在组件里调用this.$emit('update:XXXXXX', value);不生效

以下是组件代码，简单地模拟一个tab控件：

<template>  
  <scroll-view class="wuc-tab" scroll-with-animation scroll-x :scroll-left="scrollLeft">  
    <div>  
      <div  
        v-for="(item, index) in tabList" :key="index"  
        class="wuc-tab-item"  
        :class="index == currentTab ? 'cur' : ''"  
        @tap="onTabChange(index, $event)"  
      >  
        <span>{{item.name}}</span>  
      </div>  
    </div>  
  </scroll-view>  
</template>  

<script>  
export default {  
    props: {  
        tabList: {  
            type: Array,  
            default: []  
        },  
        currentTab: {  
            type: Number,  
            default: 0  
        }  
    },  
    data() {  
        return {  
        };  
    },  
    computed: {  
        scrollLeft() {  
            return (this.currentTab - 1) * 60;  
        }  
    },  
    methods: {  
        onTabChange(index, e) {  
            if (this.currentTab == index) return false;  
            this.$emit('update:currentTab', index);  
            this.$emit('change', index);  
        }  
    }  
};  
</script>  

<style scope>  
div,  
scroll-view {  
    box-sizing: border-box;  
}  
.wuc-tab {  
    white-space: nowrap;  
}  
.wuc-tab-item {  
    display: inline-block;  
    margin: 5upx 10upx;  
    padding: 0 10upx;  
}  

.wuc-tab-item.cur {  
    border-bottom: 4upx solid;  
}  
</style>

以下是调用组件的代码：

<template>  
    <view>  
        <wuc-tab  
            :tabList="candidateTypes"  
            :currentTab="currentTab"  
            @change="onTabChange"/>  
    </view>  
</template>  

<script>  
    import WucTab from './component/wuc-tab.vue'  

    export default {  
        components: {  
            WucTab  
        },  
        data() {  
            return {  
                candidateTypes: [  
                    {  
                        id: 0,  
                        name: "全部"  
                    },  
                    {  
                        id: 1,  
                        name: "子分类1"  
                    },  
                    {  
                        id: 2,  
                        name: "子分类2"  
                    }  
                ],  
                currentTab: 0  
            }  
        },  
        methods: {  
            onTabChange(index) {  
                // 测试在wuc-tab组件中执行this.$emit('update:currentTab', index)并未生效，因此必须在此手工更新  
                this.currentTab = index;  
            }  
        }  
    }  
</script>

经过测试，在组件的onTabChange方法中执行 this.$emit('update:currentTab', index); 语句，currentTab属性的值并没有改变；必须在调用者的onTabChange方法中执行 this.currentTab = index; ，该属性的值才改变。这是怎么回事呢？