获取UUID，类似于Microsoft的GUID

- (NSString *)makeUUID{    CFUUIDRef uuidRef = CFUUIDCreate(NULL);    CFStringRef uuidStringRef = CFUUIDCreateString(NULL, uuidRef);    CFRelease(uuidRef);//__bridge: 只是负责两者形式的转换，不涉及内存权限的转移。只涉及在ARC环境    NSString *uuid = [NSString stringWithString:(__bridge NSString*)uuidStringRef];    CFRelease(uuidStringRef);    return uuid;//返回类似于Microsoft的GUID，例如：7B02E2D2-C8DA-4B32-8B1F-AD48B61A18B3}