My codebase contains a lot of Objective-C but I'm slowly replacing it with Swift. I rarely write Objective-C (largely because it's so easy to introduce bugs with release/retain shenanigans) unless I really have to. Interoperability between the languages does work but is fragile, with Xcode often getting into a chicken-and-egg state where a compilation error in Swift prevents the Objective-C from compiling, and vice versa, stuffing the interface with errors that can make it hard to discover where the problem is.(edit: typo)
Klonoar|2 years ago