In the lead-up to WWDC, you could follow the rumor mill — or you could learn Swift and get more out of the conference.
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Like any advanced college course, Stanford suggests you familiarize yourself with C programming via the Programming Abstractions track as a prerequisite, and complete a Programming Paradigms course as well.
The course is free, and you can subscribe to it any time as classes remain available long after they land in iTunes. While it’s definitely a good lead-in to WWDC, neither the conference of Swift is going anywhere, so you’ve got time.