onChange (val, event) {
      const { checked, value } = event.target
      let newValues
      if (checked) {
        newValues = [...this._values, value]
      } else {
        newValues = this._values.filter(val => val !== value)
      }

      if (!this.isControlled) {
        this.values = newValues
      }
      this.$emit('change', newValues)
    }
Handles change event for checkbox group
@param {Event} event Event object