[WIP] Reimagining how subscribing observers works

[elihart]

@felipecsl I've been brainstorming how to addressthe shortcomings of rxgroups for a while and think I'm onto something decent. This is a working proof of concept, but doesn't have updated tests/styles/comments etc.

Problems this tries to solve:

• Resubscribing happens on background thread so race conditions can cause crashes. Plus the process is slower than having a generated class
• Devs have to manually specify a tag, and that is error prone and extra boilerplate. For airbnb that is usually the request class, and I've seen several times where people put the wrong class.
• Handling multiple observables that share the same observer is messy and complicated
• There is no build time detection for using @AutoResubscribe improperly

The previous tag system required you to specify the tag when creating an observable and when creating an observer. It made sense to me to instead have the observer be the main definition for the tag, and to have that tag be generated by an annotation processor. In the code below I call that class AutoResubscribingObserver. For a 1-1 mapping of observer to observable you just provide that observer field instead of the tag that used to be provided. This makes usage clearer and less error prone.

For a many observables to one observer scenario I set up a two tag system. The first tag is the observer tag. All observables executed with the same observer are grouped under that observer's tag and are keyed again by a second tag that they specify. Our AirRequest already includes a notion of a tag so it should be easy to have that tagging definition exist in the request object and keep the class executing the requests clean.

There's still a lot of work to be done, but the basics are working in the sample app! Let me know what you think.

[elihart]

I obviously changed the interface a bunch. It would be hard to make all of these changes in the best way possible while providing backwards compatibility, so I'm thinking we can release a 1.0.0 version finally and do these breaking API changes then.

[elihart]

Case in point

[felipecsl]

@elihart just had the chance to finally scan though these changes and I like where it's going. It clearly makes sense to get rid of the manual and error-prone tag system we have to day for something more automatic and smart that "just works". We'd have to be able to still manually specify a tag if needed, so we'd need to take that into account. I think this could make for a nice 1.0 release once it's finished 👍

[elihart]

@felipecsl Thanks for looking! I've love to walk through it with you in person some time to outline what changes are needed and talk through some design decisions. Maybe next week sometime? Or I could probably do Friday

[felipecsl]

Friday works for me!

[elihart]

Example of the generated code

public class MainActivity_ObservableResubscriber {
  public MainActivity_ObservableResubscriber(MainActivity target, ObservableGroup group) {
    target.observer.tag = "MainActivity_observer";
    group.resubscribe(target.observer);
  }
}