It seems to have been used internally for some time. The public WWDC app was written in Swift, and many Apple engineers had moved to using Swift entirely by the

It seems to have been used internally for some time. The public WWDC app was written in Swift, and many Apple engineers had moved to using Swift entirely by the time it was announced. The most glaring problems exist in the IDE support for the language, not the language itself. There is minuscule risk of Swift not gaining widespread adoption amongst iOS and OS X developers.