Safe keypaths without macros!

Example.m

NSString *safe = self.keys.url.port.stringValue;
NSString *unsafe = @"url.port.stringValue";
assert([safe isEqualToString:unsafe]);

SafeKeypaths.h

@interface NSObject (SafeKeypaths)
+ (instancetype)keys;
- (instancetype)keys;
@end

SafeKeypaths.m

@interface JSKeypathRecorder : NSString {
    NSMutableString *_path;
}
- (void)appendPathComponent:(NSString *)component;
@end


@implementation NSObject (SafeKeypaths)
+ (instancetype)keys { return [JSKeypathRecorder new]; }
- (instancetype)keys { return self.class.keys; }
@end


@implementation JSKeypathRecorder {
  NSMutableString *_path;
}

- (id)init {
  if ((self = [super init]))
    _path = [NSMutableString new];
  
  return self;
}


#pragma mark - NSString Primitives

- (NSUInteger)length {
  return [_path length];
}

- (unichar)characterAtIndex:(NSUInteger)index {
  return [_path characterAtIndex:index];
}


#pragma mark - Auxiliary

- (void)appendPathComponent:(NSString *)component {
  [_path appendFormat:@"%@%@", _path.length > 0 ? @"." : @"",component];
}

#pragma mark - Invocation Recording

- (void)forwardInvocation:(NSInvocation *)anInvocation {
  [self appendPathComponent:NSStringFromSelector(anInvocation.selector)];
  anInvocation.returnValue = (id __strong*)(&self);
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
  NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
  if (signature)
    return signature;
  
  // if we don't already have a signature, let's use one that is the 
  // same as all the messages we intend to accept
  return [NSNumber instanceMethodSignatureForSelector:@selector(stringValue)];
}

@end