var new_scroll_position = 0;

var last_scroll_position;

onPageScroll: function (e) {

    var that = this

    last_scroll_position = e.scrollTop;

    if (new_scroll_position < last_scroll_position && last_scroll_position > 20) {

        setTimeout(function () {

            that.setData({

            write_style: 'slideDown'

            })

        }, 100);

    } else {

        setTimeout(function () {

            that.setData({

            write_style: 'slideUp'

            })

        }, 100);

    }

    new_scroll_position = last_scroll_position;

},

这是我的写法，但始终无法完美实现，有一些Bug