data () {
  return {
   ……
   sms: {
    numbers: 4,
    msg: [],
    show: [],
    position: 0
   },
  ……
  }
 }

_setSmsInputDisplay () {
   var arr = []
   for (var i = 0; i < this.sms.numbers * 2 - 1; i++) {
    arr.push(i >= this.sms.numbers - 1 - this.sms.position && i < this.sms.numbers - 1 - this.sms.position + this.sms.numbers ? 1 : 0)
   }
   this.sms.show = arr
  },
  _resetSms () {
   this.sms.msg = []
   for (var i = 0; i < this.sms.numbers; i++) {
    this.sms.msg.push(null)
   }
   this.sms.position = 0
   this._setSmsInputDisplay()
   this.$nextTick(function () {
    this.$refs.sms_input.focus()
   })
  }
  submit () {
   this.$api.post('signin', {
    mobile: this.mobile,
    captcha: this.captcha
   }, r => {
    this.$router.push('/main')
    console.log(r)
   })
  },
  sms_input (e) {
   if (e.keyCode === 8) { // 删除
    if (this.sms.position > 0) {
     this.sms.position--
     this.sms.msg.splice(-2, 1)
     this.sms.msg.unshift(null)
     this._setSmsInputDisplay()
    }
   } else if (e.keyCode >= 48 && e.keyCode <= 57) { // 仅可以输入数字
    if (this.sms.position < this.sms.numbers - 1) {
     this.sms.position++
     this.sms.msg.splice(-1, 1, String.fromCharCode(e.keyCode))
     this.sms.msg.shift()
     this.sms.msg.push(null)
     this._setSmsInputDisplay()
    } else {
     document.activeElement.blur() // hide keyboard for iOS
     console.log(this.sms.msg.join(''))
     this.submit()
    }
   } else {
    this.$refs.sms_input.value = '' // remove NaN
   }
  }