Last year, in the run up to WWDC, I published a post called Why I don’t write Swift. Since then, I’ve shipped two major contracting projects, both in Swift, over the last six months. With this newfound experience in small to medium Swift codebases, I have some new observations and comments on writing codebases or large portions thereof in Swift.

It’s been a big year. Besides Swifts 2.0, 2.1, and 2.2, we’ve also seen the open-sourcing of Swift, a roadmap with public timelines, and plenty of open discussion about the future of the language. I’m hopeful about it for sure, and I have little doubt that it’ll be the only language that we write iOS and Mac apps in.

Primarily, the big thing I got wrong last year was that Swift is just plain fun. This weekend, I had the pleasure of helping a friend with code-review. His project is completely in Objective-C, and our code review session reminded me how rough the rough edges of that language are, especially after spending any time in Swift. Listening to myself try to explain the difference between the NSNumber , int , and NSInteger types made me long for Swift. While helping him write new code, I kept forgetting to include semicolons, missed typed arrays greatly, and wish I’d had map and filter handy. Swift has plenty of its own quirks and complexities, but it rounds off so many of Objective-C’s rough edges that writing code in it becomes joyful. I love enums, I love protocols, and I love optionals.

Despite this newfound revelation, I still think the prudent choice is to continue writing apps in Objective-C. Those who are fully on board with Swift commonly reply: every line of Objective-C you write is a legacy. While this is true, Objective-C code will still be viable for at least another five years, and probably won’t stop working for closer to 10 or 15. This is based the fact that every one of Apple’s OSes is written in it, as well as the frameworks themselves. (How long did Carbon code work for? How long before Finder was rewritten in Cocoa?)

However, your Swift 2.2 project is also going to become legacy code, and that transition will happen this September. Your migration will be a bloodbath, and it will have to happen in one fell swoop (including your dependencies!) because of a lack of ABI compatibility. Latent bugs will creep in from the migrator, and you’ll have to keep your swift-3 branch updated with regular, messy merges. In the end, git bisect will stop working, and your compile times will still be awful.

Despite this brutal reality, every client I’ve talked to wants their app in pure Swift. I’m happy to give them my professional advice, which is that it’ll cause more problems that it’ll solve. I don’t fight that hard, though, because I know the industry is moving in a Swifty direction, and there’s not really anything that one person can do about it. When I’m done on the project, the next developer that comes on board would just scoff and overrule any restraint I brought to the project. He or she would probably also curse me while they’re at it.

My post-WWDC takeaway last year was:

My new plan is that once Swift is source compatible with previous versions, the next new app I write after that will be all Swift. I don’t want my code to break when I go back in git, and I don’t want to deal with the context switching of having Objective-C and Swift in one app.

This basically still seems right to me. As clients want it, I’m happy to write Swift. That decision isn’t hard, given that I enjoy it so much, and given that it won’t hurt to keep my skills sharp. My own projects will probably stay in Objective-C, at least until ABI compatibility, and probably until source compatibility.

Unlike last year, we already know what’s coming in Swift 3, and what else is in the pipeline. Despite my short-term hesitance and despite the language’s seemingly slow pace of advancement, I think that Swift in general is on the right track. Getting ABI resilience right is important, and shouldn’t be rushed. The features in the pipeline will solve real pain points that users have (you’ll note I’m currently ignoring the static vs dynamic brouhaha, but I would love to write a post about it soon). The long-term future of Swift is bright, and I can’t wait to get there.