單例引發(fā)奔潰
dispatch_once(predicate, block); Thread 1: EXC_BREAKPOINT (code=1, subcode=0
代碼如下，
首先， 調(diào)用了Querulous中的某個(gè)方法，[[Querulous sharedInstance] crenellated]，這個(gè)時(shí)候已經(jīng)開(kāi)始調(diào)用了通過(guò)實(shí)例調(diào)用了 [instance th], 如果在 [instance th] 還沒(méi)完成的情況下再調(diào)用[Querulous sharedInstance]，就很容易出現(xiàn)卡死的狀態(tài)，原理可以看看這篇文章濫用單例dispatch_once而造成的死鎖問(wèn)題

+(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"]) {
            
        }
    }
}