methods: {
    sendDrag (val) {
      console.log(val)
    }
[...]



    mounted () {
        this.$nextTick(() => {
          this.$refs.flickity.on('dragStart', function () {
            this.stageDragging = true
            this.sendDrag('started')
          })

mounted () {
        this.$nextTick(() => {
          const that = this;
          this.$refs.flickity.on('dragStart', function () {
            that.stageDragging = true
            that.sendDrag('started')
          })