在功能范围之外解决Javascript Promise

我一直在使用ES6 Promise。

通常，Promise是这样构造和使用的

new Promise(function(resolve, reject){
    if (someCondition){
        resolve();
    } else {
        reject();
    } 
});

但是为了灵活性起见，我一直在做以下类似的事情来解决问题。

var outsideResolve;
var outsideReject;
new Promise(function(resolve, reject) { 
    outsideResolve = resolve; 
    outsideReject = reject; 
});

然后

onClick = function(){
    outsideResolve();
}

这可以正常工作，但是有更简单的方法吗？如果不是，这是个好习惯吗？

不，没有其他方法可以执行此操作-我唯一可以说的是这种用例不是很常见。就像费利克斯（Felix）在评论中所说-您所做的将始终如一。

值得一提的是，promise构造器以这种方式运行的原因是抛出安全性-如果在您的代码在promise构造器中运行时未预料到的异常发生，它将变成拒绝，这种形式的throw安全-将抛出的错误转换为拒绝很重要，有助于维护可预测的代码。

出于这种抛出安全的原因，promise构造函数被选择为deferreds（这是允许您执行操作的另一种promise构造方式）-至于最佳实践，我将传递该元素并使用promise构造函数：

var p = new Promise(function(resolve, reject){
    this.onclick = resolve;
}.bind(this));

因此，每当您可以在输出函数中使用promise构造函数时，建议您使用它。只要您可以避免两者都使用-避免同时使用和连锁。

请注意，对于诸如之类的东西if(condition)，绝对不要使用promise构造函数，第一个示例可以写成：

var p = Promise[(someCondition)?"resolve":"reject"]();

简单：

var promiseResolve, promiseReject;

var promise = new Promise(function(resolve, reject){
  promiseResolve = resolve;
  promiseReject = reject;
});

promiseResolve();

在这里参加聚会有点晚，但是另一种方法是使用Deferred对象。您基本上具有相同数量的样板，但是如果您希望将它们传递出去并可能在其定义之外进行解析，这将非常方便。

天真的实现：

class Deferred {
  constructor() {
    this.promise = new Promise((resolve, reject)=> {
      this.reject = reject
      this.resolve = resolve
    })
  }
}

function asyncAction() {
  var dfd = new Deferred()

  setTimeout(()=> {
    dfd.resolve(42)
  }, 500)

  return dfd.promise
}

asyncAction().then(result => {
  console.log(result) // 42
})

ES5版本：

function Deferred() {
  var self = this;
  this.promise = new Promise(function(resolve, reject) {
    self.reject = reject
    self.resolve = resolve
  })
}

function asyncAction() {
  var dfd = new Deferred()

  setTimeout(function() {
    dfd.resolve(42)
  }, 500)

  return dfd.promise
}

asyncAction().then(function(result) {
  console.log(result) // 42
})

我在2015年针对自己的框架提出了一个解决方案。我称这种诺言任务

function createPromise(handler){
  var _resolve, _reject;

  var promise = new Promise(function(resolve, reject){
    _resolve = resolve; 
    _reject = reject;

    handler(resolve, reject);
  })

  promise.resolve = _resolve;
  promise.reject = _reject;

  return promise;
}

var promise = createPromise()
promise.then(function(data){ alert(data) })

promise.resolve(200) // resolve from outside

我喜欢@JonJaques的答案，但我想更进一步。

如果绑定then和catch随后的Deferred对象，那么它完全实现了PromiseAPI，你可以把它当作承诺，await它与这样的。

class DeferredPromise {
  constructor() {
    this._promise = new Promise((resolve, reject) => {
      // assign the resolve and reject functions to `this`
      // making them usable on the class instance
      this.resolve = resolve;
      this.reject = reject;
    });
    // bind `then` and `catch` to implement the same interface as Promise
    this.then = this._promise.then.bind(this._promise);
    this.catch = this._promise.catch.bind(this._promise);
    this[Symbol.toStringTag] = 'Promise';
  }
}

const deferred = new DeferredPromise();
console.log('waiting 2 seconds...');
setTimeout(() => {
  deferred.resolve('whoa!');
}, 2000);

async function someAsyncFunction() {
  const value = await deferred;
  console.log(value);
}

someAsyncFunction();

一个辅助方法将减轻这种额外的开销，并给您同样的jQuery感觉。

function Deferred() {
    let resolve;
    let reject;
    const promise = new Promise((res, rej) => {
        resolve = res;
        reject = rej;
    });
    return { promise, resolve, reject };
}

用法是

const { promise, resolve, reject } = Deferred();
displayConfirmationDialog({
    confirm: resolve,
    cancel: reject
});
return promise;

类似于jQuery

const dfd = $.Deferred();
displayConfirmationDialog({
    confirm: dfd.resolve,
    cancel: dfd.reject
});
return dfd.promise();

尽管在用例中，这种简单的本机语法很好

return new Promise((resolve, reject) => {
    displayConfirmationDialog({
        confirm: resolve,
        cancel: reject
    });
});

