- (void)viewDidLoad {

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{

[selftestLock];

});

}

#define semaphore_signal 0

- (void)testLock {

#if semaphore_signal

dispatch_semaphore_t sem = dispatch_semaphore_create(0);

#else

dispatch_group_t group =dispatch_group_create();

#endif

NSLog(@"begin");

#if semaphore_signal

#else

dispatch_group_enter(group);

#endif

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{

NSLog(@"loading....");

[imageView setImageWithURL:[NSURL URLWithString:@"http://www.pptbz.com/pptpic/UploadFiles_6909/201110/20111014111307895.jpg"] placeholder:nil options:0 completion:^(UIImage*image,NSURL*url,YYWebImageFromType from,YYWebImageStage stage,NSError*error) {

NSLog(@"loading finish ... ");

#if semaphore_signal

dispatch_semaphore_signal(sem);

#else

dispatch_group_leave(group);

#endif

}];

});

#if semaphore_signal

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

#else

dispatch_group_wait(group,DISPATCH_TIME_FOREVER);

#endif

NSLog(@"end");

}

2016-08-05 16:38:51.616 TableViewControllerDemo[16337:2393186] begin

2016-08-05 16:38:51.617 TableViewControllerDemo[16337:2393186] loading....

2016-08-05 16:38:51.703 TableViewControllerDemo[16337:2392956] loading finish ...

2016-08-05 16:38:51.704 TableViewControllerDemo[16337:2393186] end