Swift version: 5.1
Swift 3.0 introduced the biggest changes to the language since it was released, and it’s 100% guaranteed to cause your current code to break unless you had written some extraordinarily trivial apps.
I went over the changes in more detail in my article What’s new in Swift 3.0?, but here are the headline changes:
completion: nilmuch any more.
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Available from iOS 7.0
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.