Introduction to Commands

Introduction to Commands

What is it?

The XRP Minibot template code includes many commands to control the Arm and DriveTrain of the robot. For example, an important command for the minibot is called DriveDistance, which allows the robot to drive for a specific distance and velocity.

Commands have various capabilities. For example, commands can run initial autonomous movement, drive the robot a certain distance, and turn the robot for a certain number of degrees or seconds. Using these pre-created commands, you can move the robot’s Drivetrain whichever way you want as long as it’s within the robot’s constraints. Some of these pre-created commands include DriveDistance, DriveTime, TurnDegrees, and TurnTime.

There is also the SequentialCommandGroup. Classes that extend or inherit from SequentialCommandGroup run commands in sequential order. In the constructor of those classes, the method addCommands() is called where the commands are passed as arguments and will be run in the exact order they are listed. Some examples of classes that inherit from SequentialCommandGroup include AutonomousDistance and AutonomousTime.

This is a format of a basic command. All commands created extend the Command class, and the methods in the subclass, CommandName, override methods from the superclass Command. For example, the initialize(), execute(), end(), and isFinished() methods can be overridden. Here is a format of a potential command named CommandName.

Command Structure

package frc.robot.commands;
import edu.wpi.first.wpilibj2.command.Command;


public class CommandName extends Command {
  public CommandName() {}

  // Called when the command is initially scheduled.
  @Override
  public void initialize() {}

  // Called every time the scheduler runs while the command is scheduled.
  @Override
  public void execute() {}

  // Called once the command ends or is interrupted.
  @Override
  public void end(boolean interrupted) {}

  // Returns true when the command should end.
  @Override
  public boolean isFinished() {}
}

This is a format of a basic command. All commands created extend the Command class and inherit its methods. To specify what a command will do in its states, the initialize(), execute(), and end() methods must be overriden. To tell the scheduler when the command has finished execution, the isFinished() method should be overriden.

The initialize(), execute(), and end() methods are defaulted to do nothing, while isFinished() is defaulted to return false. This means it will continue to run forever, unless interrupted.

initialize()

The initialize() method is called once when a command is scheduled, setting it up for execution. It prepares the command’s state and resources, which should not be done in the constructor, as the constructor is only called once, while the command may be used multiple times.

execute()

The execute() method runs repeatedly while the command is scheduled, typically called every 20 milliseconds in the robot’s main periodic method.

end(bool interrupted)

The end(bool interrupted) method is called once when the command ends, either by completing or being interrupted. It is used to clean up the command’s state and reset any necessary values.

isFinished()

The isFinished() method is called repeatedly during execution. The isFinished() method checks if the command has completed its task. It runs after the execute() method and when it returns true, the command ends, triggering the end() method.