AppDelegate.rb

class AppDelegate
  
  def applicationDidFinishLaunching(a_notification)
    foo = FooBar.new
    
    # call Objective-C method from MacRuby which calls a MacRuby method.
    foo.getBarFromRuby
    # call a second Objective-C method from MacRuby which calls a MacRuby 
    # method returning a Hash
    foo.getBarContentFromRuby
  end
end

FooBar.h

#import <Foundation/Foundation.h>

@interface FooBar : NSObject
{
  // the Ruby class instance. Has to be a "general" data type.
  id _rubyInstance;
}

// Constructor that inits a Ruby instance
- (id) init;
// calls Ruby method;
- (void) getBarFromRuby;
// calls another Ruby method which returns a Hash
- (void) getBarContentFromRuby;

@end

FooBar.m

#import "FooBar.h"

@implementation FooBar

- (id) init {
  self = [super init];
  if (self) {
    // initialize the Ruby instance from its class.
    _rubyInstance = [[NSClassFromString(@"Mixbar") alloc] init];
  }
  return self;
}

- (void) getBarFromRuby {
  NSLog(@"The drink we got from Ruby: %@", [_rubyInstance drink]); // Xcode did not know about #drink.
}

// A Ruby Hash is a NSDictionary in Objective-C. 
- (void) getBarContentFromRuby {
  NSDictionary *dict = [_rubyInstance content];
  for (id key in dict) {
    NSLog(@"key: %@, value: %@ \n", key, [dict objectForKey:key]);
  } 
}

@end

log.txt

2012-12-27 12:51:35.909 Mix[9880:503] The drink we got from Ruby: Whisky
2012-12-27 12:51:35.919 Mix[9880:503] key: first, value: Brandy, Whisky 
2012-12-27 12:51:35.920 Mix[9880:503] key: second, value: Cola

Mixbar.rb

class Mixbar

  def drink
    "Whisky"
  end

  def content
    # I don't like the new Hash syntax. If you're not using a Symbol as key
    # you have to use the rocket arrow. So, why not using them all time?
    {:first => "Brandy, Whisky", :second => "Cola"}
  end

end