// 首先我们先创建一个待迭代的对象
let obj = {0:'Gu',1:'Yan',2:'No.1',length:3};
console.log([...obj]);// 报错 Uncaught TypeError: obj is not iterable
console.log(Array.from(obj));// ["Gu", "Yan", "No.1"]
// 接下来我们给待迭代对象添加一个迭代器
obj[Symbol.iterator] = function(){
 let index = 0;
 let self = this;
 return {
 next(){
  return {value:self[index],done:index++ === self.length}
 }
 }
}
console.log([...obj]) // ["Gu", "Yan", "No.1"]
console.log(Array.from(obj));// ["Gu", "Yan", "No.1"]

function* guYan(){
 
}
console.log(guYan()) // Object [Generator] {}
console.log(guYan().next) // [Function: next]
console.loh(guYan().next()) // { value: undefined, done: true }

function* guYan(){
 yield 1,
 yield 2,
 yield 3
}
let it = guYan();
console.log(it.next()) // { value: 1, done: false }
console.log(it.next()) // { value: 2, done: false }
console.log(it.next()) // { value: 3, done: false }
console.log(it.next()) // { value: undefined, done: true }

obj[Symbol.iterator] = function* (){
 // 每次浏览器都会不停的调用next方法 把yield的结果作为值
 let index = 0;
 while(index !== this.length){
 yield this[index++]
 }
}
console.log([...obj]) // ["Gu", "Yan", "No.1"]
console.log(Array.from(obj));// ["Gu", "Yan", "No.1"]

function* guYan(){
 let a = yield 1;
 console.log('a',a);
 let b = yield 2;
 console.log('b',b);
 let c = yield 3;
 console.log('c',c);
}
let it = guYan();
//第一次调用it.next()
it.next() // 什么都没有输出
// 第二次调用
it.next() // a undefined 
/*如果我们第二次是传入参数调用*/
it.next(100) // a 100
// 第三次调用
it.next(200) // b 200
// 第四次调用 
it.next(300) // c 300

// 实现前提 同级目录下创建name.txt age.txt 文件；name.txt中存储age.txt，age.txt中存储20
let fs = require('mz/fs');//我们直接使用mz包来实现fs的promise化
let path = require('path');
function* guYan() {
 let name = yield fs.readFile(path.resolve(__dirname, './name.txt'), 'utf-8');
 name = yield './' + name;
 let age = yield fs.readFile(path.resolve(__dirname, name), 'utf-8');
 return age;
}
let it = guYan();
let { value } = it.next();
value.then(data => {
 let { value } = it.next(data);
 Promise.resolve(value).then(data => {
  let { value } = it.next(data)
  value.then(data => {
   let { value } = it.next(data);
   console.log(value) // 20
  })
 })
})

let fs = require('mz/fs');
let path = require('path');
function* guYan() {
 let name = yield fs.readFile(path.resolve(__dirname, './name.txt'), 'utf-8');
 name = yield './' + name;
 let age = yield fs.readFile(path.resolve(__dirname, name), 'utf-8');
 return age;
}
function co(it){
 return new Promise((resolve,reject)=>{
  function next(val){
   let {value , done} = it.next(val);
   if(done){
    return resolve(value);
   }
   Promise.resolve(value).then(data=>{
    next(data)
   })
  }
  next();
 })
}
co(guYan()).then(data=>{
 console.log(data); // 20
})

// 上述代码可以简化为
let fs = require('mz/fs');
let path = require('path');
async function guYan() {
 let name = await fs.readFile(path.resolve(__dirname, './name.txt'), 'utf-8');
 name = './' + name;
 let age = await fs.readFile(path.resolve(__dirname, name), 'utf-8');
 return age;
}
// async 函数执行后返回一个promise
// 可以使用try + catch ，但如果使用try + catch 返回的就是真
guYan().then(data=>{
 console.log(data);
})