[commands] Add RepeatCommand (#4009)

Co-authored-by: Starlight220 <53231611+Starlight220@users.noreply.github.com>

wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/Command.java

@@ -257,6 +257,22 @@ public interface Command {
     return new PerpetualCommand(this);
   }
 
+  /**
+   * Decorates this command to run repeatedly, restarting it when it ends, until this command is
+   * interrupted. The decorated command can still be canceled.
+   *
+   * <p>Note: This decorator works by composing this command within a CommandGroup. The command
+   * cannot be used independently after being decorated, or be re-decorated with a different
+   * decorator, unless it is manually cleared from the list of grouped commands with {@link
+   * CommandGroupBase#clearGroupedCommand(Command)}. The decorated command can, however, be further
+   * decorated without issue.
+   *
+   * @return the decorated command
+   */
+  default RepeatCommand repeat() {
+    return new RepeatCommand(this);
+  }
+
   /**
    * Decorates this command to run "by proxy" by wrapping it in a {@link ProxyScheduleCommand}. This
    * is useful for "forking off" from command groups when the user does not wish to extend the

wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/RepeatCommand.java

@@ -0,0 +1,70 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.wpilibj2.command;
+
+import static edu.wpi.first.wpilibj2.command.CommandGroupBase.registerGroupedCommands;
+import static edu.wpi.first.wpilibj2.command.CommandGroupBase.requireUngrouped;
+
+/**
+ * A command that runs another command repeatedly, restarting it when it ends, until this command is
+ * interrupted. While this class does not extend {@link CommandGroupBase}, it is still considered a
+ * CommandGroup, as it allows one to compose another command within it; the command instances that
+ * are passed to it cannot be added to any other groups, or scheduled individually.
+ *
+ * <p>As a rule, CommandGroups require the union of the requirements of their component commands.
+ *
+ * <p>This class is provided by the NewCommands VendorDep
+ */
+public class RepeatCommand extends CommandBase {
+  protected final Command m_command;
+
+  /**
+   * Creates a new RepeatCommand. Will run another command repeatedly, restarting it whenever it
+   * ends, until this command is interrupted.
+   *
+   * @param command the command to run repeatedly
+   */
+  public RepeatCommand(Command command) {
+    requireUngrouped(command);
+    registerGroupedCommands(command);
+    m_command = command;
+    m_requirements.addAll(command.getRequirements());
+  }
+
+  @Override
+  public void initialize() {
+    m_command.initialize();
+  }
+
+  @Override
+  public void execute() {
+    m_command.execute();
+    if (m_command.isFinished()) {
+      // restart command
+      m_command.end(false);
+      m_command.initialize();
+    }
+  }
+
+  @Override
+  public boolean isFinished() {
+    return false;
+  }
+
+  @Override
+  public void end(boolean interrupted) {
+    m_command.end(interrupted);
+  }
+
+  @Override
+  public boolean runsWhenDisabled() {
+    return m_command.runsWhenDisabled();
+  }
+
+  @Override
+  public RepeatCommand repeat() {
+    return this;
+  }
+}