//循环状态标记,0为初始状态,1为正常,2为出错
let status = 0;

/* 新建Filereader,读取文件切片,返回一个promise
* 把读取成功的arraybuffer通过reslove传出
*/
const createReader = ()=> {
 return new Promise ((reslove, reject)=> {
  let reader = new Filereader();
  ...
  reader.onload = ()=> {
   reslove(reader.result)
  }
  reader.onerror = ()=> reject()
 })
}

// ajax发送createReader方法读取到的Buff
const createXhr = ()=> {
 const xhr= new XMLHttpRequest();
 return new Promise ((reslove, reject)=> {
  ...
  xhr.onreadystatechange= ()=> {
   ...
   //如果readyState == 4,status == 200且服务器的状态码存在,更改全局标记为1
   status = 1;
   reslove()
  }
 })
}

//每一轮循环开始前都检查一次全局状态标记
const checkStatus = ()=> {
 ...
 if (status == 1) {
  loop()
 }
}

//循环过程的链式调用
const loop = ()=> {
 createReader().then(()=> createXhr()).then(()=> checkStatus());
}

//模仿node.js的EventEmitter
class EventEmitter {
 constructor() {
  this.handler = {};
 }
 on(eventName, callback) {
  if (!this.handles){
   this.handles = {};
  }
  if (!this.handles[eventName]) {
   this.handles[eventName] = [];
  }
  this.handles[eventName].push(callback);
 }
 emit(eventName,...arg) {
  if (this.handles[eventName]) {
   for (var i=0;i<this.handles[eventName].length;i++) {
    this.handles[eventName][i](...arg);
   }
  }
 }
 }

let ev= new EventEmitter();
...
//监听createReader事件,如果读取buffer成功就触发toajax事件来上传切片
ev.on('createReader', ()=> {
 let reader = new Filereader();
 ...
 reader.onload = ()=> {
  ev.emit('toajax')
 }
})

//监听toajax事件,如果上传成功,就触发createReader事件开始读取下一切片
ev.on('toajax', ()=> {
 let xhr= new XMLHttpRequest();
 ...
 xhr.onreadystatechange = ()=> {
 //如果readyState == 4,status == 200且服务器的状态码存在
  ev.emit('createReader')
 }
})

//全局状态标记
let status = 0;

//读取切片
const createReader = ()=> {
 let reader = new Filereader();
 ...
 reader.onload = ()=>status = 1
}

//上传切片
const createXhr = ()=> {
 let xhr= new XMLHttpRequest();
 ...
 xhr.onreadystatechange = ()=> {
  ...
  //如果readyState == 4,status == 200且服务器的状态码存在
  status = 2
 }
}

/* 设置一个间隔时间极短的计时器,根据status决定下一步的任务,
* 上传完成后定时器自动清除自己
* 另外有判断文件是否上传完成的方法,这里就不写了
*/
let timer = setInterval(()=> {
 if (status == 2) {
  createReader();
 } else if (status == 1) {
  createXhr();
 } else if (status == 3) {
  clearInterval(timer);
 }
},10)