// 回調(diào)地獄實(shí)例
// 奶茶函數(shù)
function getTea(fn) {
  setTimeout(() => {
    fn('獲取到一杯奶茶')
  },2000)
}
// 面包函數(shù)
function getBread(fn) {
  setTimeout(() => {
    fn('獲取到一個(gè)面包')
  },100)
}
// 如果必須按照順序獲取，而不是根據(jù)時(shí)間，要求是先獲取到奶茶后獲取到面包。
getTea(function(data) {
  console.log(data);
  getBread(function(data) {
    console.log(data);
  }) 
})

// 下面解釋下，如何通過Promise來(lái)解決回調(diào)地獄的問題
function getTea() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve('獲取到一杯奶茶')
    }, 2000)
  })
}
function getBread() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve('獲取到一個(gè)面包')
    }, 500)
  })
}
getTea()
  .then(res => {
    console.log(res);
    return getBread();
  })
  .then(res => {
    console.log(res);
  })

function myPromiseAll(iterable) {
  // 首先明確要返回的對(duì)象是一個(gè)Promise
  return new Promise((resolve,reject) => {
    // 首先將可迭代對(duì)象轉(zhuǎn)換為數(shù)組
    const promises = Array.from(iterable);
    let flag = 0;
    const result = [];
    // 開始遍歷執(zhí)行
    for (let i = 0; i < promises.length; i++) {
      Promise.resolve(promises[i]).then(res => {
        result[i] = res;
        flag++;
        if (flag === promises.length) {
          resolve(result)
        }
      }).catch(err => {
        reject(err)
      })
    }
  })  
}

// 手寫promise.race
function myPromiseRace(iterator) {
  // 首先返回的是一個(gè)promise對(duì)象
  return new Promise((resolve,reject) => {
    for (let item of iterator) {
      Promise.resolve(item).then(res => {
        resolve(item);
      }).catch(err => {
        reject(err);
      })
    }
  })
}
let p1 = new Promise(resolve => {
  setTimeout(resolve, 105, 'p1 done')
})
let p2 = new Promise(resolve => {
  setTimeout(resolve, 100, 'p2 done')
})
myPromiseRace([p1, p2]).then(data => {
  console.log(data); // p2 done
})

let p = new Promise((resolve,reject) => {
  setTimeout(() => {
    resolve(111);
  },2000)
})
p.then(res => {
  console.log(res);  // 111
}).finally(() => {
  console.log('無(wú)論如何這里都會(huì)被執(zhí)行');  // 無(wú)論如何這里都會(huì)被執(zhí)行
})

Promise.prototype.finally = function(f) {
  return this.then((value) => {
    return Promise.resolve(f()).then(() => value)
  },(err) => {
    return Promise.resolve(f()).then(() => {
      throw err;
    })
  })
}

// 借助reduce函數(shù)來(lái)實(shí)現(xiàn)Promise的串行執(zhí)行
const funcArr = [
  () => {
    return new Promise((resolve) => {
      setTimeout(() => {resolve(1)},2000)
    })
  },
  () => {
    return new Promise((resolve) => {
      setTimeout(() => {resolve(2)},1000)
    })
  },
  () => {
    return new Promise((resolve) => {
      setTimeout(() => {resolve(3)},3000)
    })
  },
];
function inOrder(arr) {
  const res = [];
  return new Promise((resolve) => {
    arr.reduce((pre,cur) => {
      return pre.then(cur).then(data => res.push(data))
    },Promise.resolve()).then(data => resolve(res))
  })
}
inOrder(funcArr).then(data => console.log(data))   // [1,2,3]

Promise.resolve(1)
  .then(function(){return 2})
  .then(Promise.resolve(3))
  .then(console.log)   // 2

// 使用Promise封裝Ajax請(qǐng)求
const res = new Promise((resolve,reject) => {
  // 1. 創(chuàng)建一個(gè)XMLHttpRequest對(duì)象
  const xhr = new XMLHttpRequest();
  // 2. 初始化請(qǐng)求方法和URL
  xhr.open('GET','https://api.apiopen.top/getJoke');
  // 3. 發(fā)送請(qǐng)求
  xhr.send();
  // 4. 綁定事件，處理響應(yīng)結(jié)果
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4) {
      // 這里4代表的就是說服務(wù)端返回了全部的結(jié)果
      // 如果服務(wù)端返回的狀態(tài)碼是2開頭的，我們就resolve這個(gè)返回的結(jié)果，反之則reject對(duì)應(yīng)的狀態(tài)碼
      if (xhr.status >= 200 && xhr.status < 300) {
        resolve(xhr.response)
      } else {
        reject(xhr.status)
      }
    }
  }
})
res.then(function(value) {
  console.log(value);
},function(err) {
  console.log(err);
})

const p = new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log('resolve前的狀態(tài)：', p);
    resolve();
    console.log('resolve之后的狀態(tài)', p);
  })
})

const p = new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log('reject前的狀態(tài)：', p);
    reject();
    console.log('reject之后的狀態(tài)', p);
  })
})

const fs = require('fs');
fs.readFile('./temp.md',(err,data) => {
  console.log(data.toString());
})

const fs = require('fs');
async function myReadFile() {
  let result = await new Promise((resolve,reject) => {
    fs.readFile('./temp.md',(err,data) => {
      resolve(data.toString());
    })
  })
  console.log(result);   // xxxxx
  return result;
}
myReadFile()