Dynamic User Defaults

Demo.m

@interface NSUserDefaults (idk)
@property (nonatomic, copy) NSString *jam;
@end

@implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)notification {
    [NSUserDefaults standardUserDefaults].jam = @"strawberry";
    NSLog(@"%@", [NSUserDefaults standardUserDefaults].jam);
}
@end

@implementation NSUserDefaults (idk)
@dynamic jam; // so we can do this

/* // instead of this
- (NSString *)jam {
    return [self objectForKey:@"jam"];
}

- (void)setJam:(NSString *)jam {
    [self setObject:jam forKey:@"jam"];
}
 */
@end

HackingSwizzlingAndProxyingOhMy.m

#import "HackingSwizzlingAndProxyingOhMy.h"

#import <objc/runtime.h>

@interface NSUserDefaults (hsp_lol)
+ (instancetype)hsp_instance;
@end

@implementation NSUserDefaults (hsp_lol)
+ (void)load {
    Method old = class_getClassMethod([NSUserDefaults class], @selector(standardUserDefaults));
    Method new = class_getClassMethod([HackingSwizzlingAndProxyingOhMy class], @selector(hsp_instance));

    method_exchangeImplementations(old, new);
}

+ (instancetype)hsp_instance {
    static HackingSwizzlingAndProxyingOhMy *instance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[HackingSwizzlingAndProxyingOhMy alloc] init];
    });
    return instance;
}
@end

@implementation HackingSwizzlingAndProxyingOhMy
- (BOOL)_isGetter:(SEL)sel {
    NSString *s = NSStringFromSelector(sel);
    if ([s rangeOfString:@":"].location != NSNotFound) {
        return NO;
    }

    return YES;
}

- (BOOL)_isSetter:(SEL)sel {
    NSString *s = NSStringFromSelector(sel);
    NSRange range = [s rangeOfString:@":"];
    if (![s hasPrefix:@"set"] || [s rangeOfString:@":"].location == NSNotFound) {
        return NO;
    }

    s = [s substringFromIndex:NSMaxRange(range)];
    range = [s rangeOfString:@":"];
    if (range.location != NSNotFound) {
        return NO;
    }

    return YES;
}

#pragma mark -

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    NSMethodSignature *signature = [super methodSignatureForSelector:sel];
    if (signature) {
        return signature;
    }

    if ([self _isGetter:sel]) {
        return [self methodSignatureForSelector:@selector(objectForKey:)];
    }

    if ([self _isSetter:sel]) {
        return [self methodSignatureForSelector:@selector(setObject:forKey:)];
    }

    return nil;
}

- (void)forwardInvocation:(NSInvocation *)inv {
    SEL sel = inv.selector;

    if ([self _isGetter:sel]) {
        inv.selector = @selector(objectForKey:);

        id arg = NSStringFromSelector(sel);
        [inv setArgument:&arg atIndex:2];
        [inv retainArguments];
    } else if ([self _isSetter:sel]) {
        inv.selector = @selector(setObject:forKey:);

        NSString *key = NSStringFromSelector(sel);
        key = [[key substringFromIndex:3] substringToIndex:key.length - 4];

        NSString *initial = [[key substringToIndex:1] lowercaseString];
        key = [key stringByReplacingCharactersInRange:NSMakeRange(0, 1) withString:initial];

        [inv setArgument:&key atIndex:3];
        [inv retainArguments];
    } else {
        return [super forwardInvocation:inv];
    }

    [inv invoke];
}
@end

HackingSwizzlingProxyingOhMy.h

#import <Foundation/Foundation.h>

@interface HackingSwizzlingAndProxyingOhMy : NSUserDefaults
@end