pop_from_queue.hpp

/*  Copyright (C) 2022-2025 Davide Faconti -  All Rights Reserved
*
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*/
 
#pragma once
 
#include "behaviortree_cpp/action_node.h"
#include "behaviortree_cpp/decorator_node.h"
 
#include <list>
#include <mutex>
 
/**
 * Template Action used in ex04_waypoints.cpp example.
 *
 * Its purpose is to do make it easy to create while loops which consume the elements of a queue.
 *
 * Note that modifying the queue is not thread safe, therefore the action that creates the queue
 * or push elements into it, must be Synchronous.
 *
 * When ticked, we pop_front from the "queue" and insert that value in "popped_item".
 * Return FAILURE if the queue is empty, SUCCESS otherwise.
 */
namespace BT
{
 
template <typename T>
struct ProtectedQueue
{
  std::list<T> items;
  std::mutex mtx;
};
 
/*
 * Few words about why we represent the queue as std::shared_ptr<ProtectedQueue>:
 *
 * Since we will pop from the queue, the fact that the blackboard uses
 * a value semantic is not very convenient, since it would oblige us to
 * copy the entire std::list from the BB and than copy again a new one with one less element.
 *
 * We avoid this using reference semantic (wrapping the object in a shared_ptr).
 * Unfortunately, remember that this makes our access to the list not thread-safe!
 * This is the reason why we add a mutex to be used when modifying the ProtectedQueue::items
 *
 * */
 
template <typename T>
class PopFromQueue : public SyncActionNode
{
public:
  PopFromQueue(const std::string& name, const NodeConfig& config)
    : SyncActionNode(name, config)
  {}
 
  NodeStatus tick() override
  {
    std::shared_ptr<ProtectedQueue<T>> queue;
    if(getInput("queue", queue) && queue)
    {
      std::unique_lock<std::mutex> lk(queue->mtx);
      auto& items = queue->items;
 
      if(items.empty())
      {
        return NodeStatus::FAILURE;
      }
      T val = items.front();
      items.pop_front();
      setOutput("popped_item", val);
      return NodeStatus::SUCCESS;
    }
    return NodeStatus::FAILURE;
  }
 
  static PortsList providedPorts()
  {
    return { InputPort<std::shared_ptr<ProtectedQueue<T>>>("queue"), OutputPort<T>("poppe"
                                                                                   "d_"
                                                                                   "ite"
                                                                                   "m") };
  }
};
 
/**
 * Get the size of a queue. Useful when you want to write something like:
 *
 *  <QueueSize queue="{waypoints}" size="{wp_size}" />
 *  <Repeat num_cycles="{wp_size}" >
 *      <Sequence>
 *          <PopFromQueue  queue="{waypoints}" popped_item="{wp}" >
 *          <UseWaypoint   waypoint="{wp}" />
 *      </Sequence>
 *  </Repeat>
 */
template <typename T>
class QueueSize : public SyncActionNode
{
public:
  QueueSize(const std::string& name, const NodeConfig& config)
    : SyncActionNode(name, config)
  {}
 
  NodeStatus tick() override
  {
    std::shared_ptr<ProtectedQueue<T>> queue;
    if(getInput("queue", queue) && queue)
    {
      std::unique_lock<std::mutex> lk(queue->mtx);
      auto& items = queue->items;
 
      if(items.empty())
      {
        return NodeStatus::FAILURE;
      }
      setOutput("size", int(items.size()));
      return NodeStatus::SUCCESS;
    }
    return NodeStatus::FAILURE;
  }
 
  static PortsList providedPorts()
  {
    return { InputPort<std::shared_ptr<ProtectedQueue<T>>>("queue"),
             OutputPort<int>("size") };
  }
};
 
}  // namespace BT