NSTimer

1.在子线程添加NSTimer

NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(next) userInfo:nil repeats:YES];
    
    NSLog(@"主线1");
    
    dispatch_async(dispatch_queue_create("123", DISPATCH_QUEUE_SERIAL), ^{
        NSLog(@"分1");
        [[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
        
        //注意:一定要加这句，不然不会进行循环
        [[NSRunLoop currentRunLoop] run];
        
        //这个不会运行,[[NSRunLoop currentRunLoop] run]后面的代码读不会执行
        NSLog(@"分2");
    });
    
    NSLog(@"主线2");

- (void)next
{
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"---");
    });
}

2.或者NSRunLoopCommonModes,这样在滑动的时候定时器照样运行

[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];