用dispatch_semaphore_t解决block异步执行返回结果问题

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.