Singletons 实现

gistfile1.m

Here's what the GCD (and ARC) version looks like:
+ (id)sharedInstance
{
  static dispatch_once_t pred = 0;
  __strong static id _sharedObject = nil;
  dispatch_once(&pred, ^{
    _sharedObject = [[self alloc] init]; // or some other init method
  });
  return _sharedObject;
}

//define at 
+ (id)sharedInstance
{
  DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
    return [[self alloc] init];
  });
}