2.x: rename emitters #isCancelled to #isDisposed

[Mauin]

Renames Observable/Single/CompletableEmitter#isCancelled to #isDisposed.

This now causes a bit of a weird situation where most (not all) Emitter classes implement Disposable as well, so both interfaces expose the same #isDisposed method. Before the #isCancelled methods were basically just calling #isDisposed or had the same implementation.

Suggestions on how to improve this?

[codecov-io]

Current coverage is 77.91% (diff: 0.00%)

Merging #4490 into 2.x will increase coverage by 0.07%

@@                2.x      #4490   diff @@
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  Lines         33909      33905     -4   
  Methods           0          0          
  Messages          0          0          
  Branches       5325       5325          
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+ Hits          26392      26416    +24   
+ Misses         5547       5517    -30   
- Partials       1970       1972     +2   

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[JakeWharton]

I'm tempted to remove setDisposable from the emitter and keep this method named isCanceled(). If you want to set a disposable why not use unsafeCreate and ObservableSource directly?

[akarnokd]

@JakeWharton these are the safe versions that also manages a resource for you without the need to worry about (and lose) backpressure or cancellation in general from unsafeCreate.

[akarnokd]

Otherwise 👍

[JakeWharton]

I'm not sure how that is an argument against what I said. Having both setCancelable and setDisposable in the API is ugly and redundant.

[akarnokd]

If you believe users don't really need the Disposable overload then I'm open to remove them. Also consider dropping Disposables.from variants not in use and adding Cancellables.from.

[Mauin]

As for setCancelable and setDisposable I agree with @JakeWharton. Would probably be best to somehow merge them or get rid of one to clean the API. But that would be a separate issue/PR, I guess?

[akarnokd]

Yes, let's have a separate PR for that.