Preface

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Enums

In Objective-C, there is no dedicated type for defining string enums. The enums we commonly use are integer-based and auto-increment by +1.

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typedef NS_ENUM(NSInteger, YZAnimationType) {
    YZAnimationTypeDefault = 0,
    YZAnimationType1       = 1,
    YZAnimationType2       = 2,
    YZAnimationType3       = 3,
    YZAnimationType4       = 4,
    YZAnimationTypeCount,
};

C++ has dedicated enum classes, but Objective-C in iOS does not have the equivalent of C++ enum classes. Today let’s learn about an enum type defined using constant string literals.

Let’s create a new class called YZEnumConst.h and write the following code:

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#import <Foundation/Foundation.h>

typedef NSString * const kComponentMessage NS_STRING_ENUM;
FOUNDATION_EXPORT kComponentMessage const kComponentMessageXXXXX;

In YZEnumConst.m, write:

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kComponentMessage const kComponentMessageXXXXX = @"ComponentMessageXXXXX";

This way, a string enum type in Objective-C is constructed using type aliases.

Note: The declaration must be in the .h file and the implementation must be in the .m file; otherwise, you’ll get a symbol-not-found compilation error.

Here you’ll notice the keywords NS_STRING_ENUM and FOUNDATION_EXPORT.

• NS_STRING_ENUM indicates the type is specifically for string enums
• FOUNDATION_EXPORT represents an externally exposed string constant declaration.

Combining the above usage rules, we can look at Apple’s internal definitions—for example, the commonly used timing function constants in animation.

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typedef NSString * CAMediaTimingFunctionName NS_TYPED_ENUM;
CA_EXTERN CAMediaTimingFunctionName const kCAMediaTimingFunctionLinear
    API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
CA_EXTERN CAMediaTimingFunctionName const kCAMediaTimingFunctionEaseIn
    API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
CA_EXTERN CAMediaTimingFunctionName const kCAMediaTimingFunctionEaseOut
    API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
CA_EXTERN CAMediaTimingFunctionName const kCAMediaTimingFunctionEaseInEaseOut
    API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
CA_EXTERN CAMediaTimingFunctionName const kCAMediaTimingFunctionDefault
    API_AVAILABLE(macos(10.6), ios(3.0), watchos(2.0), tvos(9.0));

CA_EXTERN is defined in <CoreGraphics/CGBase.h>

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#ifndef CA_EXTERN
# define CA_EXTERN extern __attribute__((visibility("default")))
#endif

This implementation follows Apple’s timing function string enum pattern.

Summary

Recording valuable knowledge during development allows you to complete work more quickly and improve efficiency when you need it.

Reference: Exploring NS_STRING_ENUM