I wrote an application in Objective-c

I'm finishing up my first Swift 3 project, a macOS app. The language is well designed, and it took a lot of the ergonomics of Objective C and made them safe. Clearly, Apple learned a lot in Swift 1 and 2, and it is a quite a piece of work.

That said, this is a challenging environment to code in. No attempt at backwards compatibility was made in either the libraries or the language itself. The difference between Swift 2 and 3 is at least 100 times larger than the Python 2.7 to 3 transition everybody complains about. This means that for most of the examples you'll find, they will be in Swift 2, Swift 1, or Objective C; o copy/paste here. Many APIs in the Cocoa and Foundation API are simply not documented other than the parameter names.

Additionally there's "automatic bridging" which can easily convert between new library type and old library types, so there's a nest of transitions (along with the typical inheritance hierarchy for each type) that one must navigate to accomplish even simple things.

However, Swift is clearly in Apple's future plans, so you might as well start learning it now if you have to code for their ecosystem.