Writing your library in Objective-C is silly if you mean to use it from Swift. The languages are incredibly different (value types, powerful protocols, generics vs. just objects and simple C types), so you'll heavily limit yourself by writing in Objective-C. Nullability annotations don't really make up for that. If you're using Swift, don't limit yourself by writing your components in Objective-C.
You miss his point. It is much easier to consume Objective-C API from Swift then consuming swift API from Objective-C. Even more so if you need the library integrated at a C++ layer of your app.
I didn't miss his point. I disagree with it. Using Objective-C sacrifices so much that is great about using Swift, and the continually-shrinking Objective-C-only developer market doesn't make up for the loss of all of the Swiftyness.
C++ layers are rare for framework authors, so I don't think that the idea of them should be used to dissuade people from writing Swift frameworks. Obviously, if you need to integrate with C++, use a language that can.
There is still a massive, massive amount of enterprise software (see: everything at Apple) that has millions of lines of code in Objective-C. That code will never be rewritten. If you have millions of dollars riding on a framework/app in Objective-C the business needs outweigh any perceived language niceties you get in swift.
This aside - I also disagree with you, Swift is a worse language in a lot of ways, not to mention the extreme compile times and purely unfinished concepts, as well as bugs related to seemingly foundational things like auto-created initializers for structs not getting picked up properly by sourcekit.
If you already have millions of lines of code, "how to write a framework" is not very relevant in your case. You've already written one, obviously you won't rewrite it. The subject is new frameworks, if you're already using Swift in your app (and thus have paid the bundled frameworks price), why limit yourself to Objective-C because some other developers aren't using it yet?
I don't necessarily agree with that - many people/companies build frameworks for primary use in their own work that they open-source as a courtesy - with benefits of course, more eyes make shallow bugs, advertising, etc.
e.g. Github employees probably (originally) wrote ReactiveCocoa and Carthage because they were useful for developing Github apps, not necessarily to gift fantastic tools to the iOS world.
Your code won't be as fast if your Swift types are copy-on-write wrappers around NSObject subclasses, instead of pure-Swift types.
I agree that you'll get the most coverage of potential developers by supporting both languages, but you're already doing the world a service by releasing open-source software. I don't think putting additional demands on maintainers (you must support CocoaPods, you must support Objective-C) is a reasonable request. If a consumer of the framework wants those features, they can submit a pull request (or just maintain the Podspec independently, as is done with ReactiveCocoa).
Performance is the least compelling reason IMO because if it's slow, you should optimize the slow bits. Otherwise it won't even register as a percentage of total time; looking at it this way is just arguing for premature optimization.
Using obj-c from swift is easier than using swift from obj-c in my experience. So if your goal is to distribute the project, then doing it in obj-c has real benefits.
If you use Swift and consume the library in an Objective-C project (which is perfectly possible) you will add the Swift ABI to your project automatically which adds about 10MB to your project. The ABI needs to be added because the ABI isn't stable yet.
For mobile apps that's still a big consideration.
The other way around is much easier, that's why he proposes Swift wrappers around Objective-C code that allow a Swifty interface with your libraries. But often using generics and nullability will already help a lot.
I think the point is distributability. For an internal library, use whatever makes sense. If you want to make it available for all iOS developers, write it in ObjC, because many iOS apps are still using ObjC.
Maybe he had to deal with C/C++ APIs. He does flat out say the two main modern languages are Swift and Objective-C. I know using OpenCV from iOS is a real nightmare for this reason.
C libraries are fine in Swift, but I feel that dealing with a C++ library (and non-CF/system C libraries) is usually a rare exception when "writing an iOS SDK". Most iOS libraries are just written on top of Foundation and the appropriate platform interface kit (if it's a UI library).
