/**
+ * Extends the IO base class to implement Queue for synchronous
+ * transaction processing.
+ * @module io
+ * @submodule io-queue
+ */
+
+ /**
+ * @description Array of transactions queued for processing
+ *
+ * @property _yQ
+ * @private
+ * @static
+ * @type Object
+ */
+ var _q = new Y.Queue(),
+
+ /**
+ * @description Reference to "io:complete" event handler.
+ *
+ * @property _e
+ * @private
+ * @static
+ * @type Object
+ */
+ _e,
+
+ _activeId,
+ /**
+ * @description Property to determine whether the queue is set to
+ * 1 (active) or 0 (inactive). When inactive, transactions
+ * will be stored in the queue until the queue is set to active.
+ *
+ * @property _qState
+ * @private
+ * @static
+ * @type int
+ */
+ _qState = 1;
+
+ /**
+ * @description Method for requesting a transaction, and queueing the
+ * request before it is sent to the resource.
+ *
+ * @method _queue
+ * @private
+ * @static
+ * @return Object
+ */
+ function _queue(uri, c) {
+ var o = { uri: uri, id: Y.io._id(), cfg:c };
+
+ _q.add(o);
+ if (_qState === 1) {
+ _shift();
+ }
+
+ Y.log('Object queued. Transaction id is' + o.id, 'info', 'io');
+ return o;
+ }
+
+ /**
+ * @description Method Process the first transaction from the
+ * queue in FIFO order.
+ *
+ * @method _shift
+ * @private
+ * @static
+ * @return void
+ */
+ function _shift() {
+ var o = _q.next();
+
+ _activeId = o.id;
+ _qState = 0;
+ Y.io(o.uri, o.cfg, o.id);
+ }
+
+ /**
+ * @description Method for promoting a transaction to the top of the queue.
+ *
+ * @method _unshift
+ * @private
+ * @static
+ * @return void
+ */
+ function _unshift(o) {
+ _q.promote(o);
+ }
+
+ function _next(id) {
+ _qState = 1;
+ if (_activeId === id && _q.size() > 0) {
+ _shift();
+ }
+ }
+
+ /**
+ * @description Method for removing a specific, pending transaction from
+ * the queue.
+ *
+ * @method _remove
+ * @private
+ * @static
+ * @return void
+ */
+ function _remove(o) {
+ _q.remove(o);
+ }
+
+ function _start() {
+ _qState = 1;
+
+ if (_q.size() > 0) {
+ _shift();
+ }
+ Y.log('Queue started.', 'info', 'io');
+ }
+
+ /**
+ * @description Method for setting queue processing to inactive.
+ * Transaction requests to YUI.io.queue() will be stored in the queue, but
+ * not processed until the queue is reset to "active".
+ *
+ * @method _stop
+ * @private
+ * @static
+ * @return void
+ */
+ function _stop() {
+ _qState = 0;
+ Y.log('Queue stopped.', 'info', 'io');
+ };
+
+ /**
+ * @description Method to query the current size of the queue.
+ *
+ * @method _size
+ * @private
+ * @static
+ * @return int
+ */
+ function _size() {
+ return _q.size();
+ };
+
+ _e = Y.on('io:complete', function(id) { _next(id); }, Y.io);
+
+ /**
+ * @description Method to query the current size of the queue, or to
+ * set a maximum queue size. This is the interface for _size().
+ *
+ * @method size
+ * @public
+ * @static
+ * @param {number} i - Specified maximum size of queue.
+ * @return number
+ */
+ _queue.size = _size;
+
+ /**
+ * @description Method for setting the queue to active. If there are
+ * transactions pending in the queue, they will be processed from the
+ * queue in FIFO order. This is the interface for _start().
+ *
+ * @method start
+ * @public
+ * @static
+ * @return void
+ */
+ _queue.start = _start;
+
+ /**
+ * @description Method for setting queue processing to inactive.
+ * Transaction requests to YUI.io.queue() will be stored in the queue, but
+ * not processed until the queue is restarted. This is the
+ * interface for _stop().
+ *
+ * @method stop
+ * @public
+ * @static
+ * @return void
+ */
+ _queue.stop = _stop;
+
+ /**
+ * @description Method for promoting a transaction to the top of the queue.
+ * This is the interface for _unshift().
+ *
+ * @method promote
+ * @public
+ * @static
+ * @param {Object} o - Reference to queued transaction.
+ * @return void
+ */
+ _queue.promote = _unshift;
+
+ /**
+ * @description Method for removing a specific, pending transaction from
+ * the queue. This is the interface for _purge().
+ *
+ * @method purge
+ * @public
+ * @static
+ * @param {Object} o - Reference to queued transaction.
+ * @return void
+ */
+ _queue.remove = _remove;
+
+ Y.mix(Y.io, {
+ queue: _queue
+ }, true);
+