[commands] Refactor ProxyScheduleCommand, SelectCommand into ProxyCommand (#4534)

wpilibNewCommands/src/main/native/cpp/frc2/command/ProxyCommand.cpp

@@ -0,0 +1,37 @@
+// 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.
+
+#include "frc2/command/ProxyCommand.h"
+
+using namespace frc2;
+
+ProxyCommand::ProxyCommand(wpi::unique_function<Command*()> supplier)
+    : m_supplier(std::move(supplier)) {}
+
+ProxyCommand::ProxyCommand(Command* command)
+    : m_supplier([command] { return command; }) {}
+
+ProxyCommand::ProxyCommand(std::unique_ptr<Command> command)
+    : m_supplier([command = std::move(command)] { return command.get(); }) {}
+
+void ProxyCommand::Initialize() {
+  m_command = m_supplier();
+  m_command->Schedule();
+}
+
+void ProxyCommand::End(bool interrupted) {
+  if (interrupted) {
+    m_command->Cancel();
+  }
+  m_command = nullptr;
+}
+
+void ProxyCommand::Execute() {}
+
+bool ProxyCommand::IsFinished() {
+  // because we're between `initialize` and `end`, `m_command` is necessarily
+  // not null but if called otherwise and m_command is null, it's UB, so we can
+  // do whatever we want -- like return true.
+  return m_command == nullptr || !m_command->IsScheduled();
+}