//异步主线程运行，不会产生生死锁
dispatch_sync(dispatch_get_main_queue(), ^{
});
//同步主线程运行，会产生生死锁
dispatch_async(dispatch_get_main_queue(), ^{
});
//从子线程异步返回，网络活动监视器
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[UIApplication sharedApplication].networkActivityIndicatorVisible=YES;
NSURLRequest *webRequest =[NSURLRequest requestWithURL:[NSURL URLWithString:@"https://www.baidu.com"] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20];
[_webView loadRequest:webRequest];
// dispatch_async(dispatch_get_main_queue(), ^{
// });
});
//保证线程只运行一次，用于单例模式
dispatch_once_t oneToken;
dispatch_once(&oneToken, ^{
//alloc init
});
//按照顺序执代码－－之前代码执行完毕之后在执行这个代码
dispatch_barrier_async(dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT), ^{
});
//延迟执行10秒执行Block
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
});
