iOS如何判断app是否为第一次启动

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    // Override point for customization after application launch.    if(![[NSUserDefaultsstandardUserDefaults] boolForKey:@"first"]) {        [[NSUserDefaultsstandardUserDefaults] setBool:YESforKey:@"first"];       NSLog(@"first");    }else {       NSLog(@"not first");    }    return YES;}