objc-class-enum

Nicer enums for objective-c with class properties. Or singletons if you will.

A common way of creating more expressive enum-like structures in languages that don't support such constructs is to use static or class properties to encapsulate the enum logic. The idea is to expose specific instances of the object encapsulating the data as singletons. The approach has some merit as it gets rid of the need to have arrays and dictionaries of associated values or to have massive switch statements, since all the data is encapsulated in the perticular instance. Additionally it can make the code that uses such class enums more expressive and easier to read. None of this is particular to Objective-C.

@interface Color : NSObject
@property (class, readonly) Color *red;
@property (class, readonly, weak) Color *green;
@end

The hurdle with this approach, in Objective-C, is that there is a lot of boilerplate code to setup the class properties and to provide additional functionality, such as iterating through the enum members. This can be manageable if there are only a few singletons to setup, but there are frequent cases where the members of such enums number in the tens, and sometimes even in the hundreds. It can get cumbersome to write >50 getters/setters that all look the same and to setup arrays or dictionaries to keep track of mapping various other values to enum members.

This little library aims to get rid of that boilerplate. I wanted to be able to have one function call that would take care of setting up the boilerplate. Luckily this is Objective-C, so we can use the runtime to accomplish just that.

Setting up a Class Enum

+ (void)initialize {
    assert(class_createEnum(self));
}

The class_createEnum function is the only thig you'll need to use. It will dothe following:

• setup getter implementations
• setup setter implementations if the property is readwrite
• setup an all members NSHashTable property that we can use to iterate over the enum mebers.

BOOL class_createEnum(Class cls);

class_createEnum will consider properties that satisfy the following conditions as members of the enum:

1. property is a class property
2. property is @dynamic
3. property type is an instance of the class

@interface Color : NSObject
@property (class) Color *red;
@end

@implementation Color
@dynamic red;
@end

Iterating Memebers

@property (class, readonly) NSHashTable<Color *> *allColors;

Additionally, class_createEnum will setup an all members property if it encounters a property that satisfies the following conditions:

1. property is a class property
2. property is readonly
3. property is @dynamic
4. property type is NSHashTable *
5. property is named one of the following: all, allMembers, allValues, all[ClassName]s (ex. allColors for the Color class)

@interface Color : NSObject
@property (class, readonly) NSHashTable<Color *> *allColors;
@end

@implementation Color
@dynamic allValues;
@end

Debugging

Two additional functions are offered for debugging purposes:

objc_property_t  * class_copyEnumPropertyList(Class cls, unsigned int *outCount)

This is similar to the runtime's class_copyPropertyList except that it returns the properties included in the enum. It requires a prior call to class_createEnum. You must free the pointer returned using free().

id class_getEnumValue(Class cls, objc_property_t property)

Will get the value of the enum member of the class cls, described by property.

Here's an example of debugging using the functions described above.

unsigned int count;
objc_property_t *properties;
if((properties = class_copyEnumPropertyList(cls, &count))) {
    for (unsigned int i = 0; i < count; i++) {
        objc_property_t property = properties[i];
        NSLog(@"%s: %@", property_getName(property), class_getEnumValue(cls, property));
    }
    free(properties);
}

Example

The code in the TestApp is a good example of how to set up such a class, and how to debug it's properties. Check it out at https://github.com/thecatalinstan/objc-class-enum/blob/master/TestApp/main.m.

Here's a minimal example that uses the Color class to structure more pieces of information associated with a color:

@interface Color : NSObject

// Class enum (singletons)
@property (class, nonatomic, strong) Color *red;
@property (class, nonatomic, strong) Color *green;

// Example payload
@property NSColor *rawValue;
@property BOOL allowed;

@end

@implementation Color

@dynamic red, green;

+ (void)initialize {
    assert(class_createEnum(self));
    self.red = [Color initWithColor:NSColor.redColor allowed:NO];
    self.green = [Color initWithColor:NSColor.greenColor allowed:YES];
}

@end