Subsystems - Alt sistemler

Subsystems are the basic unit of robot organization in the command-based paradigm. A subsystem is an abstraction for a collection of robot hardware that operates together as a unit. Subsystems form an encapsulation for this hardware, “hiding” it from the rest of the robot code and restricting access to it except through the subsystem’s public methods. Restricting the access in this way provides a single convenient place for code that might otherwise be duplicated in multiple places (such as scaling motor outputs or checking limit switches) if the subsystem internals were exposed. It also allows changes to the specific details of how the subsystem works (the “implementation”) to be isolated from the rest of robot code, making it far easier to make substantial changes if/when the design constraints change.

Subsystems also serve as the backbone of the CommandScheduler’s resource management system. Commands may declare resource requirements by specifying which subsystems they interact with; the scheduler will never concurrently schedule more than one command that requires a given subsystem. An attempt to schedule a command that requires a subsystem that is already-in-use will either interrupt the currently-running command or be ignored, based on the running command’s Interruption Behavior.

Subsystems can be associated with “default commands” that will be automatically scheduled when no other command is currently using the subsystem. This is useful for “background” actions such as controlling the robot drive, keeping an arm held at a setpoint, or stopping motors when the subsystem isn’t used. Similar functionality can be achieved in the subsystem’s periodic() method, which is run once per run of the scheduler; teams should try to be consistent within their codebase about which functionality is achieved through either of these methods. Subsystems are represented in the command-based library by the Subsystem interface (Java, C++).

Bir Subsystem - Alt Sistem Oluşturma

The recommended method to create a subsystem for most users is to subclass the abstract SubsystemBase class (Java, C++), as seen in the command-based template (Java, C++):

JavaC++

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

public class ExampleSubsystem extends SubsystemBase {
  /** Creates a new ExampleSubsystem. */
  public ExampleSubsystem() {}

  /**
   * Example command factory method.
   *
   * @return a command
   */
  public CommandBase exampleMethodCommand() {
    // Inline construction of command goes here.
    // Subsystem::RunOnce implicitly requires `this` subsystem.
    return runOnce(
        () -> {
          /* one-time action goes here */
        });
  }

  /**
   * An example method querying a boolean state of the subsystem (for example, a digital sensor).
   *
   * @return value of some boolean subsystem state, such as a digital sensor.
   */
  public boolean exampleCondition() {
    // Query some boolean state, such as a digital sensor.
    return false;
  }

  @Override
  public void periodic() {
    // This method will be called once per scheduler run
  }

  @Override
  public void simulationPeriodic() {
    // This method will be called once per scheduler run during simulation
  }
}

Bu sınıf, temel Subsystem arayüzünün üstünde birkaç kolaylık sağlar : alt sistemi zamanlayıcıya kaydetmek için constructor daki register() metodunu otomatik olarak çağırır (bu, periodic() zamanlayıcı çalıştığında çağrılacak yöntem) ve ayrıca ilgili durum bilgilerini görüntülemek / günlüğe kaydetmek için gösterge panosuna gönderilebilmesi için Sendable arayüzünü uygular.

Daha fazla esneklik arayan ileri düzey kullanıcılar, Subsystem arayüzünü uygulayan bir sınıf oluşturabilir.

Basit Subsystem Örneği

What might a functional subsystem look like in practice? Below is a simple pneumatically-actuated hatch mechanism from the HatchBotTraditional example project (Java, C++):

JavaC++ (Header)C++ (Source)

package edu.wpi.first.wpilibj.examples.hatchbottraditional.subsystems;

import static edu.wpi.first.wpilibj.DoubleSolenoid.Value.kForward;
import static edu.wpi.first.wpilibj.DoubleSolenoid.Value.kReverse;

import edu.wpi.first.util.sendable.SendableBuilder;
import edu.wpi.first.wpilibj.DoubleSolenoid;
import edu.wpi.first.wpilibj.PneumaticsModuleType;
import edu.wpi.first.wpilibj.examples.hatchbottraditional.Constants.HatchConstants;
import edu.wpi.first.wpilibj2.command.SubsystemBase;

/** A hatch mechanism actuated by a single {@link DoubleSolenoid}. */
public class HatchSubsystem extends SubsystemBase {
  private final DoubleSolenoid m_hatchSolenoid =
      new DoubleSolenoid(
          PneumaticsModuleType.CTREPCM,
          HatchConstants.kHatchSolenoidPorts[0],
          HatchConstants.kHatchSolenoidPorts[1]);

  /** Grabs the hatch. */
  public void grabHatch() {
    m_hatchSolenoid.set(kForward);
  }

