将异步转换为Rx.js

Question

So, we are trying to rewrite our express server into Rx . It is currently using async for all stream operations. The code looks like the following:

var async = require('async');

function getCountAndChannels(name, cb){
    var tasks = [
        function(cb) {
             //does a mongoDB search and returns count
        },
        function(cb) {
            //does a findOne mongoDB search and returns 
        }
    ];
    async.parallel(tasks, cb);
}

router.get('data', function(req, res) { //router is the express router
    var recorders = req.query.recorders.split(',');

    async.map(recorders, function(name, cb) {
        getCountAndChannels(name, cb);
    }, function(err, countsAndChannels) {
        if(err) throw err;

        // here countsAndChannels is an array with first element the count
        // and second element the document.

        // do other async stuff based on the results

        res.status(200).json('send some calculations');
});

The thing here I have to do is loop over the array of recorders and for each one calculate the two mongoDB searches. I have tried using Rx.Observable.merge which doesn't return the results in an array but in 2 different calls of the callback. So, then I tried Rx.Observable.zip which I believe is what I'm looking for.

The problem is I don't know how to loop over the recorders and send the result when all operations are finished. Because a simple forEach loop will throw a Cannot set headers after they are sent error.

This is what I have so far:

recorders.forEach(recorder => {        
    Rx.Observable.zip([
        search1,
        search2
    ]).subscribe(
        (countsAndChannels) => {                
            // do stuff
            res.send('the results');
        },
        err => res.status(500).json(err),
        () => res.send('OK')
    );  
});

Haven't used Rx before, so any help is appreciated.

Answer 1

It might be easier to convert your list of recorders to an Observable stream, then flatMap over each recorder (ie perform your async processing), then call toArray to store all the results into an array:

var recorder$ = Rx.Observable.from(recorders);
var countsAndChannels$ = recorder$
    .flatMap(performAsyncTask);

// allResults$ will emit once all of the async work is complete
var allResults$= countsAndChannels$.toArray();

allResults$.subscribe(results => {
    // Send response to client;
});