NS_ASSUME_NONNULL_BEGIN Macro

Question

It's a convenience macro to save you typing nonnull in your headers. From the Swift blog detailing how new safety features have been incorporated back in Objective-C:

To ease adoption of the new annotations, you can mark certain regions of your Objective-C header files as audited for nullability. Within these regions, any simple pointer type will be assumed to be nonnull.

See Nullability and Objective-C - https://developer.apple.com/swift/blog/?id=25