iOS单例引发奔溃

单例引发奔溃
dispatch_once(predicate, block); Thread 1: EXC_BREAKPOINT (code=1, subcode=0
代码如下，
首先， 调用了Querulous中的某个方法，[[Querulous sharedInstance] crenellated]，这个时候已经开始调用了通过实例调用了 [instance th], 如果在 [instance th] 还没完成的情况下再调用[Querulous sharedInstance]，就很容易出现卡死的状态，原理可以看看这篇文章滥用单例dispatch_once而造成的死锁问题

+(Querulous *)sharedInstance {
    static Querulous *instance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[Querulous alloc] init];
        instance.level = 3;
        [instance th];
    });
    return instance;
}

- (void)th
{
    NSTimer *timer = [NSTimer timerWithTimeInterval:0 target:self selector:@selector(hill) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
    [timer fire];
    self.timer = timer;
}

- (void)hill
{
    if (self.trainers) {
        [self.timer invalidate];
        if ([self.trainers.allKeys containsObject:@"status"]) {
            
        }
    }
}