Objective-C Notification.Name with Swift interoperability

by saiday

NSNotification name has evolved from Objective-C's plain NSString type to a dedicated Notification.Name type, which is succinct and intuitive.

My project is still in the state of mixed Objective-C and Swift.

I accidentally found a LLVM glamour (correct me if I'm wrong!) that handles old Objective-C notification names been used with new Swift NotificationCenter observer names with perfect interoperability.

If your Objective-C notification names are named this style:

// ObjCFile.h
extern NSString *const kSomeNotification;  
// ObjCFile.m
NSString *const kSomeNotification = @"kSomeNotification";





Build your project, you now have a Notification.Name.some type LLVM generated for you: 

NotificationCenter.default.addObserver(self, selector: #selector(someSelector), name: Notification.Name.some, object: nil)



If you follow certain Objective-C Notification name naming convention revealed below, you don't have to change a bit of your codebase and Swift is ready to use all Notification identifiers defined in Objective C.

Objective-C Notification name naming convention

The convention I found out is when Objective-C .h file has a NSString typed field with Notification suffix, LLVM would pick it and generate a corresponding Swift Notification.Name type.

Have you noticed that k prefix from Objective-C field (which is a constant field naming convention used by C developers) is ripped off at the Swift Notification.Name type. This is mesmerized.

The Notification suffix was formally recommended by Apple.

[Name of associated class] + [Did | Will] + [UniquePartOfName] + Notification For example:

NSApplicationDidBecomeActiveNotification NSWindowDidMiniaturizeNotification

NSTextViewDidChangeSelectionNotification

NSColorPanelColorDidChangeNotification

from Naming Properties and Data Types

Formats

The essential rules are NSString typed and Notification suffix, nothing else.

So it could be any of the formats: 

extern NSString *const kSomeNotification;  
NSString *const kSomeNotification;  
NSString *someNotification;