我正在使用辅助函数来创建所谓的“统一承诺”-

function flatPromise() {

    let resolve, reject;

    const promise = new Promise((res, rej) => {
      resolve = res;
      reject = rej;
    });

    return { promise, resolve, reject };
}

我正在这样使用它-

function doSomethingAsync() {

    // Get your promise and callbacks
    const { resolve, reject, promise } = flatPromise();

    // Do something amazing...
    setTimeout(() => {
        resolve('done!');
    }, 500);

    // Pass your promise to the world
    return promise;

}

查看完整的工作示例-

function flatPromise() {

    let resolve, reject;

    const promise = new Promise((res, rej) => {
        resolve = res;
        reject = rej;
    });

    return { promise, resolve, reject };
}

function doSomethingAsync() {
    
    // Get your promise and callbacks
    const { resolve, reject, promise } = flatPromise();

    // Do something amazing...
    setTimeout(() => {
        resolve('done!');
    }, 500);

    // Pass your promise to the world
    return promise;
}

(async function run() {

    const result = await doSomethingAsync()
        .catch(err => console.error('rejected with', err));
    console.log(result);

})();

编辑：我创建了一个名为flat-promise的NPM软件包，该代码也可以在GitHub上获得。

您可以将Promise包装在一个类中。

class Deferred {
    constructor(handler) {
        this.promise = new Promise((resolve, reject) => {
            this.reject = reject;
            this.resolve = resolve;
            handler(resolve, reject);
        });

        this.promise.resolve = this.resolve;
        this.promise.reject = this.reject;

        return this.promise;
    }
    promise;
    resolve;
    reject;
}

// How to use.
const promise = new Deferred((resolve, reject) => {
  // Use like normal Promise.
});

promise.resolve(); // Resolve from any context.

这里的许多答案与本文的最后一个示例相似。我正在缓存多个Promises，并且resolve()and reject()函数可以分配给任何变量或属性。结果，我可以使这段代码更加紧凑：

function defer(obj) {
    obj.promise = new Promise((resolve, reject) => {
        obj.resolve = resolve;
        obj.reject  = reject;
    });
}

这是使用此版本的defer()来将FontFace负载承诺与另一个异步过程结合在一起的简化示例：

function onDOMContentLoaded(evt) {
    let all = []; // array of Promises
    glob = {};    // global object used elsewhere
    defer(glob);
    all.push(glob.promise);
    // launch async process with callback = resolveGlob()

    const myFont = new FontFace("myFont", "url(myFont.woff2)");
    document.fonts.add(myFont);
    myFont.load();
    all.push[myFont];
    Promise.all(all).then(() => { runIt(); }, (v) => { alert(v); });
}
//...
function resolveGlob() {
    glob.resolve();
}
function runIt() {} // runs after all promises resolved 

更新：如果要封装对象，有两种选择：

function defer(obj = {}) {
    obj.promise = new Promise((resolve, reject) => {
        obj.resolve = resolve;
        obj.reject  = reject;
    });
    return obj;
}
let deferred = defer();

和

class Deferred {
    constructor() {
        this.promise = new Promise((resolve, reject) => {
            this.resolve = resolve;
            this.reject  = reject;
        });
    }
}
let deferred = new Deferred();

接受的答案是错误的。使用范围和引用非常容易，尽管这可能会使Promise 纯粹主义者感到愤怒：

const createPromise = () => {
    let resolver;
    return [
        new Promise((resolve, reject) => {
            resolver = resolve;
        }),
        resolver,
    ];
};

const [ promise, resolver ] = createPromise();
promise.then(value => console.log(value));
setTimeout(() => resolver('foo'), 1000);

创建诺言时，我们实质上是在获取对resolve函数的引用，然后将其返回，以便可以在外部进行设置。

一秒钟后，控制台将输出：

> foo

是的你可以。通过将CustomEventAPI用于浏览器环境。并在node.js环境中使用事件发射器项目。由于问题中的代码段是针对浏览器环境的，因此这里是一个可行的示例。

function myPromiseReturningFunction(){
  return new Promise(resolve => {
    window.addEventListener("myCustomEvent", (event) => {
       resolve(event.detail);
    }) 
  })
}


myPromiseReturningFunction().then(result => {
   alert(result)
})

document.getElementById("p").addEventListener("click", () => {
   window.dispatchEvent(new CustomEvent("myCustomEvent", {detail : "It works!"}))
})

<p id="p"> Click me </p>

我希望这个答案是有用的！

我们的解决方案是使用闭包存储解析/拒绝函数，并附加一个函数来扩展promise本身。

这是模式：

function getPromise() {

    var _resolve, _reject;

    var promise = new Promise((resolve, reject) => {
        _reject = reject;
        _resolve = resolve;
    });

    promise.resolve_ex = (value) => {
       _resolve(value);
    };

    promise.reject_ex = (value) => {
       _reject(value);
    };

    return promise;
}

并使用它：

var promise = getPromise();

promise.then(value => {
    console.info('The promise has been fulfilled: ' + value);
});

promise.resolve_ex('hello');  
// or the reject version 
//promise.reject_ex('goodbye');

