Add fromSharedPreferencesChanges Observable

[Yarikx]

This is wrapper around SharedPreferences.OnSharedPreferenceChangeListener
It's pretty simple and straightforward, but is useful to observe SharedPreferences changes.

[nsk-mironov]

I usually use OnSharedPreferenceChangeListener to observe only one particular key. Maybe we should also provide an overload that observes the specified key and emits its value? Something like:

Observable<T> fromSharedPreferencesChanges(SharedPreferences preferences, String key, Function2<SharedPreferences, String, T> extractor);

Observable<Integer> observable = fromSharedPreferencesChanges(preferences, "key_to_observer", new Function2<SharedPreferences, String, Integer> () {
    @Override
    public Integer call(SharedPreferences preferences, String key) {
      return preferences.getInt(key, 0);
    }
};

Anyway, LGTM.

[Yarikx]

@mironov-nsk I was thinking about it too. Actually I also use modified version of this code that listen only for particular key.
But such approach can be simply achieved via .filter operator.
So it's better to ship more general sollution that can be adapted to particular case.
But anyway we can add one more method to AndroidObservables that will combine this operator with .filter

[mttkay]

just a style remark: the indentation is off, and we should always use curly braces even for one liners. It makes it more readable.

[mttkay]

Actually, is this check necessary? RxJava makes a best effort at unsubscribing, e.g. when the sequence completes or fails. In general it shouldn't be necessary to check a subscription before emitting a notification

[Yarikx]

About subscriber.isUnsubscribed() check:
I was just following instructions from https://github.com/ReactiveX/RxJava/wiki/Implementing-Your-Own-Operators#other-considerations.

Sometimes I'm not sure should I check for it or not.

[mttkay]

👍 except for the remark above

Let me know if or how you want to address it, then I'll land it

[Yarikx]

And one more point I want to discuss:
Should it be named as 'fromSharedPreferencesChanges' ?
I assume that number of operators will increase, so we need to make convention about names and location of new operators.
Is static method of AndroidObservables is good place for it?
Is methods with prefix 'from...' is good names for operators?

[mttkay]

Good questions. I think it's something we have to find out with time. That's exactly why I want the project to remain in pre-release status for a while, since I anticipate there to be breaking API changes still

I'll merge this, but maybe you can open a question issue where we can gather feedback around this, and if necessary, rename things in one fell swoop.