Promise

Posted 2020-11-20 liangcheng11

// 回调
const ajax  = function (cb) {
    setTimeout(()=>{
        cb();
    });
};

// 基本用法
const ajax = function() {
    const res = ‘‘;
    return new Promise((resolve, reject) => {
        if (flag) {
            resolve(res);
        } else {
            reject(res);
        }
    });
};
const resolve = (res) => {
    // toddo
}
const reject = (res) => {
    // todo
}
ajax.then(resolve, resolve).catch(err => {
    console.log(err);
});

1. 链式

// promise

const ajax = function() {
    const res = ‘‘;
    return new Promise((resolve, reject) => {
        resolve(res);
    });
};
const resolve = (res) => {
    return new Promise((resolve, reject) => {
        resolve();
    });
}
ajax.then(resolve).then(() => {
    // todo
});

2. 群发——Promise.all(FnAry);

// 所有图片加载完再添加到页面
  function loadImg(src){
    return new Promise((resolve,reject)=>{
      let img=document.createElement(‘img‘);
      img.src=src;
      img.onload=function(){
        resolve(img);
      }
      img.onerror=function(err){
        reject(err);
      }
    })
  }

  function showImgs(imgs){
    imgs.forEach(function(img){
      document.body.appendChild(img);
    })
  }

  Promise.all([
    loadImg(‘http://i4.buimg.com/567571/df1ef0720bea6832.png‘),
    loadImg(‘http://i4.buimg.com/567751/2b07ee25b08930ba.png‘),
    loadImg(‘http://i2.muimg.com/567751/5eb8190d6b2a1c9c.png‘)
  ]).then(showImgs)

3. 在多个请求中，只处理最先返回响应的——Promise.race(FnAry)

// 有一个图片加载完就添加到页面
  function loadImg(src){
    return new Promise((resolve,reject)=>{
      let img=document.createElement(‘img‘);
      img.src=src;
      img.onload=function(){
        resolve(img);
      }
      img.onerror=function(err){
        reject(err);
      }
    })
  }

  function showImgs(img){
    let p=document.createElement(‘p‘);
    p.appendChild(img);
    document.body.appendChild(p)
  }

  Promise.race([
    loadImg(‘http://i4.buimg.com/567571/df1ef0720bea6832.png‘),
    loadImg(‘http://i4.buimg.com/567751/2b07ee25b08930ba.png‘),
    loadImg(‘http://i2.muimg.com/567751/5eb8190d6b2a1c9c.png‘)
  ]).then(showImgs)