Using one subject to propagate completly different event streams through

Question

I need to proxy all the different event streams through one subject.

I came up with this code:

var mySubject,
    getObservable;

getObservable = function (subject, eventName) {
    return subject
        .asObservable()
        .filter(function (x) {
            return x.EventName === eventName;
        })
        .flatMap(function (x) {
            if (x.Type === 'onNext') {
                return Rx.Observable.return(x.Data);
            }

            if (x.Type === 'onError') {
                return Rx.Observable.throw(x.Data);
            }

            return Rx.Observable.empty();
        });
};

mySubject = new Rx.Subject();

getObservable(mySubject, 'foo')
    .subscribe(function(x){ 
        console.log('foo onNext ' + x); 
    }, function(x){ 
        console.log('foo onError ' + x); 
    }, function(){ 
        console.log('foo onComplete');
    });

getObservable(mySubject, 'bar')
    .subscribe(function(x){ 
        console.log('bar onNext ' + x); 
    }, function(x){ 
        console.log('bar onError ' + x); 
    }, function(){ 
        console.log('bar onComplete');
    });

mySubject.onNext({Type: 'onNext', EventName: 'foo', Data: 5});
mySubject.onNext({Type: 'onCompleted', EventName: 'foo'});

mySubject.onNext({Type: 'onNext', EventName: 'bar', Data: 5});
mySubject.onNext({Type: 'onError', EventName: 'bar', Data: 'Error message'});

Got output:

foo onNext 5

bar onNext 5
bar onError Error message

Expected output:

foo onNext 5  
foo onCompleted

bar onNext 5
bar onError Error message

For the bar event, that works like a charm: onNext will be propagated and as soon as the error raises the onError function gets called and the event stream finishes. However, I can't get it to work for the onComplete.

Whenever a complete notification raises I do see that Rx.Observable.empty() gets called but that doesn't cause the subscribers onComplete handler to be called. Instead, its calling its onNext handler.

Answer 1

The getObservable function returns an observable that subscribes to a eventName event sent through a subject.

let getObservable = function (subject, eventName) {
    return Rx.Observable.create(function (observer) {
        subject
            .asObservable()
            .filter(function(x) {
                return x.EventName === eventName;
            })
            .map(function(x) {
                if (x.Type === 'onNext') {
                    observer.onNext(x.Data);
                }

                if (x.Type === 'onError') {
                    observer.onError(x.Data);
                }

                if (x.Type === 'onCompleted') {
                    observer.onCompleted();
                }

                return x;
            })
            .subscribe();
    });
};

This is a working example using data from the original question:

var mySubject,
    getObservable;

getObservable = function (subject, eventName) {
    return Rx.Observable.create(function (observer) {
        subject
            .asObservable()
            .filter(function(x) {
                return x.EventName === eventName;
            })
            .map(function(x) {
                if (x.Type === 'onNext') {
                    observer.onNext(x.Data);
                }

                if (x.Type === 'onError') {
                    observer.onError(x.Data);
                }
                
                if (x.Type === 'onCompleted') {
                    observer.onCompleted();
                }
                
                return x;
            })
            .subscribe();
    });
};

mySubject = new Rx.Subject();

getObservable(mySubject, 'foo')
    .subscribe(function(x){ 
        console.log('SomethingHappened onNext ' + x); 
    }, function(x){ 
        console.log('SomethingHappened onError ' + x); 
    }, function(){ 
        console.log('SomethingHappened onComplete');
    });


getObservable(mySubject, 'bar')
    .subscribe(function(x){ 
        console.log('DataUpdated onNext ' + x); 
    }, function(x){ 
        console.log('DataUpdated onError ' + x); 
    }, function(){ 
        console.log('DataUpdated onComplete');
    });

mySubject.onNext({Type: 'onNext', EventName: 'foo', Data: 5});
mySubject.onNext({Type: 'onCompleted', EventName: 'foo'});

mySubject.onNext({Type: 'onNext', EventName: 'bar', Data: 5});
mySubject.onNext({Type: 'onError', EventName: 'bar', Data: 'Error message'});

<script src='https://rawgit.com/Reactive-Extensions/RxJS/master/dist/rx.all.js'></script>

Answer 2

in .NET Observable.SelectMany uses Observable.Merge to merge streams together to one composite observable. IMHO Observable.Merge completes only when any of the merged observables are completed.

e.g. http://theburningmonk.com/2010/02/rx-framework-iobservable-merge/

Thats could be the reason for issue.