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我正在尝试使用套接字和流向另一台设备发送消息，使用此处的代码：

http://mobileorchard.com/tutorial-networking-and-bonjour-on-iphone/

这工作正常，我可以从一个设备发送一条消息，让另一台设备接收它并毫无问题地阅读它。对于较大的消息，包括诸如 NSData 之类的内容，它会在发送消息时冻结我的 UI，因此我想在后台线程上运行它。我的代码看起来一样，除了实现了后台线程：

dispatch_queue_t queue = dispatch_queue_create("com.App.SendMessage", NULL);
    dispatch_async(queue, ^{

    NSMutableDictionary *mutable = [[NSMutableDictionary alloc] init];

    [mutable setObject:@"Test" forKey:@"TestKey"];

    [self.server sendMessage:mutable toService:service];

});

dispatch_release(queue);

消息代码如下所示：

  NSData* rawPacket = [NSKeyedArchiver archivedDataWithRootObject:packet];

  // Write header: lengh of raw packet
  int packetLength = [rawPacket length];
  [outgoingDataBuffer appendBytes:&packetLength length:sizeof(int)];

  // Write body: encoded NSDictionary
  [outgoingDataBuffer appendData:rawPacket];

  // Try to write to stream
  [self writeOutgoingBufferToStream];



- (void)writeOutgoingBufferToStream {
  // Is connection open?
  if ( !readStreamOpen || !writeStreamOpen ) {
    // No, wait until everything is operational before pushing data through
    return;
  }

  // Do we have anything to write?
  if ( [outgoingDataBuffer length] == 0 ) {
    return;
  }

  // Can stream take any data in?
  if ( !CFWriteStreamCanAcceptBytes(writeStream) ) { 
    return;
  }

  // Write as much as we can
  CFIndex writtenBytes = CFWriteStreamWrite(writeStream, [outgoingDataBuffer bytes], [outgoingDataBuffer length]);

  if ( writtenBytes == -1 ) {
    // Error occurred. Close everything up.
    [self close];
    [delegate connectionTerminated:self];
    return;
  }

  NSRange range = {0, writtenBytes};
  [outgoingDataBuffer replaceBytesInRange:range withBytes:NULL length:0];
}

它因错误而崩溃：

*** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[NSConcreteMutableData replaceBytesInRange:withBytes:length:]: range {0, 1855} exceeds data length 0'

有人知道为什么这可能会在后台线程上崩溃吗？

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