * @param leftSpark the left-side SparkMax motor controller
   * @param rightSpark the right-side SparkMax motor controller
   * @param imu the Systemcore onboard IMU
   */
  public DrivetrainSim(SparkMax leftSpark, SparkMax rightSpark) {
    this.leftSpark = leftSpark;
    this.rightSpark = rightSpark;
  }

  /**
   * Call once after construction. SparkMax inversion/orientation is handled through motor controller
   * configuration (e.g. {@link com.revrobotics.spark.config.SparkBaseConfig#inverted}) rather than
   * in simulation setup.
   */
  public void init() {
    this.leftSim = new SparkMaxSim(leftSpark, DCMotor.getNEO(2));
    this.rightSim = new SparkMaxSim(rightSpark, DCMotor.getNEO(2));
  }

  /**
   * Call every 20ms from {@code simulationPeriodic()}. Reads the applied output from each
   * SparkMaxSim (one-tick delayed from the previous iterate), feeds the resulting motor voltage into
   * the physics model, then writes the resulting velocity and position back to the SparkMaxSim and
   * the heading to the OnboardIMU simulation.
   */
  public void periodic() {
    leftSim.setBusVoltage(kBusVoltage);
    rightSim.setBusVoltage(kBusVoltage);

    double leftMotorVoltage = leftSim.getAppliedOutput() * kBusVoltage;
    double rightMotorVoltage = rightSim.getAppliedOutput() * kBusVoltage;

    m_driveSim.setInputs(leftMotorVoltage, rightMotorVoltage);
    m_driveSim.update(0.02);

    double leftRPM = metersPerSecToRPM(m_driveSim.getLeftVelocity());
    double rightRPM = metersPerSecToRPM(m_driveSim.getRightVelocity());

    leftSim.iterate(leftRPM, kBusVoltage, 0.02);
    rightSim.iterate(rightRPM, kBusVoltage, 0.02);

    OnboardIMUSim.setYaw(m_driveSim.getHeading().getRadians());

    simPosePublisher.set(m_driveSim.getPose());
  }

  /**
   * Converts a linear wheel velocity (m/s) into motor RPM. Uses the same wheel radius and gearing
   * ratio as the physics simulation. The SparkMax relative encoder defaults to a velocity
   * conversion factor of 1 (RPM); if a custom factor is configured, this method must be updated
   * accordingly.
   */
  private double metersPerSecToRPM(double metersPerSecond) {
    return metersPerSecond / kWheelRadiusMeters * kGearRatio * 60.0 / (2.0 * Math.PI);
  }
}

Error for reference:

********** Robot program startup complete **********
Error at com.revrobotics.spark.SparkSim.setBusVoltage(SparkSim.java:502): Unhandled exception: java.lang.NullPointerException: Cannot invoke "org.wpilib.hardware.hal.SimDouble.set(double)" because "this.m_busVoltage" is null
        at com.revrobotics.spark.SparkSim.setBusVoltage(SparkSim.java:502)
        at first.robot.simulation.DrivetrainSim.periodic(DrivetrainSim.java:76)
        at first.robot.Robot.simulationPeriodic(Robot.java:74)
        at org.wpilib.framework.OpModeRobot.loopFunc(OpModeRobot.java:756)
        at org.wpilib.internal.PeriodicPriorityQueue.runCallbacks(PeriodicPriorityQueue.java:176)
        at org.wpilib.framework.OpModeRobot.startCompetition(OpModeRobot.java:786)
        at org.wpilib.framework.RobotBase.runRobot(RobotBase.java:359)
        at org.wpilib.framework.RobotBase.lambda$startRobot$0(RobotBase.java:428)
        at java.base/java.lang.Thread.run(Thread.java:1474)

Warning at org.wpilib.driverstation.DriverStationErrors.reportWarning(DriverStationErrors.java:40): The robot program quit unexpectedly. This is usually due to a code error.
  The above stacktrace can help determine where the error occurred.
  See https://wpilib.org/stacktrace for more information.
Error at org.wpilib.driverstation.DriverStationErrors.reportError(DriverStationErrors.java:20): The startCompetition() method (or methods called by it) should have handled the exception above.
NT: Got a NT4 connection from 127.0.0.1 port 54076