我发现自己在某些情况下也缺少Deferred模式。您始终可以在ES6 Promise的顶部创建一个：

export default class Deferred<T> {
    private _resolve: (value: T) => void = () => {};
    private _reject: (value: T) => void = () => {};

    private _promise: Promise<T> = new Promise<T>((resolve, reject) => {
        this._reject = reject;
        this._resolve = resolve;
    })

    public get promise(): Promise<T> {
        return this._promise;
    }

    public resolve(value: T) {
        this._resolve(value);
    }

    public reject(value: T) {
        this._reject(value);
    }
}

感谢在此主题中发布的每个人。我创建了一个模块，其中包括前面所述的Defer（）对象以及在其上构建的其他一些对象。它们都利用Promises和简洁的Promise回调语法在程序内实现通信/事件处理。

• 推迟：可以远程解决的承诺失败（在主体外部）
• 延迟：在给定时间后自动解决的承诺
• 超时：在给定时间后自动失败的承诺。
• 周期：可重触发的Promise语法来管理事件

• 队列：基于Promise链接的执行队列。

  rp = require("repeatable-promise")

  https://github.com/CABrouwers/repeatable-promise

我为此写了一个小库。https://www.npmjs.com/package/@inf3rno/promise.exposed

我用的是工厂方法的方法别人写的，但我推翻了then，catch，finally方法也一样，这样你就可以解决这些以及原来的承诺。

在没有外部执行者的情况下解决Promise：

const promise = Promise.exposed().then(console.log);
promise.resolve("This should show up in the console.");

从外部与执行者的setTimeout赛跑：

const promise = Promise.exposed(function (resolve, reject){
    setTimeout(function (){
        resolve("I almost fell asleep.")
    }, 100000);
}).then(console.log);

setTimeout(function (){
    promise.resolve("I don't want to wait that much.");
}, 100);

如果您不想污染全局名称空间，则有一个无冲突模式：

const createExposedPromise = require("@inf3rno/promise.exposed/noConflict");
const promise = createExposedPromise().then(console.log);
promise.resolve("This should show up in the console.");

我制作了一个名为的库manual-promise，该库可以代替Promise。Promise由于使用代理或包装器，此处的其他答案都不能作为替代的替代品。

yarn add manual-promise

npn install manual-promise

import { ManualPromise } from "manual-promise";

const prom = new ManualPromise();

prom.resolve(2);

// actions can still be run inside the promise
const prom2 = new ManualPromise((resolve, reject) => {
    // ... code
});


new ManualPromise() instanceof Promise === true

https://github.com/zpxp/manual-promise#readme

如何创建劫持拒绝并返回的函数？

function createRejectablePromise(handler) {
  let _reject;

  const promise = new Promise((resolve, reject) => {
    _reject = reject;

    handler(resolve, reject);
  })

  promise.reject = _reject;
  return promise;
}

// Usage
const { reject } = createRejectablePromise((resolve) => {
  setTimeout(() => {
    console.log('resolved')
    resolve();
  }, 2000)

});

reject();

我已经汇总了完成该工作的要点：https : //gist.github.com/thiagoh/c24310b562d50a14f3e7602a82b4ef13

使用方法如下：

import ExternalizedPromiseCreator from '../externalized-promise';

describe('ExternalizedPromise', () => {
  let fn: jest.Mock;
  let deferredFn: jest.Mock;
  let neverCalledFn: jest.Mock;
  beforeEach(() => {
    fn = jest.fn();
    deferredFn = jest.fn();
    neverCalledFn = jest.fn();
  });

  it('resolve should resolve the promise', done => {
    const externalizedPromise = ExternalizedPromiseCreator.create(() => fn());

    externalizedPromise
      .promise
      .then(() => deferredFn())
      .catch(() => neverCalledFn())
      .then(() => {
        expect(deferredFn).toHaveBeenCalled();
        expect(neverCalledFn).not.toHaveBeenCalled();
        done();
      });

    expect(fn).toHaveBeenCalled();
    expect(neverCalledFn).not.toHaveBeenCalled();
    expect(deferredFn).not.toHaveBeenCalled();

    externalizedPromise.resolve();
  });
  ...
});

首先在浏览器或节点上启用--allow-natives-syntax

const p = new Promise(function(resolve, reject){
    if (someCondition){
        resolve();
    } else {
        reject();
    } 
});

onClick = function () {
    %ResolvePromise(p, value)
}

另一个从外部解决Promise的解决方案

 class Lock {
        #lock;  // Promise to be resolved (on  release)
        release;  // Release lock
        id;  // Id of lock
        constructor(id) {
            this.id = id
            this.#lock = new Promise((resolve) => {
                this.release = () => {
                    if (resolve) {
                        resolve()
                    } else {
                        Promise.resolve()
                    }
                }
            })
        }
        get() { return this.#lock }
    }

用法

let lock = new Lock(... some id ...);
...
lock.get().then(()=>{console.log('resolved/released')})
lock.release()  // Excpected 'resolved/released'