Add new C++ Command framework (#1785)

This is the C++ version of #1682.

The old command framework is still available, but will be deprecated.

Due to name conflicts, the new framework is in the frc2 namespace.
Eventually (after the old command framework is removed in a future year)
it will be moved into the main frc namespace.

wpilibc/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<
+                         Command, std::remove_reference_t<T>>::value>>
+  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