Backing ObjC class's properties with a dictionary automatically.

DictBack.m

/*
 *  Demo of backing class properties with a dictionary instead of individual
 *  ivars.
 *
 *  http://stackoverflow.com/questions/29158157/
 *
 *  Created by Josh Caswell on 3/19/15.
 *  Released to the public domain on that date.
 */

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

NSString * setterNameForGetterName(NSString * getterName)
{
    NSString * first = [getterName substringToIndex:1];
    NSString * rest = [getterName substringFromIndex:1];
    return [NSString stringWithFormat:@"set%@%@:",
                                            [first uppercaseString], rest];
    
}

NSString * propNameFromSetterName(NSString * setterName)
{
    NSString * first = [setterName substringWithRange:(NSRange){3, 1}];
    NSRange restRange = (NSRange){4, [setterName length]-5};
    NSString * rest = [setterName substringWithRange:restRange];
    return [NSString stringWithFormat:@"%@%@", [first lowercaseString], rest];
}

@interface DictBack : NSObject

/* Sample properties.
 * N.B. That the ivars are _still created_. They're just not used.
 */
@property (strong, nonatomic) NSString * userID;
@property (strong, nonatomic) NSNumber * privilegeLevel;

- (void)readOutAttributes;

@end

@implementation DictBack
{
    NSMutableDictionary * attributes;
}

/* Both of these rely on the accessor names being standard:
 * -set<PropName>: and -<propName>
 */
id redirectedGetter(DictBack * self, SEL _cmd)
{
    NSString * propName = NSStringFromSelector(_cmd);
//    NSLog(@"Getting %@", propName);
    return self->attributes[propName];
}

void redirectedSetter(DictBack * self, SEL _cmd, id val)
{
    NSString * propName = propNameFromSetterName(NSStringFromSelector(_cmd));
//    NSLog(@"Setting %@", propName);
    self->attributes[propName] = val;
}

+ (void)initialize
{
    // Prevent subclasses from being drawn into this crazy scheme.
    if( self != [DictBack class] ) return;
    
    unsigned int numProps;
    // class_copyPropertyList() only returns properties declared in the
    // given class, not anything inherited, so this is safe.
    objc_property_t * props = class_copyPropertyList(self, &numProps);
    for( unsigned int i = 0; i < numProps; i++ ){
        objc_property_t prop = props[i];
        const char * attrs = property_getAttributes(prop);
        NSString * getterName = [NSString stringWithUTF8String:property_getName(prop)];
        class_replaceMethod(self, NSSelectorFromString(getterName),
                            (IMP)redirectedGetter, attrs);
        
        NSString * setterName = setterNameForGetterName(getterName);
        class_replaceMethod(self, NSSelectorFromString(setterName),
                            (IMP)redirectedSetter, attrs);
    }
    
}

- (id)init {
    
    self = [super init];
    if( !self ) return nil;
    
    attributes = [NSMutableDictionary dictionary];
    
    return self;
}

- (void)readOutAttributes
{
    NSLog(@"%@", attributes);
}

@end

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        
        DictBack * d = [DictBack new];
        
        // Set properties; this goes through to the dictionary.
        d.userID = @"Alistair";
        d.privilegeLevel = @7;
        
        [d readOutAttributes];
        
    }
    return 0;
}