Async support for when, whenError and onRetry

[beroso]

Addresses issue #645 .

In a general way, it add support for assigning async callbacks to when, whenError and onRetry

[kevmoo]

Sadly, we cannot do this without releasing a major version. This is a breaking change.

CC @natebosch

[natebosch]

I do think this looks like a nice change, but we can't do a major bump right now.

If you file an issue we can tag it so that it gets attention once we are ready to push through a major version.

[beroso]

Thanks for the feedback.

Sorry, but I thought it wasn't a breaking change because I'm using FutureOr. The previous tests are passing and also tried to subclass the RetryClient with these changes and it worked.

There is a chance to reconsider this PR, because in my opinion it is backwards compatible?

@kevmoo , @natebosch, could you show me an example of api breaking with these changes? Maybe I'm missing the point.

[natebosch]

Any code with @override of these methods will break. This is non breaking for uses of the client, but it is breaking for implementations of the client. Both are supporting by this package.

[beroso]

@natebosch I don't get it, because I just changed the type of private variables, not methods. The private vars are not exposed by the API. And the implementations constructors continues to working properly.

Could you show a concrete example of breaking?

[natebosch]

Ah, I think @kevmoo and I were mistaken that there were changed parameters on methods instead of constructors.

I do think this is non-breaking. I'll need to investigate whether it has impact on my current approach for #424

[beroso]

Thank you!

[lrhn]

I believe this is practically non-breaking (almost everything is technically breaking, but it requires people to go through some hoops to be broken).
Changes look good to me, so just need @natebosch to be fine too,