我想使用promise,但我有一个回调API,格式如下:

1.DOM加载或其他一次性事件:

window.onload; // set to callback
...
window.onload = function() {

};

2.普通回调:

function request(onChangeHandler) {
    ...
}
request(function() {
    // change happened
    ...
});

3.节点样式回调(“nodeback”):

function getStuff(dat, callback) {
    ...
}
getStuff("dataParam", function(err, data) {
    ...
})

4.具有节点样式回调的整个库:

API;
API.one(function(err, data) {
    API.two(function(err, data2) {
        API.three(function(err, data3) {
            ...
        });
    });
});

我如何在promise中使用API,如何“promise”它?


当前回答

也许已经回答了,但我通常是这样做的:

// given you've defined this `Future` fn somewhere:
const Future = fn => {return new Promise((r,t) => fn(r,t))}

// define an eventFn that takes a promise `resolver`
const eventFn = resolve => {
  // do event related closure actions here. When finally done, call `resolve()`
  something.oneventfired = e => {resolve(e)}
}

// invoke eventFn in an `async` workflowFn using `Future`
// to obtain a `promise` wrapper
const workflowFn = async () => {await Future(eventFn)}

特别是对于indexedDb事件包装器之类的东西,以简化使用。

或者你可能会发现未来的这种变化更具普遍性

class PromiseEx extends Promise {
  resolve(v,...a) {
    this.settled = true; this.settledValue = v;
    return(this.resolve_(v,...a))
  }
  reject(v,...a) {
    this.settled = false; this.settledValue = v;
    return(this.reject_(v,...a))
  }
  static Future(fn,...args) {
    let r,t,ft = new PromiseEx((r_,t_) => {r=r_;t=t_})
    ft.resolve_ = r; ft.reject_ = t; fn(ft,...args);
    return(ft)
  }
}

其他回答

您可以在ES6中使用本机Promise,例如处理setTimeout:

enqueue(data) {

    const queue = this;
    // returns the Promise
    return new Promise(function (resolve, reject) {
        setTimeout(()=> {
                queue.source.push(data);
                resolve(queue); //call native resolve when finish
            }
            , 10); // resolve() will be called in 10 ms
    });

}

在本例中,Promise没有失败的理由,因此永远不会调用reject()。

在内置promise和异步的节点v7.6+下:

// promisify.js
let promisify = fn => (...args) =>
    new Promise((resolve, reject) =>
        fn(...args, (err, result) => {
            if (err) return reject(err);
            return resolve(result);
        })
    );

module.exports = promisify;

如何使用:

let readdir = require('fs').readdir;
let promisify = require('./promisify');
let readdirP = promisify(readdir);

async function myAsyncFn(path) {
    let entries = await readdirP(path);
    return entries;
}

我通常使用的一个简单的泛型函数。

const promisify = (fn, ...args) => {
  return new Promise((resolve, reject) => {
    fn(...args, (err, data) => {
      if (err) {
        return reject(err);
      }
      resolve(data);
    });
  });
};

如何使用它

函数promify接受带有回调的函数:

   const cb = (result) => `The result is ${result}`;

   const sum = (a, b, cb) => {
    const result = a + b;
    cb(result); // passing args to the callback function
   }


  // using the util
  promise = promisify(sum, 3, 1, cb);
  promise.then(x => console.log(x)) // 4

您可能不需要这个答案,但这将有助于了解可用实用程序的内部工作原理

Node.js 8.0.0包含一个新的util.provisify()API,它允许将标准Node.js回调样式的API封装在返回Promise的函数中。下面显示了util.provisify()的示例用法。

const fs = require('fs');
const util = require('util');

const readFile = util.promisify(fs.readFile);

readFile('/some/file')
  .then((data) => { /* ... */ })
  .catch((err) => { /* ... */ });

请参阅改进对承诺的支持

你可以这样做

// @flow

const toPromise = (f: (any) => void) => {
  return new Promise<any>((resolve, reject) => {
    try {
      f((result) => {
        resolve(result)
      })
    } catch (e) {
      reject(e)
    }
  })
}

export default toPromise

然后使用它

async loadData() {
  const friends = await toPromise(FriendsManager.loadFriends)

  console.log(friends)
}