I'm looking for a certification as an "Apple developer" but I don't know if that exists.
I found http://training.apple.com/certification but it only offers certification for OSX and Apple software like FinalCut.
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There is no official certification. There are, though, non-Apple-official courses which are highly reputed, such as Big Nerd Ranch's, for instance.
Apple now offer training with software engineering focused courses for Swift and iOS:
Swift is a powerful and intuitive programming language for macOS, iOS, watchOS, and tvOS. Writing Swift code is interactive and fun, the syntax is concise yet expressive, and Swift includes modern features developers love. Swift code is safe by design, yet also produces software that runs lightning fast.
With regard to Apple certification, they do not offer software engineering specific certification.
only https://certiport.pearsonvue.com/Certifications/Apple/App-Dev-With-Swift/Overview has an official partnership with Apple to certify students on their Swift Programming knowledge and skills.
Their year-long Swift certification program covers high school and college-level knowledge of the programming language, which when attained can be a golden stamp on your iOS developer resume that can lead you to your dream iOS developer role!
I don’t think it’s absolutely necessary but if you’re all about the certifications, then there you go!