Rxswift: Can't bind ObservableType to BehaviorRelay

[biboran]

Short description of the issue:

It's not possible to bind ObservableType to BehaviorRelay as it was possible to bind ObservableType to Variable.

Expected outcome:

Since BehaviorRelay is considered a replacement for Variable, it should offer a hassle free migration. ObservableType should know how to bind itself to BehaviorRelay since that's the intention of bind(to:) method.

Self contained code example that reproduces the issue:

//This doesn't compile
Observable.just(false).bind(to: BehaviorRelay(value: true))
//This still compiles
Observable.just(false).bind(to: Variable(true))

RxSwift/RxCocoa/RxBlocking/RxTest version/commit

4.0

Platform/Environment

• [x] iOS
• [ ] macOS
• [ ] tvOS
• [ ] watchOS
• [ ] playgrounds

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

• [x] easy, 100% repro
• [ ] sometimes, 10%-100%
• [ ] hard, 2% - 10%
• [ ] extremely hard, %0 - 2%

Xcode version:

9.0.1

[kzaher]

Ok, we can add that API.

[dakeshi]

@kzaher Additionally, it needs to give a warning message. ex) Use BehaviorRelay. Variable is deprecated.

[kzaher]

@dakeshi We've for now only put Variable in Deprecated.swift because it's meant to be deprecated in the following version.

We haven't marked it deprecated right now because I'm assuming people will have hard time migrating to 4.0 APIs, and unfortunately I'm assuming that there is a lot of Variable usage.

We wanted to help people for now by just informing them about the deprecation. It is possible we'll move Variable in RxCocoa in the following version and just do typealias Variable = BehaviorRelay.

That would enable newcomers to find relays more easily and we would have some consistency with behavior and publish relays.

We still haven't decided how exactly we'll deprecate Variable, so this is just preparation for that move.

When the exact deprecation path is decided, we'll potentially add deprecation warnings if that API will be removed completely or in case of typealias we'll just perform the move to RxCocoa without deprecation warnings.

[dakeshi]

@kzaher Thanks for your detailed response.

What I feel strange is something which will be deprecated is placed in Deprecated.swift
Your intension isVariable will be deprecated but someone could understand Variable is deprecated.

And how do developers or new comers know Variable will be deprecated?
From Issue? Release page description? or Slack channel?
To make clear and smooth transition, I think a document should be updated to provide this info.
It is very helpful to add how to handle some deprecated methods or your plan to handle this(migration guide would be a good name) you mentioned in docs or README file.

[kzaher]

I can't think of a better way of informing people some API will be deprecated than adding it to a file named Deprecated.swift. People don't read docs a lot as far as I can tell.

When migration path is decided it will get it's availability attribute and become deprecated.

[hmlongco]

If the current thinking is that Variable will be replaced by BehaviorRelay, will BehaviorRelay move to RxSwift from RxCocoa?

This would smooth migration as all of the current files that use Variable import RxSwift and not RxCocoa.