Starts a show or hide animation.
function _processAnimations(event) {
        var prevItem;
        for (var i = 0; i < this._viewStack.length; i++) {
            var item = this._viewStack[i];
            switch (item.state) {
                case ItemState.HIDE:
                    item.state = ItemState.HIDING;
                    _initHideAnimation.call(this, item, prevItem, event.size);
                    _updateState.call(this);
                    break;
                case ItemState.SHOW:
                    item.state = ItemState.SHOWING;
                    _initShowAnimation.call(this, item, prevItem, event.size);
                    _updateState.call(this);
                    break;
            }
            prevItem = item;
        }
    }