24.单例的严谨写法

@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