ThreadedMessageQueue.h

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#ifndef INCLUDED_ThreadedMessageQueue_h_
#define INCLUDED_ThreadedMessageQueue_h_

#include "Thread.h"
#include "Shared/MarkScope.h"
#include <list>
#include <stdexcept>
#include <algorithm>

//! Provides a mechanism for exchanging messages between threads
/*! Receivers are responsible for message cleanup/deletion. */
template<class T>
class ThreadedMessageQueue {
public:
  //! Constructor
  ThreadedMessageQueue() : lock(), signal(), msgs(), receiver(NULL) {}
  
  //! Destructor, calls finishCallback()
  ~ThreadedMessageQueue() { finishCallback(); }
  
  //! Adds a message to the queue
  void send(const T& msg) { MarkScope l(lock); msgs.push_back(msg); signal.broadcast(); }
  
  //! Applies the specified predicate to each of the messages (you can modify the queue value itself via pass-by-reference)
  template<class F> void for_each(const F& f) { std::for_each(msgs.begin(),msgs.end(),f); }
  
  //! Applies the specified predicate to each of the message, removing those for which the predicate returns true
  template<class F> void remove(const F& f) { msgs.remove_if(f); }
  
  //! Returns number of messages in #msgs
  size_t size() const { return msgs.size(); }
  
  //! Clears any backlog
  void clear() { MarkScope l(lock); msgs.clear(); }
  
  //! Returns the next message (does not remove from the queue), blocking until available
  const T& front() const {
    MarkScope l(lock); 
    while(msgs.size()==0)
      signal.wait(lock);
    return msgs.front();
  }
  
  //! Removes the front message, if any
  void pop() { MarkScope l(lock); if(msgs.size()>0) msgs.pop_front(); }
  
  //! Spawns a thread to trigger a class member callback with each message - only a single receiver is supported, so this replaces any previous receiver
  template<typename F, typename C>
  void spawnCallback(F fn, C& cl) {
    stopCallback();
    receiver = new ReceiverThread<F,C>(*this, fn, cl);
    receiver->start();
  }
  
  //! Sends a thread cancellation to the receiver thread to halt processing
  void stopCallback() {
    if(receiver!=NULL) {
      {
#ifdef USE_SIGNAL_TO_CANCEL_THREAD
        // lock is necessary on stop to ensure cancellation does
        // not arrive between the testCancel and the system wait call
        MarkScope l(lock);
#endif
        receiver->stop();
        receiver->keepRunning=false;
      } // must release lock for wait cancellation to go through
      receiver->join();
      delete receiver;
      receiver=NULL;
    }
  }
  
  //! Sets a flag to exit the receiver thread at the completion of the current callback
  /*! If no callback is active, cancels the receiver immediately -- doesn't wait
   *  for another message first */
  void finishCallback() {
    if(receiver==NULL)
      return;
    {
      MarkScope l(lock);
      if(msgs.size()==0)
        receiver->stop();
      receiver->keepRunning=false;
    } // must release lock for wait cancellation to go through
    receiver->join();
    delete receiver;
    receiver=NULL;
  }
  
  //! Sets a flag that when the receiver gets to the end of the queue, it will exit
  /*! If the receiver is already blocking at the end of the queue, stops the thread now */
  void finishQueue() {
    if(receiver==NULL)
      return;
    {
      MarkScope l(lock);
      if(msgs.size()==0)
        receiver->stop();
      receiver->block=false;
    } // must release lock for wait cancellation to go through
    receiver->join();
    delete receiver;
    receiver=NULL;
  }
  
protected:
  mutable Thread::Lock lock; //!< provides mutual exclusion on #msgs operations and #signal reception
  Thread::Condition signal; //!< connects notification of send() in #receiver
  std::list<T> msgs; //!< unprocessed messages
  
  //! Holds controller flags for the receiver thread to indicate exit conditions
  class ReceiverThreadBase : public Thread {
  public:
    ReceiverThreadBase() : Thread(), keepRunning(true), block(true) {} //!< constructor
    bool keepRunning; //!< if cleared, indicates receiver should stop processing messages and exit its thread
    bool block; //!< if cleared, indiciates receiver should exit its thread if/when it runs out of messages to process
  };
  
  //! Pulls messages out of the queue and gives them to the specified callback.
  template<typename F, class C>
  class ReceiverThread : public ReceiverThreadBase {
  public:
    //! constructor
    ReceiverThread(ThreadedMessageQueue<T>& tmq, F f, C& c) : ReceiverThreadBase(), q(tmq), fn(f), cl(c) {}
    
  protected:
    ThreadedMessageQueue& q; //!< the queue being monitored
    F fn; //!< function pointer
    C& cl; //!< class pointer
    
    virtual void* run() {
      while(ReceiverThreadBase::keepRunning && (ReceiverThreadBase::block || q.size()>0)) {
        (cl.*fn)(q.front());
        q.pop();
      }
      return NULL;
    }
  };
  ReceiverThreadBase* receiver; //!< currently only a single receiver is supported
  
private:
  ThreadedMessageQueue(const ThreadedMessageQueue& o); //!< Do not call
  ThreadedMessageQueue& operator=(const ThreadedMessageQueue& o); //!< Do not call
};

/*! @file
 * @brief Describes ThreadedMessageQueue, which 
 * @author ejt (Creator)
 */

#endif