Expose a method and add a handler in websocketpp.

* Expose websocketpp::transport::asio::connection::get_strand().
* Add new send_empty_handler to websocketpp::endpoint.

src/websocketpp/websocketpp/connection.hpp

@@ -150,6 +150,13 @@ typedef lib::function<bool(connection_hdl)> validate_handler;
  */
 typedef lib::function<void(connection_hdl)> http_handler;
 
+/// The type and function signature of an send_empty handler
+/**
+ * The send_empty handler is called each time the send queue becomes empty of
+ * messages.
+ */
+typedef lib::function<void(connection_hdl)> send_empty_handler;
+
 //
 typedef lib::function<void(lib::error_code const & ec, size_t bytes_transferred)> read_handler;
 typedef lib::function<void(lib::error_code const & ec)> write_frame_handler;
@@ -458,6 +465,16 @@ public:
         m_message_handler = h;
     }
 
+    /// Set send empty handler
+    /**
+     * The send empty handler is called when the message queue is empty.
+     *
+     * @param h The new message_handler
+     */
+    void set_send_empty_handler(send_empty_handler h) {
+        m_send_empty_handler = h;
+    }
+
     //////////////////////////////////////////
     // Connection timeouts and other limits //
     //////////////////////////////////////////
@@ -1382,6 +1399,7 @@ private:
     http_handler            m_http_handler;
     validate_handler        m_validate_handler;
     message_handler         m_message_handler;
+    send_empty_handler      m_send_empty_handler;
 
     /// constant values
     long                    m_open_handshake_timeout_dur;

src/websocketpp/websocketpp/endpoint.hpp

@@ -271,6 +271,11 @@ public:
         scoped_lock_type guard(m_mutex);
         m_message_handler = h;
     }
+    void set_send_empty_handler(send_empty_handler h) {
+        m_alog.write(log::alevel::devel,"set_send_empty_handler");
+        scoped_lock_type guard(m_mutex);
+        m_send_empty_handler = h;
+    }
 
     //////////////////////////////////////////
     // Connection timeouts and other limits //
@@ -560,6 +565,7 @@ private:
     http_handler                m_http_handler;
     validate_handler            m_validate_handler;
     message_handler             m_message_handler;
+    send_empty_handler          m_send_empty_handler;
 
     long                        m_open_handshake_timeout_dur;
     long                        m_close_handshake_timeout_dur;

src/websocketpp/websocketpp/impl/connection_impl.hpp

@@ -1575,7 +1575,9 @@ void connection<config>::write_frame() {
         // pull off all the messages that are ready to write.
         // stop if we get a message marked terminal
         message_ptr next_message = write_pop();
+        bool saw_message = false;
         while (next_message) {
+            saw_message = true;
             m_current_msgs.push_back(next_message);
             if (!next_message->get_terminal()) {
                 next_message = write_pop();
@@ -1586,6 +1588,10 @@ void connection<config>::write_frame() {
         
         if (m_current_msgs.empty()) {
             // there was nothing to send
+            // If we just made the transition to empty, send out
+            // "send_empty" callback.
+            if (saw_message)
+                m_send_empty_handler(m_connection_hdl);
             return;
         } else {
             // At this point we own the next messages to be sent and are

src/websocketpp/websocketpp/transport/asio/connection.hpp

@@ -339,12 +339,13 @@ public:
             callback(lib::error_code());
         }
     }
-protected:
+
     /// Get a pointer to this connection's strand
     strand_ptr get_strand() {
         return m_strand;
     }
 
+protected:
     /// Initialize transport for reading
     /**
      * init_asio is called once immediately after construction to initialize