wxml文件:

<view>
  <button type="primary" bindtap="countdown" disabled="{{isDisabled}}">{{title}}</button>
</view>
<view>
  <button type="primary" bindtap="countdown1" disabled="{{isDisabled}}">{{title}}</button>
</view>

js文件

  data: {
    title:"发送验证码", // 按钮中显示的字符
    titleConst:"发送验证码", // 重置后的按钮中显示的字符
    count:3,   // 倒计时的秒数
    countConst:3, // 重置后的倒计时的秒数
    isDisabled:false // 按钮是否禁用
  },  

  // setInterval中用箭头函数，保证this和外部一致
  countdown: function(){
    let count = this.data.count;
    // 当count不为0开始倒计时，当count为0就关闭倒计时
    // 设置定时器
    var countdown = setInterval(() => {
      if( count == 0 ){
        this.setData({
          title:this.data.titleConst,
          count: this.data.countConst,
          isDisabled:false
        });
        // 取消定时器
        clearInterval(countdown);
      } else {
        this.setData({
          title:count-- + "s后重试",
          isDisabled:true
        });
      }
    }, 1000);
  },
  // 用that保存this，防止在setInterval中this被替换
  countdown1: function(){
    let that = this;
    let count = this.data.count;
    // 当count不为0开始倒计时，当count为0就关闭倒计时
    // 设置定时器
    var countdown = setInterval(function(){
      if( count == 0 ){
        that.setData({
          title:that.data.titleConst,
          count: that.data.countConst,
          isDisabled:false
        });
        // 取消定时器
        clearInterval(countdown);
      } else {
        that.setData({
          title:count-- + "s后重试",
          isDisabled:true
        });
      }
    }, 1000);
  }