23-写个单例

利用GCD构建

DataCenter.h

#import

@interface DataCenter:NSObject

+(id)sharedDataCenter;

@end

DataCenter.m

#import “DataCenter.h”

@implementation DataCenter

+(id)sharedDataCenter{

          static DataCenter *sharedDataCenter=nil;

          static dispatch_once_t token;

          dispatch_once(&token, ^{

                     sharedDataCenter = [[self alloc]init];

           });

       return sharedDataCenter;

}