Promisify导入类（构造函数）与蓝鸟在ES6 + babel [英] Promisify imported class (constructor) with bluebird in ES6 + babel

问题描述

假设我创建或有一个node.js库 lib.js

Suppose I created or have a node.js library lib.js

export class C {
    constructor(value, callback) {
        callback(false, `Hello ${value}`);
    }

    task(value, callback) {
        callback(false, "returned " + value);
    }
}

重要的是类的构造函数需要接受回调，因为它执行数据库连接和文件I / O。如果我现在导入和使用库回调风格，一切都很好（见下面 c1 ）。

The important part is that the classes' constructor needs to accept a callback as it does database connections and file I/O. If I now import and use the library callback-style, everything is fine (see c1 below).

我真的想宣传图书馆，我用它来使对象构造更方便（在现实中，它是一大堆课程，方法）。

I would really like to promisify the library where I use it to make object construction more convenient (in reality it's a whole bunch of classes and methods).

但是，我找不到一个方法来新类中正确的promise安全很好。

However, I can't find a way to new the class properly in a promise-safe nicely.

import Promise from 'bluebird';
import * as lib from './lib';


Promise.promisifyAll(lib);


// old style -- works as expected
const c1 = new lib.C("c1", (e, v) => {
    console.log(c1, e, v); 
});


// assuming c1 got initialized, .task() also works
c1.task("t1", console.log);
c1.taskAsync("t2").then(() => console.log("also works"));


// But how to do this properly with promises?
const c2 = new lib.C("c2"); c2.then(console.log); // clearly doesn't work, lack of callback
const c3 = new lib.CAsync("c3"); c3.then(console.log); // "cannot read property apply of undefined"
const c4 = ???

我该如何做到最好？更改库签名不是一个好的选择，创建工厂方法也似乎是难看的。

How would I do this best? Changing library signature isn't a good option, creating factory methods also seems to be ugly.

推荐答案

我将从它开始：不要在构造函数中做IO是绑定io和构造在一起的一个坏主意。

I feel strongly about this so I'll start with it: Don't do IO in constructors it's a bad idea to bind io and construction together.

也就是说，如果必须这样做，因为库是不受你的控制，并且可以失去以同步方式构建对象的能力，你可以：

That said, if you must do this because the library is out of your control and are OK with losing the ability to build objects in a sync way, you can:

export class C {
    constructor(value, callback) {
        callback(false, `Hello ${value}`);
    }

    task(value, callback) {
        callback(false, "returned " + value);
    }
}

并且当promisifying时：

And when promisifying:

import Promise from 'bluebird';
import * as lib from './lib';


Promise.promisifyAll(lib);

var old = lib.C; // reference the constructor
lib.C = function(value){ // override it
  o; // object we'll later return, populate in promise constructor
  var p = new Promise(function(resolve, reject){ 
    // the promise constructor is always sync, so the following works
    o = new old(value, function(err, data) {
      if(err) return reject(err);
      resolve(data);   
    });
  }); 
  // THIS IS THE IMPORTANT PART
  o.then = p.then.bind(p); // make the object a thenable, 
  return o
};

这将允许你使用返回值和promise，promise只会有一个然后所以你可能想要 Promise.resolve 它得到一个真正的promise而不是一个对象的属性和 a promise。

Which would let you both use the return value and the promise, the promise will only have a then so you might want to Promise.resolve it to get a "real" promise rather than an object with properties and a promise.

var o = new lib.C(); // get object
o.then(function(data){
    // access data
});

可以提取到以下模式：

 function promisifyConstructor(cons){
   return function(...args) => { // new constructor function
     let o;
     let p = new Promise((resolve, reject) => {
         // delegate arguments
        o = new cons(...args, (err, data) => err ? reject(err) : resolve(data));
     });
     o.then = p.then.bind(p);
     return o;
   }
 }

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