Slight alteration of Any Matuschak's BlockObservation for MacRuby.

NSObject+ProcObservation.h

//
//  NSObject+ProcObservation.h
//  Version 1.0
//
//  Andy Matuschak
//  andy@andymatuschak.org
//  Public domain because I love you. Let me know how you use it.
//
//  NTW 2009-Oct-21: Added selectors with an options argument.
//  NTW 2009-Oct-30: Transplanted new observation key from MYUtilities's KVUtils.
//  NTW 2010-Jan-06: Changed to accept Ruby proc instead of C block. Got rid of block token typedef so MacRuby isn't confused. See NSObject_ProcObservation.rb for handy Ruby interface.
 
#import <Cocoa/Cocoa.h>

// Or MYKeyValueObservingOptionOnce with your observation options to cause the 
// observer to remove itself upon the first change notification.
// Idea and line of code from MYUtilities.
enum {
    MYKeyValueObservingOptionOnce = 1<<31
};
 
@interface NSObject (AMRubyBlockObservation)

- (NSString *)addObserverForKeyPath:(NSString *)keyPath proc:(id)proc;

- (NSString*)addObserverForKeyPath:(NSString*)keyPath
                           options:(NSKeyValueObservingOptions)options
                              proc:(id)proc;

- (NSString*)addObserverForKeyPath:(NSString*)keyPath
                           onQueue:(NSOperationQueue*)queue
                              proc:(id)proc;

- (NSString*)addObserverForKeyPath:(NSString*)keyPath
                           options:(NSKeyValueObservingOptions)options
                           onQueue:(NSOperationQueue*)queue
                              proc:(id)proc;

- (void)removeObserverWithProcToken:(NSString *)token;

@end

NSObject+ProcObservation.m

//
//  NSObject+ProcObservation.m
//  Version 1.0
//
//  Andy Matuschak
//  andy@andymatuschak.org
//  Public domain because I love you. Let me know how you use it.
//
 
#import "NSObject+ProcObservation.h"
#import <dispatch/dispatch.h>
#import <objc/runtime.h>
#import <macruby/macruby.h>
 
@interface AMRubyObserverTrampoline : NSObject
{
	__weak id observee;
	NSString *keyPath;
	id proc;
	NSOperationQueue *queue;
  NSKeyValueObservingOptions options;
}
 
- (AMRubyObserverTrampoline*)initObservingObject:(id)obj
                                         keyPath:(NSString*)newKeyPath
                                         options:(NSKeyValueObservingOptions)newOptions
                                         onQueue:(NSOperationQueue*)newQueue
                                            proc:(id)newProc;

- (void)cancelObservation;

@end
 
 
@implementation AMRubyObserverTrampoline
 
static NSString *AMRubyObserverTrampolineContext = @"AMRubyObserverTrampolineContext";
 
- (AMRubyObserverTrampoline*)initObservingObject:(id)obj
                                     keyPath:(NSString*)newKeyPath
                                     options:(NSKeyValueObservingOptions)newOptions
                                     onQueue:(NSOperationQueue*)newQueue
                                        proc:(id)newProc
{
  self = [super init];
  if (self != nil)
  {
  	proc = newProc;
  	keyPath = [newKeyPath copy];
  	queue = [newQueue retain];
  	observee = obj;
    options = newOptions;
  	
  	// Clear out our customized options before passing them on.
  	// From MYUtilities.
    newOptions &= ~MYKeyValueObservingOptionOnce;
  	
  	[observee addObserver:self
               forKeyPath:keyPath
                  options:newOptions
                  context:AMRubyObserverTrampolineContext];
	}
	return self;
}
 
- (void)observeValueForKeyPath:(NSString*)aKeyPath
                      ofObject:(id)object
                        change:(NSDictionary*)change
                       context:(void*)context
{
  if (context != AMRubyObserverTrampolineContext)
	{
		[super observeValueForKeyPath:aKeyPath ofObject:object change:change context:context];
    return;
  }
  
	// Not sure if we really need this on the next run loop iteration. Seemed right.
	if (options & MYKeyValueObservingOptionOnce)
    [self performSelector:@selector(cancelObservation) withObject:nil afterDelay:0.0];
  
	if (queue)
		[queue addOperationWithBlock:^{ [proc performRubySelector:@selector(call:) withArguments:object, change]; }];
	else
    [proc performRubySelector:@selector(call:) withArguments:object, change];
}
 
- (void)cancelObservation
{
	[observee removeObserver:self forKeyPath:keyPath];
}
 
- (void)dealloc
{
	[self cancelObservation];
	[proc release];
	[keyPath release];
	[queue release];
	[super dealloc];
}
 
@end
 

static NSString *AMRubyObserverMapKey = @"org.andymatuschak.observerMapRuby";

 
@implementation NSObject (AMRubyBlockObservation)
 
- (NSString *)addObserverForKeyPath:(NSString *)keyPath proc:(id)proc
{
	return [self addObserverForKeyPath:keyPath options:0 onQueue:nil proc:proc];
}

- (NSString*)addObserverForKeyPath:(NSString*)keyPath
                               options:(NSKeyValueObservingOptions)options
                                  proc:(id)proc
{
  return [self addObserverForKeyPath:keyPath options:options onQueue:nil proc:proc];
}

- (NSString*)addObserverForKeyPath:(NSString*)keyPath
                               onQueue:(NSOperationQueue*)queue
                                  proc:(id)proc
{
  return [self addObserverForKeyPath:keyPath options:0 onQueue:queue proc:proc];
}
 
- (NSString*)addObserverForKeyPath:(NSString*)keyPath
                               options:(NSKeyValueObservingOptions)options
                               onQueue:(NSOperationQueue*)queue
                                  proc:(id)proc
{
	NSString *token = [[NSProcessInfo processInfo] globallyUniqueString];
	dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
		if (!objc_getAssociatedObject(self, AMRubyObserverMapKey))
			objc_setAssociatedObject(self, AMRubyObserverMapKey, [NSMutableDictionary dictionary], OBJC_ASSOCIATION_RETAIN);
		AMRubyObserverTrampoline *trampoline = [[[AMRubyObserverTrampoline alloc] initObservingObject:self keyPath:keyPath options:options onQueue:queue proc:proc] autorelease];
		[objc_getAssociatedObject(self, AMRubyObserverMapKey) setObject:trampoline forKey:token];
	});
	return token;
}
 
- (void)removeObserverWithProcToken:(NSString *)token
{
	NSMutableDictionary *observationDictionary = objc_getAssociatedObject(self, AMRubyObserverMapKey);
	AMRubyObserverTrampoline *trampoline = [observationDictionary objectForKey:token];
	if (!trampoline)
	{
		NSLog(@"Tried to remove non-existent observer on %@ for token %@", self, token);
		return;
	}
	[trampoline cancelObservation];
	[observationDictionary removeObjectForKey:token];
}
 
@end

NSObject+ProcObservation.rb

# NSObject+ProcObservation.rb
# Public Domain
#
# Created by Nolan Waite on 10-01-06.
# Copyright 2010 Nolan Waite. All rights reserved.


# Replicated from NSObject+ProcObservation.h
MYKeyValueObservingOptionsOnce = 1<<31

class NSObject
  def observe(keyPath, options=0, &block)
    self.addObserverForKeyPath(keyPath, options:options, proc:block)
  end
  def stopObserving(token)
    self.removeObserverWithProcToken token
  end
end