iOS 延时和循环

1.nstimer
可取消

[NSTimer scheduledTimerWithTimeInterval:1/30.0f repeats:true block:^(NSTimer * _Nonnull timer) {
    //代码    
}];

2. dispatch
   多线程延迟，缺点->无法取消

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delayTime * NSEC_PER_SEC), dispatch_get_main_queue(), block);

参考：https://www.jianshu.com/p/6b0a7d4ec1a8