Lesson 2: Intro to WPILib

What is WPILib?

• WPILib = The library/framework we use to program our robot in Java
• Built for FRC
• Contains useful classes and methods for interacting with many parts of the FRC control system
  • Handles: motors, sensors, joysticks, the driver station, etc.

Our job: use WPILib to design how the robot thinks and behaves

• E.g. organizing subsystems, defining commands, linking controls

Goals For Today

• Understand how WPILib projects are structured
• Learn about how commands are defined and scheduled
• Make a small command to test it out

WPILib Code Structure Overview

WPILib Program Flow Overview

• Command Sources: Responsible for starting commands
• Commands: Define robot behavior
• Subsystems: Responsible for the hardware, e.g. translating commands into motor output

Command Sources: Triggers and Default Commands

• We bind Commands to Triggers (like button presses) in the configureBindings method inside the RobotContainer class
• We can also specify a default command for a Subsystem that runs continuously

Example:

When the leftBumper button on the joystick is pressed, run the seedFieldCentric command

By default, continuously run the roller Subsystem based on what the joystick triggers read

Commands

• Commands define robot actions/behaviors that tell subsystems what to do
  • Use subsystem methods to move motors, etc.
• Each command typically controls one or more subsystems
  • Only one command can control a subsystem at a time

Declaring a Command object

• means defining an action
• DOES NOT RUN BY ITSELF

Examples:

Commands.run(() -> { ... })
Commands.runOnce(() -> { ... })
Commands.sequence(cmd1, cmd2, ...)
cmd1.andThen(cmd2)
new InstantCommand(() -> { ... })

Scheduling a Command

• means telling WPILib to actually run it

Examples:

button.onTrue(cmd)        // binding to a Trigger
subsystem.setDefaultCommand(cmd)

Creating a Command Object: Example

Commands.runOnce(() -> intake.retract(), intake)

• Commands.runOnce(...): Creates a command that runs once, then ends immediately
• () -> intake.retract(): An anonymous function (lambda) that calls the retract() method on the intake subsystem
• intake: Specifies the subsystem this command requires (ensures only one command controls it at a time)

This does not run the command. Pass it into something like intake.setDefaultCommand() to schedule.

Commands and Method Chaining

• Complex commands can be built using method chaining
• Method chaining: calling methods on the return values of other methods (in this case, Command objects)
• You don’t need to memorize syntax now: just understand that complex behavior can be chained together.

Example:

Commands.runOnce(() -> shooter.spinUp(), shooter)
        .andThen(Commands.waitSeconds(2))
        .andThen(Commands.runOnce(() -> shooter.fire(), shooter));

What this Command does:

• Starts by spinning up the shooter wheels
• Waits 2 seconds for the wheels to reach speed
• Then fires once

It uses method chaining (specifically .andThen()) to run these actions one after the other.

Command Factories

• Instead of creating commands inline everywhere, we often write methods that return Command objects
• These are called command factories
  • Keeps code organized and reusable
  • Placed inside the Subsystem class (if specific) or a class inside the commands/ folder

Example (inside a Climber subsystem):

public Command readyClimb() {
    return Commands.runOnce(() -> {if (climbIsReady()) climbState = ClimbState.PULL; }, this);
}

Usage (in RobotContainer):

driverController.a().onTrue(climber.readyClimb());

Subsystems

• Represent robot parts like drivetrain, shooter, climber
• Own the hardware (motors, sensors)
  • Responsible for converting abstract Commands into real motor actions
• Expose methods that Commands use to control the robot
  • E.g. elevator.goToPosition()

More details in the next lesson

Code Structure Tour

Let’s trace through how input (button) turns into output (motor) in our wrist robot code.

See: the 971-robot-2025 repo.

Task for This Lesson

• Create a command factory method inside the Climber subsystem
• Bind it to the A button on the joystick
• The command should print "Climb starting" when A is pressed
• Test out your code in a simulation

A template project is provided for you here (TODO)