AFNetworking等待网络请求，继续同步操作

dispatch_semaphore_wait()

- (id)sendForUrl:(NSURL *)url {
AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];

manager.completionQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

__block id response;

[manager GET:url.absoluteString parameters:nil success: ^(AFHTTPRequestOperation *operation, id responseObject) {
response = responseObject;
NSLog(@"JSON: %@", responseObject);
dispatch_semaphore_signal(semaphore);
} failure: ^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(@"Error: %@", error);
dispatch_semaphore_signal(semaphore);
}];

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

return response;
}