当事件触发时触发 Promise

Question

我的整个项目都使用 (Bluebird) Promises，但有一个特定的库使用 EventEmitter。

我想实现这样的目标：

Promise.on('connect', function() {
    x.doSomething();
}).then(function() {
    return new Promise(function(resolve) {
        y.doAction(resolve); // this will result in `eventB` getting emitted
    });
}).on('eventB', function() {
    z.handleEventB();
}).then(function() {
    z.doSomethingElse();
});

我阅读了 EventEmitter in the middle of a chain of Promises 的答案。这为我提供了一种执行 'connect' 事件回调的方法。这是我到目前为止的进展

var p = new Promise(function(resolve) {
    emitter.on('connect', resolve);
});
p.on = function() {
    emitter.on.apply(emitter, arguments);
    return p;
};
p.on('connect', function() {
    x.doSomething();
}).then(function() {
    return new Promise(function(resolve) {
        y.doAction(resolve); // this will result in eventB getting emitted
    });
});

现在如何为 'eventB' 进一步链接？

Answer 1

我假设您想为每个事件做不同的事情链。即使 eventB 是由 connect 的动作触发的，您也可以将其视为另一个逻辑流。

旁注：为了避免让您和任何其他必须阅读此代码库的人感到困惑，我建议您不要使用其他方法来补充承诺，除非您非常[=33] =] 彻底记录它们。

从您的示例来看，以下内容似乎可行。

var Promise = require( 'bluebird' ) var emitter = someEmitter() var connected = new Promise( function( resolve ){ emitter.on( 'connect', resolve ) }) var eventBHappened = new Promise( function( resolve ){ emitter.on( 'eventB', resolve ) }) connected.then( function(){ return x.doSomething() }).then( function(){ return y.doSomethingElse() // will trigger `eventB` eventually }) // this promise stream will begin once `eventB` has been triggered eventBHappened.then( function(){ return z.doSomething() })

如果您想简化这个常量

var p = new Promise( function( resolve ){ emitter.on( 'something', resolve ) })

你可以使用这样的东西

function waitForEvent( emitter, eventType ){ return new Promise( function( resolve ){ emitter.on( eventType, resolve ) }) }

将上面的代码解变成

var Promise = require( 'bluebird' ) var emitter = someEmitter() function waitForEvent( eventEmitter, eventType ){ return new Promise( function( resolve ){ eventEmitter.on( eventType, resolve ) }) } waitForEvent( emitter, 'connect' ).then( function(){ return x.doSomething() }).then( function(){ return y.doSomethingElse() // will trigger `eventB` eventually }) // this promise stream will begin once `eventB` has been triggered waitForEvent( emitter, 'eventB' ).then( function(){ return z.doSomething() })

并且因为 Javascript 中的函数捕获了它们被定义的范围，所以这段代码可以进一步简化为

var Promise = require( 'bluebird' ) var emitter = someEmitter() function waitForEvent( type ){ return new Promise( function( resolve ){ //emitter has been captured from line #2 emitter.on( type, resolve ) }) } waitForEvent( 'connect' ).then( function(){ return x.doSomething() }).then( function(){ return y.doSomethingElse() // will trigger `eventB` eventually }) // this promise stream will begin once `eventB` has been triggered waitForEvent( 'eventB' ).then( function(){ return z.doSomething() })

Answer 2

我遇到了同样的问题并编写了一个小型承诺包装库 (controlled-promise)，它允许承诺事件发射器。您的示例的解决方案是：

const Promise = require('bluebird');
const ControlledPromise = require('controlled-promise');

const emitter = someEmitter();
const waiting = new ControlledPromise();

function waitForEvent(type) {
    return waiting.call(() => {
       emitter.once(type, event => waiting.resolve(event));
    });
}

waitForEvent('connect')
    .then(() => x.doSomething())
    .then(() => waitForEvent('eventB'))
    .then(() => z.doSomethingElse());

这种方法的好处：

现有承诺待处理时自动return
轻松访问 resolve() / reject() 回调

当事件触发时触发 Promise

Trigger Promise when an event fires

node.js

promise

bluebird