iOS创建安全的单例

创建安全的单例

 #import "Singleton.h"

 @implementation Singleton
 static Singleton* _instance = nil;
 +(instancetype) shareInstance
 {
     static dispatch_once_t onceToken ;
     dispatch_once(&onceToken, ^{
     _instance = [[super allocWithZone:NULL] init] ;
 }) ;
 return _instance ;
 }

 +(id) allocWithZone:(struct _NSZone *)zone
 {
     return [Singleton shareInstance] ;
 }

 -(id) copyWithZone:(struct _NSZone *)zone
 {
     return [Singleton shareInstance] ;
 }
 @end