http://www.raywenderlich.com/95181/whats-new-in-swift-1-2


Dealing with optionals
If you’ve been following all the changes to Swift until now, you’ve seen arguments change from UIView to UIView! to UIView? and back again. We were spoiled with being able to message nil in Objective-C, but Swift is much stricter about things.
If you’re maintaining Objective-C code, there are some new qualifiers for you to use when specifying the type for arguments, variables, properties, etc.:
nonnull – will never be nil
nullable – can be nil
null_unspecified – unknown whether it can be nil or not (the current default)
For example, consider how these Objective-C declarations would map to Swift:
nonnull NSString *string – a regular object, String
nullable NSString *string – this is an optional, String?
null_unspecified NSString *string – unknown, thus implicitly unwrapped, String!
If you don’t have Objective-C code to maintain, you’ll still benefit from Apple adding these qualifiers to the Cocoa headers. That will make your Swift experience that much cleaner with fewer implicitly-unwrapped values.