[cmd3] Make Mechanism an interface (#8303)

Since there is no longer a requirement for Subsystems/Mechanisms to be
registered to the command scheduler via a register() call, Mechanism can
be changed to an interface instead to allow for more flexible
inheritance structures.

Specifically, this would allow compatibility with CTRE swerve (which
previously required implementing Subsystem so that it could extend
CTRE's base class).

commandsv3/src/main/java/org/wpilib/command3/Mechanism.java

@@ -10,61 +10,33 @@ import org.wpilib.annotation.NoDiscard;
 import org.wpilib.units.measure.Time;
 
 /**
- * Generic base class to represent mechanisms on a robot. Commands can require sole ownership of a
+ * Generic interface to represent mechanisms on a robot. Commands can require sole ownership of a
  * mechanism; when a command that requires a mechanism is running, no other commands may use it at
  * the same time.
  *
- * <p>Even though this class is named "Mechanism", it may be used to represent other physical
+ * <p>Even though this interface is named "Mechanism", it may be used to represent other physical
  * hardware on a robot that should be controlled with commands - for example, an LED strip or a
  * vision processor that can switch between different pipelines could be represented as mechanisms.
  */
-public class Mechanism {
-  private final String m_name;
-
-  private final Scheduler m_registeredScheduler;
-
+public interface Mechanism {
   /**
-   * Creates a new mechanism registered with the default scheduler instance and named using the name
-   * of the class. Intended to be used by subclasses to get sane defaults without needing to
-   * manually declare a constructor.
-   */
-  @SuppressWarnings("this-escape")
-  protected Mechanism() {
-    m_name = getClass().getSimpleName();
-    m_registeredScheduler = Scheduler.getDefault();
-    setDefaultCommand(idle());
-  }
-
-  /**
-   * Creates a new mechanism, registered with the default scheduler instance.
+   * Returns the scheduler under which this subsystem and its default commands are registered. The
+   * scheduler is also used to fetch running commands for the subsystem.
    *
-   * @param name The name of the mechanism. Cannot be null.
+   * @return The registered scheduler.
    */
-  public Mechanism(String name) {
-    this(name, Scheduler.getDefault());
+  default Scheduler getRegisteredScheduler() {
+    return Scheduler.getDefault();
   }
 
   /**
-   * Creates a new mechanism, registered with the given scheduler instance.
-   *
-   * @param name The name of the mechanism. Cannot be null.
-   * @param scheduler The registered scheduler. Cannot be null.
-   */
-  @SuppressWarnings("this-escape")
-  public Mechanism(String name, Scheduler scheduler) {
-    m_name = name;
-    m_registeredScheduler = scheduler;
-    setDefaultCommand(idle());
-  }
-
-  /**
-   * Gets the name of this mechanism.
+   * Gets the name of this mechanism. This will default to the name of this mechanism's class.
    *
    * @return The name of the mechanism.
    */
   @NoDiscard
-  public String getName() {
-    return m_name;
+  default String getName() {
+    return getClass().getSimpleName();
   }
 
   /**
@@ -79,8 +51,8 @@ public class Mechanism {
    *
    * @param defaultCommand the new default command
    */
-  public void setDefaultCommand(Command defaultCommand) {
-    m_registeredScheduler.setDefaultCommand(this, defaultCommand);
+  default void setDefaultCommand(Command defaultCommand) {
+    getRegisteredScheduler().setDefaultCommand(this, defaultCommand);
   }
 
   /**
@@ -89,8 +61,8 @@ public class Mechanism {
    *
    * @return The currently configured default command
    */
-  public Command getDefaultCommand() {
-    return m_registeredScheduler.getDefaultCommandFor(this);
+  default Command getDefaultCommand() {
+    return getRegisteredScheduler().getDefaultCommandFor(this);
   }
 
   /**
@@ -99,7 +71,7 @@ public class Mechanism {
    * @param commandBody The main function body of the command.
    * @return The command builder, for further configuration.
    */
-  public NeedsNameBuilderStage run(Consumer<Coroutine> commandBody) {
+  default NeedsNameBuilderStage run(Consumer<Coroutine> commandBody) {
     return new StagedCommandBuilder().requiring(this).executing(commandBody);
   }
 
@@ -111,7 +83,7 @@ public class Mechanism {
    * @param loopBody The body of the infinite loop.
    * @return The command builder, for further configuration.
    */
-  public NeedsNameBuilderStage runRepeatedly(Runnable loopBody) {
+  default NeedsNameBuilderStage runRepeatedly(Runnable loopBody) {
     return run(
         coroutine -> {
           while (true) {
@@ -131,7 +103,7 @@ public class Mechanism {
    *
    * @return A new idle command.
    */
-  public Command idle() {
+  default Command idle() {
     return run(Coroutine::park).withPriority(Command.LOWEST_PRIORITY).named(getName() + "[IDLE]");
   }
 
@@ -141,7 +113,7 @@ public class Mechanism {
    * @param duration How long the mechanism should idle for.
    * @return A new idle command.
    */
-  public Command idleFor(Time duration) {
+  default Command idleFor(Time duration) {
     return idle().withTimeout(duration);
   }
 
@@ -154,12 +126,7 @@ public class Mechanism {
    * @return The currently running commands that require the mechanism.
    */
   @NoDiscard
-  public List<Command> getRunningCommands() {
-    return m_registeredScheduler.getRunningCommandsFor(this);
-  }
-
-  @Override
-  public String toString() {
-    return m_name;
+  default List<Command> getRunningCommands() {
+    return getRegisteredScheduler().getRunningCommandsFor(this);
   }
 }