iOS 子线程下载 主线程刷新UI

dispatch_async(dispatch_get_global_queue(0,0), ^{

//执行耗时的异步操作...

dispatch_async(dispatch_get_main_queue(), ^{

//回到主线程，执行UI刷新操作

});
});

2.异步延时调用

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(.1f * NSEC_PER_SEC)), dispatch_get_global_queue(0,0), ^{
    });