vue的插件BetterScroll的getSelectedIndex方法怎么调用的?

Question

<template>
    <div class="slide" ref="slide">
        <ul class="list clearfix" id="slide-list">
            <li v-for="d in slideArr">
                <a href=""><img :src="d.pic_url" /></a>
                <div class="title">
                    <a href="">{{d.title}}</a>
                </div>
            </li>
        </ul>


        <div class="count"><strong class="cur">1</strong>/3</div>
    </div>
</template>
<script>
    import BScroll from 'better-scroll';
    export default {
        data(){
            return{
                slideArr : [
                    {
                        "title" : "宁波待拆迁住宅区突发爆炸现场",
                        "pic_url" : "http://xxx.com/test/images/ba98541462ddb36_size28_w600_h400.jpg"
                    },
                    {
                        "title" : "宁波待拆迁住宅区突发爆炸现场",
                        "pic_url" : "http://xxxx.com/test/images/C6C3B19CBE38BA8829CA8C1DEE44FAF0A18C4C62_size25_w568_h423.jpg"
                    }
                ]
            }
        },
        methods : {
            _setSlideWidth(){
                var list = document.querySelector("#slide-list"),item = list.getElementsByTagName("li"),str = 0;
                list.style.width=(640*item.length*2)+"px";
            }
        },
        mounted() {
            
            this._setSlideWidth();
            setTimeout(() => {
                

                var bScroll = new BScroll(".slide",{
                    scrollY : false,
                    scrollX : true,
                    momentum: false,
                    snap: {
                        loop: true,
                        threshold: 0.3,
                        speed: 400
                    },
                    click:true
                  });
                bScroll.on("scrollEnd",function(o){
                    console.log(bScroll.getSelectedIndex())
                })

                
            },20);
        }
    }
</script>

<style lang="scss" scoped>
    .slide{ overflow:hidden;margin:.4em 0;position: relative; }
    .list li{width:640px;float:left;position:relative;}
    .list li .title{ position:absolute;bottom:0;left:0;width:100%; padding:.5em 0; background-image: linear-gradient(rgba(132, 132, 132, 0.08), rgba(132, 132, 132, 0.29), rgba(0, 0, 0, 0.33), rgba(0, 0, 0, 0.61)); }
    .list li .title a{ font-size:1.2em;color:#dbd9da;margin-left:2%;  }
    .count{ position:absolute;bottom:.5em;right:.5em;color:#dbd9da;  }
    .cur{ color:#c2424f; font-size:1.2em; }
</style>

想在scrollEnd事件里监听console.log(bScroll.getSelectedIndex())
为什么返回一个false给我?
文档里明明说 获取当前选中的索引值 是number类型的