倒计时线程失败

Question

按我的iOS应用程序中的按钮调用changeLanguage方法。

- (void)changeLanguage:(BOOL) isMyanmar {
    if (hasRun == YES) {
        return;
    }
    hasRun = YES;

    NSUserDefaults * userdefaults = [NSUserDefaults standardUserDefaults];
    [userdefaults setBool:isMyanmar forKey:@"myanmar"];
    [userdefaults synchronize];

    [self updateCurrentLanguageText];
    if ([self isMyanmar]) {
        [self changeLanguageFlag:@"MyanmarFlagBig"];
    } else {
        [self changeLanguageFlag:@"UnitedKingdomFlagBig"];
    }

    [self countDown];
}

- (void)countDown {
    static int i = 3;
    if (i == 3) {
        i--;
        UIImage *three = [UIImage imageNamed:@"Three"];
        countDownFlag = [[UIImageView alloc] initWithImage:three];
        countDownFlag.frame = CGRectMake(0, 370, countDownFlag.frame.size.width, countDownFlag.frame.size.height);
        countDownFlag.center = CGPointMake(width / 2, countDownFlag.center.y);
        [self.view addSubview:countDownFlag];
        [self performSelector:@selector(countDown) withObject:nil afterDelay:0.5];
    } else if (i == 2) {
        i--;
        UIImage *two = [UIImage imageNamed:@"Two"];
        [countDownFlag setImage:two];
        [self performSelector:@selector(countDown) withObject:nil afterDelay:0.5];
    } else if (i == 1) {
        i--;
        UIImage *one = [UIImage imageNamed:@"One"];
        [countDownFlag setImage:one];
        [self performSelector:@selector(countDown) withObject:nil afterDelay:0.5];
    } else {
        [self dismissViewControllerAnimated:YES completion:nil];
    }
}

用于运行良好但最近运行的方法在[self countDown]处遇到线程问题。

可能导致问题的原因是什么？如何解决？

修改

如何调用changeLanguage

两种方法称为此方法：

- (void)changeLanguageToEnglish {
    [self changeLanguage:false];
}

- (void)changeLanguageToMyanmar {
    [self changeLanguage:true];
}

按钮如何调用方法：

[languageButton addTarget:self action:@selector(changeLanguageToEnglish) forControlEvents:UIControlEventTouchUpInside];