Rename `fromCallback` to `factoryFromCallback`

[benlesh]

Since Observable.fromCallback doesn't really create an Observable, but rather creates a function, the name is a little confusing. Observable.factoryFromCallback makes a little more sense, as suggested by @staltz.

Related discussion in #729 is around whether or not it should be in Rx "core". It adds a lot of weight for limited value, IMO. I personally have never used this method, but perhaps it's more popular than I know.

• Should we rename it to factoryFromCallback?
• Should it be core?

[kwonoj]

+1 for renaming, I was also hit confusion first time try to use fromCallback, thought it directly returns observable.

[staltz]

I vote for factoryFromCallback and not in core.

[luisgabriel]

+1 for renaming and should not be in core.

I don't like the new name very much, but I don't have a better suggestion right now.

[mattpodwysocki]

👎 on both renaming and removing it from core. It is essential for both Node programming but also binding to things such as jQuery etc. The from should also stay as is. After all toPromise isn't consistent either but yet I don't see any outrage over that. That and I hate the word Favtory since it sounds too Java-like.

[robwormald]

We could call it observablifyCallback (like Bluebird's "promisify" method) ;-)

[staltz]

@mattpodwysocki fair enough, about core membership. But the name? Is Java-likeness really the problem? Plenty of people have tripped over on fromCallback. The current documentation is really bad, describing it simply as "Converts a callback function to an observable sequence." which is not true.

If not a rename, we could change it to actually return an Observable, not a function. E.g.

var source = Rx.Observable.fromCallback(fs.exists, 'file.txt');

// Can be subscribed
source.subscribe(...)

If anyone needs the factory function, then simply:

function exists(filename) {
  return Rx.Observable.fromCallback(fs.exists, filename);
}

[benlesh]

Well more interesting than that, @staltz, it returns a function that returns a stateful Observable that caches it's result, similar to fromPromise. It's very different from all of its like-named siblings.

[staltz]

Hmm, indeed https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/fromcallback.js#L16
I didn't know about that. What are the problems of implementing fromCallback like I described above (stateless, with Observable.create)?

[benlesh]

No problem other than compatibility concerns, AFAICT.

[staltz]

So, let's keep the name but fix the functionality...

[benlesh]

I'm inclined to agree with @staltz, we should let users compose the behavior they want here, and with consistent tools.

[staltz]

A problem with changing the functionality is the signature starts getting really complicated:

static fromCallback: <T>(callbackFunc: Function, args: any[], ctx?: Object, selector?: Function, scheduler?: Scheduler) => Observable<T>;

args should be an array, so if callbackFunc takes 3 arguments, it would be

Observable.fromCallback(callbackFunc, ['blish', 'blesh', 'blosh'], {value: 42}, function (err, datum) { return datum; }, Scheduler.nextTick);

Should we drop ctx and selector, or do something else?

For instance perhaps

static fromCallback: <T>(callbackFunc: Function, ...argsOrScheduler: Array<any | Scheduler>) => Observable<T>;

Observable.fromCallback(callbackFunc, 'blish', 'blesh', 'blosh', Scheduler.nextTick);

[benlesh]

@staltz ... We wouldn't want to drop selector. It's an important feature for these types of creation methods because it generally saves people from creating a second subscription to map the values they're getting back. I'm unsure about the ctxt argument, but then again, I'm in favor of removing all of the thisArg-type arguments in general.

[mattpodwysocki]

@Blesh context is absolutely necessary, for example, if I were to call a method on a stateful object, it would lose its this immediately for example:

const $book = $('book');

const animate = Rx.Observable.fromCallback($book.animate, $book);
animate({
    opacity: 0.25,
    left: "+=50",
    height: "toggle"
  }, 5000
).subscribe(x => console.log(done))