  /** Releases the hatch. */
  public void releaseHatch() {
    m_hatchSolenoid.set(kReverse);
  }

  @Override
  public void initSendable(SendableBuilder builder) {
    super.initSendable(builder);
    // Publish the solenoid state to telemetry.
    builder.addBooleanProperty("extended", () -> m_hatchSolenoid.get() == kForward, null);
  }
}

Notice that the subsystem hides the presence of the DoubleSolenoid from outside code (it is declared private), and instead publicly exposes two higher-level, descriptive robot actions: grabHatch() and releaseHatch(). It is extremely important that “implementation details” such as the double solenoid be “hidden” in this manner; this ensures that code outside the subsystem will never cause the solenoid to be in an unexpected state. It also allows the user to change the implementation (for instance, a motor could be used instead of a pneumatic) without any of the code outside of the subsystem having to change with it.

Alternatively, instead of writing void public methods that are called from commands, we can define the public methods as factories that return a command. Consider the following from the HatchBotInlined example project (Java, C++):

JavaC++ (Header)C++ (Source)

package edu.wpi.first.wpilibj.examples.hatchbotinlined.subsystems;

import static edu.wpi.first.wpilibj.DoubleSolenoid.Value.kForward;
import static edu.wpi.first.wpilibj.DoubleSolenoid.Value.kReverse;

import edu.wpi.first.util.sendable.SendableBuilder;
import edu.wpi.first.wpilibj.DoubleSolenoid;
import edu.wpi.first.wpilibj.PneumaticsModuleType;
import edu.wpi.first.wpilibj.examples.hatchbotinlined.Constants.HatchConstants;
import edu.wpi.first.wpilibj2.command.CommandBase;
import edu.wpi.first.wpilibj2.command.SubsystemBase;

/** A hatch mechanism actuated by a single {@link edu.wpi.first.wpilibj.DoubleSolenoid}. */
public class HatchSubsystem extends SubsystemBase {
  private final DoubleSolenoid m_hatchSolenoid =
      new DoubleSolenoid(
          PneumaticsModuleType.CTREPCM,
          HatchConstants.kHatchSolenoidPorts[0],
          HatchConstants.kHatchSolenoidPorts[1]);

  /** Grabs the hatch. */
  public CommandBase grabHatchCommand() {
    // implicitly require `this`
    return this.runOnce(() -> m_hatchSolenoid.set(kForward));
  }

  /** Releases the hatch. */
  public CommandBase releaseHatchCommand() {
    // implicitly require `this`
    return this.runOnce(() -> m_hatchSolenoid.set(kReverse));
  }

  @Override
  public void initSendable(SendableBuilder builder) {
    super.initSendable(builder);
    // Publish the solenoid state to telemetry.
    builder.addBooleanProperty("extended", () -> m_hatchSolenoid.get() == kForward, null);
  }
}

Note the qualification of the RunOnce factory used here: this isn’t the static factory in Commands! Subsystems have similar instance factories that return commands requiring this subsystem. Here, the Subsystem.runOnce(Runnable) factory (Java, C++) is used.

For a comparison between these options, see Instance Command Factory Methods.

Periodic

Subsystems have a periodic method that is called once every scheduler iteration (usually, once every 20 ms). This method is typically used for telemetry and other periodic actions that do not interfere with whatever command is requiring the subsystem.

JavaC++ (Header)C++ (Source)

  @Override
  public void periodic() {
    // Update the odometry in the periodic block
    m_odometry.update(
        Rotation2d.fromDegrees(getHeading()),
        m_leftEncoder.getDistance(),
        m_rightEncoder.getDistance());
    m_fieldSim.setRobotPose(getPose());
  }

There is also a simulationPeriodic() method that is similar to periodic() except that it is only run during Simulation and can be used to update the state of the robot.

Default Commands

Not

In the C++ command-based library, the CommandScheduler owns the default command object.

“Default commands” are commands that run automatically whenever a subsystem is not being used by another command. This can be useful for “background” actions such as controlling the robot drive, or keeping an arm held at a setpoint.

Bir alt sistem için varsayılan bir komut ayarlamak çok kolaydır; basitçe CommandScheduler.getInstance().setDefaultCommand() veya daha basit bir şekilde Subsystem arayüzünün setDefaultCommand() metodunu çağırır:

JavaC++

CommandScheduler.getInstance().setDefaultCommand(exampleSubsystem, exampleCommand);

JavaC++

exampleSubsystem.setDefaultCommand(exampleCommand);

Not

A command that is assigned as the default command for a subsystem must require that subsystem.