The singleton pattern is useful for creating objects that are shared across the entire application,
such as an HTTP client or a notification manager,
or objects that may be expensive to create, such as formatters.
+ (instancetype)sharedInstance { static id _sharedInstance = nil; static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ _sharedInstance = [[self alloc] init]; }); return _sharedInstance; }
原文：http://www.cnblogs.com/ficow/p/5375337.html