事件触发时触发Promise

Question

我的整个项目都使用（Bluebird）Promises，但是有一个使用EventEmitter的特定库。

我想要实现以下目标：

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();
});

我在Promises链中读了EventEmitter的答案。这给了我一种执行’connect’事件的回调的方法。这是我到目前为止所到之处

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
    });
});

现在如何进一步链接“ eventB”？

Answer 1

我假设您想为每个事件做不同的事情。即使eventB由的动作触发connect，您也可以将其视为另一种逻辑流。

旁注：为避免对您和其他必须阅读此代码库的人造成混淆，我建议您不要使用其他方法来补充Promise，除非您 非常 详尽地记录了它们。

从您的示例来看，似乎可以进行以下操作。

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()
})

如果您想简化此常数

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

你可以用这样的东西

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

这将上面的代码解决方案变成

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()
})

并且由于Javascript中的函数捕获了定义它们的范围，因此可以将该代码进一步简化为

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()
})