用dispatch

1.这是AFN3.0里边的一段代码.利用的是技术信号的形式,等待异步执行的结果,并将结果返回.

- (NSArray *)tasksForKeyPath:(NSString *)keyPath

 {     __block NSArray *tasks = nil;     dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);     [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {         if ([keyPath isEqualToString:NSStringFromSelector(@selector(dataTasks))]) {             tasks = dataTasks;         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(uploadTasks))]) {             tasks = uploadTasks;         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(downloadTasks))]) {             tasks = downloadTasks;         } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(tasks))]) {             tasks = [@[dataTasks, uploadTasks, downloadTasks] valueForKeyPath:@"@unionOfArrays.self"];         }         dispatch_semaphore_signal(semaphore);     }];     dispatch_semaphore_wait(semaphore);     return tasks; }

2.Dispatch Queue没有"取消"这一概念,一旦将处理追加到Dispatch Queue中,就没有办法可将该处理去除,也没有办法可在执行中取消该处理.但是可以使用NSOperationQueue中的取消.

转载于:https://www.cnblogs.com/Sunnyheart/p/6559067.html