如何等待直到多个EventEmitter实例在Node.js中发出同一事件？

Question

I'm working on a Node.js module/utility which will allow me to scaffold some directories/files. Long story short, right now I have main function which looks something like this:

util.scaffold("FileName")

This "scaffold" method returns an EventEmitter instance, so, when using this method I can do something like this:

util.scaffold("Name")
.on("done", paths => console.log(paths)

In other words, when all the files are created, the event "done" will be emitted with all the paths of the scaffolded files.

Everything good so far.

Right now, I'm trying to do some tests and benchmarks with this method, and I'm trying to find a way to perform some operations (assertions, logs, etc) after this "scaffold" method has been called multiple times with a different "name" argument. For example:

const names = ["Name1", "Name2", "Name3"] 
const emitters = names.map(name => {
return util.scaffold(name)
})

If I was returning a Promise instead of an EventEmitter, I know that I could do something like this:

Promise.all(promises).then(()=> {
//perform assertions, logs, etc
})

However, I'm not sure how can I do the equivalent using EventEmitters. In other words, I need to wait until all these emitters have emitted this same event (ie "done") and then perform another operation.

Any ideas/suggestions how to accomplish this?

Thanks in advance.

Answer 1

With promise.all you have a unique information when "everything" is done. Of course that is when all Promises inside are fullfiled/rejected.

If you have an EventEmitter the information when "everything" is done can not be stored inside your EventEmitter logic because it doesn't know where or how often the event is emmited.

So first solution would be to manage an external state "everything-done" and when this changes to true you perform the other operation.

So like promise.all you have to wrap around it.

The second approach i could imagine is a factory where you build your EventEmitters that keeps track of the instances. Then this factory could provide the information whether all instances have been fired. But this approach could fail on many levels: One Instance->many Calls; One Instance->no Call; ...

just my 5 cent and i would be happy to see another solution

Answer 2

The simplest approach, as mentioned by others, is to return promises instead of EventEmitter instances. However, pursuant to your question, you can write your callback for the done event as follows:

const names = ['Name1', 'Name2', 'Name3']
let count = 0
util.scaffold('Name').on('done', (paths) => {
  count += 1
  if (count < names.length) {
    // There is unfinished scaffolding
  } else {
    // All scaffolding complete
  }
})

Answer 3

I ended up doing what @theGleep suggested and wrapping each of those emitters inside a Promise, like this:

const names = ["Name1", "Name2", "Name3"] 
const promises = names.map(name => {
     return new Promise((resolve) => {
       util.scaffold(name).on("done", paths => {
       resolve(paths)})
  })
})

// and then

Promise.all(promises).then(result => {
    // more operations
})

It seems to be doing what I need so far, so I'll just use this for now. Thanks everyone for your feedback :)