NullabilityMacros.h

// Xcode 6.3 defines new language features to declare nullability
#if __has_feature(nullability)
#define PSPDF_ASSUME_NONNULL_BEGIN _Pragma("clang assume_nonnull begin")
#define PSPDF_ASSUME_NONNULL_END _Pragma("clang assume_nonnull end")
#define ps_nullable nullable
#define ps_nonnull nonnull
#define ps_null_unspecified null_unspecified
#define ps_null_resettable null_resettable
#define __ps_nullable __nullable
#define __ps_nonnull __nonnull
#define __ps_null_unspecified __null_unspecified
#else
#define PSPDF_ASSUME_NONNULL_BEGIN
#define PSPDF_ASSUME_NONNULL_END
#define ps_nullable
#define ps_nonnull
#define ps_null_unspecified
#define ps_null_resettable
#define __ps_nullable
#define __ps_nonnull
#define __ps_null_unspecified
#endif

should look back to the original gist

#if !__has_feature(nullability)
#define NS_ASSUME_NONNULL_BEGIN
#define NS_ASSUME_NONNULL_END
#define nullable
#define nonnull
#define null_unspecified
#define null_resettable
#define __nullable
#define __nonnull
#define __null_unspecified
#endif