[swift-evolution] deployment targets and frameworks

[Jonathan Tang]

+1 to this.  It's really important for getting a sane third-party library
ecosystem working.

I assume that the @available annotation generated would also work for
watchOS, tvOS, etc. frameworks.  How would it work for non-iOS Swift
platforms?  I'm not terribly familiar with how #available works on Linux,
but the versioning problem exists there too.

On Mon, Apr 4, 2016 at 4:48 PM, Drew Crawford via swift-evolution <
swift-evolution at swift.org> wrote:

> Suppose **Apple** ships a framework that is only supported in iOS 9.3.
> As a direct consequence, the framework is only #available in iOS 9.3 or
> later.
>
> Suppose Jane links this framework into her iOS application.  The
> deployment target for her application *can be any value*.  She sets the
> framework to be weakly linked, and as long as the code that uses the Apple
> framework is guarded by a 9.3 availability check, she can deploy back to
> 8.0, 7.0, etc.
>
> Suppose *I* ship a custom framework that I only want to bother supporting
> for iOS 9.3 users.  I'm not testing on old OS, I don't have CI on old OS,
> and quite frankly I have no idea if it works.  And I'm not in the habit of
> shipping code that wasn't even tested on my machine.  As a direct
> consequence, I set my framework deployment target to 9.3.
>
> Now Jane links this framework into her "deployment target 8.0"
> application.  She weakly links it and uses availability checks just like
> she did with the Apple framework.  But this time the compiler says no:
>
> error: module file's minimum deployment target is ios9.3 v9.3
>
>
> Jane now has a set of bad choices:
>
> 1.  She can not use my framework at all
> 2.  She can drop support for <9.3 entirely from her application in order
> to use my framework
> 3.  She can convince me to support iOS 8, when I don't want to invest in
> the QA and test time.
> 4.  She can convince me to set my deployment target to 8, try to find all
> my public APIs, sprinkle `@available(iOS 9.3, *)` everywhere and hope I
> didn't miss any.  Spoiler alert: that's what I did all afternoon.
>
> This is too hard.  IMO Jane should be able to use my "9.3+" frameworks as
> easily as she can use Apple's.
>
> IMO, when Jane imports a "DT 9.3" framework, into her "DT 8.0"
> application, it should A) import successfully, B) link weakly, and C) have
> `@availability(9.3, *)` overlayed on the interface.
>
> There may be some subtle additional details because I don't know exactly
> the implementation of these features, but that's the general idea.
>
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>
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