使用具有块参数的NSURLConnection异步URL请求

Question

我正在尝试正确使用NSURLConnection的sendAsynchronousRequest方法，但我遇到了阻塞参数问题。我在调用之上构建了块，并使用__block变量来尝试检索其中收集的数据。我已经确认块内的数据是正确的，因此块工作正常，使得块数据的检索成为问题。我在方法结束时的赋值和返回语句没有被赋予正确的值，我认为这是因为在分配发生之前块尚未完成执行。如何在执行分配之前“等待”直到块执行完毕而不中断主线程？

- (NSData *) sendAsynchronousURLRequest: (NSMutableURLRequest *)request withResponse: (NSURLResponse *)response andError:(NSError *)error
{
    // Prepare the block handler.
    __block NSURLResponse *retrievedBlockResponse;
    __block NSError *retrievedBlockError;
    __block NSData *retrivedBlockData;
    void(^requestHandler)(NSURLResponse*, NSData*, NSError*) =
    ^(NSURLResponse *blockResponse, NSData *blockData, NSError *blockError)
    {
        retrievedBlockResponse = blockResponse;
        retrivedBlockData = blockData;
        retrievedBlockError = blockError;

        // NOTE: Correct data is here...
        NSLog(@"Data Inside Block:\n%@", [NSMutableDictionary DictionaryFromJSONData:blockData errorReport:nil]);
    };

    // Send the asynchronous request (operationQueue initialized elsewhere in class).
    [NSURLConnection sendAsynchronousRequest:request queue:operationQueue completionHandler:requestHandler];

    // NOTE: Reference to data cannot be established here!
    // Establish references to block variables and return NSData object.
    response = retrievedBlockResponse;
    error = retrievedBlockError;
    return retrivedBlockData;
}