//異步主線程運行，不會產生生死鎖
dispatch_sync(dispatch_get_main_queue(), ^{
});
//同步主線程運行，會產生生死鎖
dispatch_async(dispatch_get_main_queue(), ^{
});
//從子線程異步返回，網絡活動監視器
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(), ^{
//? ? ? ? });
});
//保證線程只運行一次，用于單例模式
dispatch_once_t oneToken;
dispatch_once(&oneToken, ^{
//alloc init
});
//按照順序執代碼－－之前代碼執行完畢之后在執行這個代碼
dispatch_barrier_async(dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT), ^{
});
//延遲執行10秒執行Block
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
});
