下面这个 是上拉加载的实现方法，当if条件为真时，会触发 回调（this.scroll.trigger(PULL_UP_HOOKS_NAME)），就是没有看明白this.scroll.movingDirectionY一直都是0，是在什么时候变成1的？
还有 this.scroll.maxScrollY 是怎么更新的？

demo地址

`PullUp.prototype.checkPullUp = function (pos) {

        var _this = this;
        var threshold = this.options.threshold;
        if (this.scroll.movingDirectionY === 1 /* Positive */ &&
            pos.y <= this.scroll.maxScrollY + threshold) {
            this.pulling = true;
            // must reset pulling after scrollEnd
            this.scroll.once(this.scroll.eventTypes.scrollEnd, function () {
                _this.pulling = false;
            });
            this.unwatch();
            this.scroll.trigger(PULL_UP_HOOKS_NAME);
        }
    };`