model.m

% cat model.m
#import <Foundation/Foundation.h>
#import <dispatch/dispatch.h>

void Init_model() {};

@interface Model : NSObject {
  //int result;
  NSString *result;
  dispatch_group_t group;
}

@end

@implementation Model

- (id)init {
  if (self = [super init]) {
    //result = 0;
    result = [NSString string];
    group = NULL;
  }
  return self;
}

- (void)dealloc {
  if (group != NULL) {
    dispatch_release(group);
  }
  [result release];
  [super dealloc];
}

- (void)preloadResult {
  group = dispatch_group_create();
  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
  dispatch_group_async(group, queue, ^{
    NSLog(@"Async dispatch!");
    sleep(2);
    //result = 42;
    result = [NSString stringWithString:@"banana!"];
  });
}

//- (int)result {
- (NSString *)result {
  if (group != NULL) {
    NSLog(@"Waiting till preloading finishes...");
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    dispatch_release(group);
    group = NULL;
  } else {
    NSLog(@"Not preloaded!");
    sleep(2);
    //result = 42;
    result = [NSString stringWithString:@"banana!"];
  }
  return result;
}

@end

// $ gcc -bundle -fobjc-gc -framework Foundation model.m -o model.bundle
// macruby -r model.bundle -e 'm = Model.new.preloadResult; sleep 1; p m.result'