Introduction to Command-Based Programming

Command-based programming offers a structured approach for controlling robot operations in the FIRST Robotics Competition (FRC). By defining individual commands and subsystems, teams can organize their code, make it more readable, and facilitate troubleshooting.

Why Command-Based Programming?

Command-based programming divides robot operations into a series of commands and subsystems. This approach provides:

1. Structured Robot Code: Each command represents a single robot operation, and each subsystem represents a collection of related hardware components. This modular approach promotes clarity and simplifies debugging.
2. Benefits: With commands acting as "tasks" and subsystems as "resources", the command-based framework provides an intuitive way to schedule and handle multiple operations concurrently.

Subsystems

Subsystems group related hardware components. For example, a drivetrain could be a subsystem composed of multiple motors.

import edu.wpi.first.wpilibj2.command.SubsystemBase;

public class Drivetrain extends SubsystemBase {
    // Example motor declaration
    private final Motor leftMotor = new Motor();
    private final Motor rightMotor = new Motor();

    public void drive(double speed) {
        leftMotor.set(speed);
        rightMotor.set(speed);
    }
}

1. Definition and Purpose: Subsystems manage related robot components as cohesive units. They also provide a structured way to organize code around these components.

2. Declaring Subsystems: Typically defined as classes, subsystems incorporate methods to control their associated components.

3. Controlling Hardware Components: Subsystems contain methods to read from and write to the physical hardware, like setting motor speeds or reading sensor data.

Commands

Commands represent individual robot operations and use subsystems to achieve their desired effects.

import edu.wpi.first.wpilibj2.command.CommandBase;

public class DriveForward extends CommandBase {
    private final Drivetrain drivetrain;

    public DriveForward(Drivetrain dt) {
        drivetrain = dt;
        addRequirements(drivetrain); // This command requires the drivetrain subsystem.
    }

    @Override
    public void initialize() {}

    @Override
    public void execute() {
        drivetrain.drive(0.5);  // Drive forward at half speed
    }

    @Override
    public boolean isFinished() {
        return false;  // This command never ends on its own.
    }

    @Override
    public void end(boolean interrupted) {
        drivetrain.drive(0);  // Stop the drivetrain when the command ends
    }
}

1. Definition and Purpose: Commands are actions or operations the robot can perform. For instance, a command might make the robot drive forward or rotate.

2. Defining Commands: Commands are usually defined as classes with methods to handle their lifecycle:

   • initialize(): Called when the command starts.
   • execute(): Repeatedly called until the command finishes.
   • end(): Called once when the command completes.
   • isFinished(): Indicates when the command should stop.

3. Interaction with Subsystems: Commands utilize subsystems to interact with hardware components. A command will "require" a subsystem, ensuring exclusive access to it for the command's duration. Default

Default Commands

Subsystems can have associated default commands that run when no other command requires the subsystem.

// In the robot's main class or robot container
drivetrain.setDefaultCommand(new DriveForward(drivetrain));

1. Importance: Default commands allow for continuous operation of a subsystem when not interrupted by other commands.

2. Setting Default Commands: The subsystem's setDefaultCommand() method assigns its default command.

Tasks:

1. Setting Up a Project: Initialize a robot project with a command-based structure using WPILib in your preferred programming environment.

2. Defining a Subsystem: Create a Drivetrain subsystem class, integrating motors as components.

3. Creating a Command: Develop a DriveForward command. Within the command's execute() method, instruct the Drivetrain subsystem to move forward.

Additional Resources:

WPILib Command-Based Programming Guide

Creating a Subsystem in WPILib

Creating Commands with WPILib