    /**
     * This is a utility class for being able to track all items of a particular type
     * inside any level at a container. This can be used in favour of bubbling add/remove events
     * which can add a large perf cost when implemented globally
     * @private
     */
    Ext.define('Ext.container.Monitor', {
        target: null,
        selector: '',

        scope: null,
        addHandler: null,
        removeHandler: null,

        disabled: 0,

        constructor: function(config){
            Ext.apply(this, config);
        },

        bind: function(target){
            var me = this;

            me.target = target;
            target.on('beforedestroy', me.disable, me);
            me.onContainerAdd(target);
        },

        unbind: function() {
            var me = this,
                target = me.target;

            if (target) {
                target.un('beforedestroy', me.disable, me);
            }
            me.items = null;
        },

        disable: function(){
            ++this.disabled;
        },

        enable: function(){
            if (this.disabled > 0) {
                --this.disabled;
            }
        },

        handleAdd: function(ct, comp) {
            if (!this.disabled) {
                if (comp.is(this.selector)) {
                    this.onItemAdd(comp.ownerCt, comp);
                }

                if (comp.isQueryable) {
                    this.onContainerAdd(comp);
                }
            }
        },

        onItemAdd: function(ct, comp){
            var me = this,
                items = me.items,
                handler = me.addHandler;

            if (!me.disabled) {
                if (handler) {
                    handler.call(me.scope || comp, comp);
                }
                if (items) {
                    items.add(comp);
                }
            }
        },

        onItemRemove: function(ct, comp){
            var me = this,
                items = me.items,
                handler = me.removeHandler;

            if (!me.disabled) {
                if (handler) {
                    handler.call(me.scope || comp, comp);
                }
                if (items) {
                    items.remove(comp);
                }
            }
        },

        onContainerAdd: function(ct, preventChildren) {
            var me = this,
                items, len,
                handleAdd = me.handleAdd,
                handleRemove = me.handleRemove,
                i, comp;

            if (ct.isContainer) {
                ct.on('add', handleAdd, me);
                ct.on('dockedadd', handleAdd, me);
                ct.on('remove', handleRemove, me);
                ct.on('dockedremove', handleRemove, me);
            }

            // Means we've been called by a parent container so the selector
            // matchers have already been processed
            if (preventChildren !== true) {
                items = ct.query(me.selector);
                for (i = 0, len = items.length; i < len; ++i) {
                    comp = items[i];
                    me.onItemAdd(comp.ownerCt, comp);
                }
            }

            items = ct.query('container');
            for (i = 0, len = items.length; i < len; ++i) {
                me.onContainerAdd(items[i], true);
            }

        },

        handleRemove: function(ct, comp) {
            var me = this;

            // During a destroy we don't want to maintain any of this information,
            // so typically we'll be disabled here
            if (!me.disabled) {
                if (comp.is(me.selector)) {
                    me.onItemRemove(ct, comp);
                }

                if (comp.isQueryable) {
                    me.onContainerRemove(ct, comp);
                }
            }
        },

        onContainerRemove: function(ct, comp){
            var me = this,
                items, i, len, item;

            // If it's not a container, it means it's a queryable that isn't a container.
            // For example a button with a menu
            if (!comp.isDestroyed && !comp.destroying && comp.isContainer) {
                me.removeCtListeners(comp);

                items = comp.query(me.selector);
                for (i = 0, len = items.length; i < len; ++i) {
                    item = items[i];
                    me.onItemRemove(item.ownerCt, item);
                }

                items = comp.query('container');
                for (i = 0, len = items.length; i < len; ++i) {
                    me.removeCtListeners(items[i]);
                }
            } else {
                // comp destroying, or we need to invalidate the collection
                me.invalidateItems();
            }
        },

        removeCtListeners: function(comp){
            var me = this;
            comp.un('add', me.handleAdd, me);
            comp.un('dockedadd', me.handleAdd, me);
            comp.un('remove', me.handleRemove, me);
            comp.un('dockedremove', me.handleRemove, me);
        },

        getItems: function(){
            var me = this,
                items = me.items;

            if (!items) {
                items = me.items = new Ext.util.MixedCollection();
                items.addAll(me.target.query(me.selector));
            }
            return items;
        },

        invalidateItems: function(){
            this.items = null;
        }
    });