<div id="app">
 <div class="captcha">
  <input v-for="(c, index) in ct" :key="index"
   type="number" v-model="ct[index]" ref="input" 
   :style="{borderBottomColor: index <= cIndex "
   @input="e => {onInput(e.target.value, index)}" 
   @keydown.delete="e=>{onKeydown(e.target.value, index)}"
   @focus="onFocus"
   :disabled="loading"
   >
 </div>
 <p>{{msg}}</p>
</div>

.captcha {
 display: flex;
 justify-content: center;
 margin-top: 40px;
}
input {
 margin-right: 20px;
 width: 45px;
 text-align: center;
 border: none;
 border-bottom: 1px solid #eee;
 font-size: 24px;
 outline: none;
}
input:last-of-type {
 margin-right: 0;
}
input:disabled {
 color: #000;
 background-color: #fff;
}
.msg {
 text-align: center;
}

var Main = {
 data() {
  return {
   ct: ['', '', '', '', '', ''],
   loading: false,
   msg: '',
  }
 },
 computed: {
  ctSize() {
   return this.ct.length;
  },
  cIndex() {
   let i = this.ct.findIndex(item => item === '');
   i = (i + this.ctSize) % this.ctSize;
   return i;
  },
  lastCode() {
   return this.ct[this.ctSize - 1];
  }
 },
 watch: {
  cIndex() {
   this.resetCaret();
  },
  lastCode(val) {
   if (val) {
    console.log('this.ctSize', this.ctSize)
    this.$refs.input[this.ctSize - 1].blur();
    this.sendCaptcha();
   }
  }
 },
 mounted() {
  this.resetCaret();
 },
 methods: {
  onInput(val, index) {
   this.msg = ''
   val = val.replace(/\s/g, '');
   if (index == this.ctSize - 1) {
    this.ct[this.ctSize - 1] = val[0];  // 最后一个码，只允许输入一个字符。
   } else if(val.length > 1) {
    let i = index;
    for (i = index; i < this.ctSize && i - index < val.length; i++) {
     this.ct[i] = val[i];
    }
    this.resetCaret();
   }
  },
  // 重置光标位置。
  resetCaret() {
   this.$refs.input[this.ctSize-1].focus();
  },
  onFocus() {
   // 监听 focus 事件，将光标重定位到“第一个空白符的位置”。
   let index = this.ct.findIndex(item => item === '');
   index = (index + this.ctSize) % this.ctSize;
   console.log(this.$refs.input)
   this.$refs.input[index].focus();
  },
  onKeydown(val, index) {
   if (val === '') {
    // 删除上一个input里的值，并对其focus。
    if (index > 0) {
     this.ct[index - 1] = '';
     this.$refs.input[index - 1].focus();
    }
   }
  },
  sendCaptcha() {
   console.log();
   this.msg = `发送验证码到服务器：${this.ct.join('')}`;
   // 此时无法操作 input。。
   this.loading = true;
   setTimeout(() => {
    this.msg = ('验证码错误')
    this.loading = false;
    this.$nextTick(() => {
     this.reset();
    })
   }, 3000)
  },

  reset() {
   // 重置。一般是验证码错误时触发。
   this.ct = this.ct.map(item => '');
   this.resetCaret();
  }
 }
}
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')