iOS5 Singleton Pattern

WWDC2011ビデオ　323 Intro to ARC のスライドから

Use dispatch_once for thread-safety

 @implementation ActivityIndicator
 + (ActivityIndicator *)sharedIndicator {
   static ActivityIndicator *sharedIndicator;
   static dispatch_once_t done;
   dispatch_once(&done, ^{ sharedIndicator = [ActivityIndicator new]; });
   return sharedIndicator;
 }
 - (void)show {...}
 - (void)hide {...}
 @end