Declaring String Constants:

If they are specific and internal to a single class, declare them as static const at the top of the .m file, like so:

static NSString *const MyThingNotificationKey = @"MyThingNotificationKey";


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If they pertain to a single class but should be public/used by other classes, declare them as extern in the header and define them in the .m:

//.h
extern NSString *const MyThingNotificationKey;

//.m
NSString *const MyThingNotificationKey = @"MyThingNotificationKey";