Split the two command implementations into separate libraries (#2012)

This will allow us at the user code side to determine to include old commands, new commands or both.

wpilibNewCommands/src/main/native/include/frc2/command/Subsystem.h

@@ -0,0 +1,88 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <frc2/command/CommandScheduler.h>
+
+#include <utility>
+
+namespace frc2 {
+class Command;
+/**
+ * A robot subsystem.  Subsystems are the basic unit of robot organization in
+ * the Command-based framework; they encapsulate low-level hardware objects
+ * (motor controllers, sensors, etc) and provide methods through which they can
+ * be used by Commands.  Subsystems are used by the CommandScheduler's resource
+ * management system to ensure multiple robot actions are not "fighting" over
+ * the same hardware; Commands that use a subsystem should include that
+ * subsystem in their GetRequirements() method, and resources used within a
+ * subsystem should generally remain encapsulated and not be shared by other
+ * parts of the robot.
+ *
+ * <p>Subsystems must be registered with the scheduler with the
+ * CommandScheduler.RegisterSubsystem() method in order for the
+ * Periodic() method to be called.  It is recommended that this method be called
+ * from the constructor of users' Subsystem implementations.  The
+ * SubsystemBase class offers a simple base for user implementations
+ * that handles this.
+ *
+ * @see Command
+ * @see CommandScheduler
+ * @see SubsystemBase
+ */
+class Subsystem {
+ public:
+  ~Subsystem();
+  /**
+   * This method is called periodically by the CommandScheduler.  Useful for
+   * updating subsystem-specific state that you don't want to offload to a
+   * Command.  Teams should try to be consistent within their own codebases
+   * about which responsibilities will be handled by Commands, and which will be
+   * handled here.
+   */
+  virtual void Periodic();
+
+  /**
+   * Sets the default Command of the subsystem.  The default command will be
+   * automatically scheduled when no other commands are scheduled that require
+   * the subsystem. Default commands should generally not end on their own, i.e.
+   * their IsFinished() method should always return false.  Will automatically
+   * register this subsystem with the CommandScheduler.
+   *
+   * @param defaultCommand the default command to associate with this subsystem
+   */
+  template <class T, typename = std::enable_if_t<std::is_base_of_v<
+                         Command, std::remove_reference_t<T>>>>
+  void SetDefaultCommand(T&& defaultCommand) {
+    CommandScheduler::GetInstance().SetDefaultCommand(
+        this, std::forward<T>(defaultCommand));
+  }
+
+  /**
+   * Gets the default command for this subsystem.  Returns null if no default
+   * command is currently associated with the subsystem.
+   *
+   * @return the default command associated with this subsystem
+   */
+  Command* GetDefaultCommand() const;
+
+  /**
+   * Returns the command currently running on this subsystem.  Returns null if
+   * no command is currently scheduled that requires this subsystem.
+   *
+   * @return the scheduled command currently requiring this subsystem
+   */
+  Command* GetCurrentCommand() const;
+
+  /**
+   * Registers this subsystem with the CommandScheduler, allowing its
+   * Periodic() method to be called when the scheduler runs.
+   */
+  void Register();
+};
+}  // namespace frc2