iOS 如何获得app的版本和系统的版本

- (NSString *)systemVersion{

    if(_systemVersion==nil)

    {

        _systemVersion = [[UIDevice currentDevice] systemVersion];

    }

    return _systemVersion;

}

- (NSString *)appVersion

{

    if(_appVersion==nil)

    {

        NSString *mainVersion   = [[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleShortVersionString"];

        NSString *minVersion    = [[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleVersion"];

        

        _appVersion = [NSString stringWithFormat:@"%@.%@", mainVersion, minVersion];

    }

    return _appVersion;

}