iOS protobuf GCDAsyncSocket

Question

我正在编写一个小应用程序，它需要通过 tcp 将 protobuf 消息发送到 netty 服务器，其中使用了 CocoaAsyncSocket/GCDAsyncSocket。

为此，我使用以下代码：

Message_LoginMessageRequest_Builder *msg_tmp_login_build=[Message_LoginMessageRequest builder];
[msg_tmp_login_build setPhoneNumber:input_phone.text];
[msg_tmp_login_build setEmail:input_email.text];
[msg_tmp_login_build setPassword:input_password.text];

Message_Builder *msg_tmp_build=[Message builder];
[msg_tmp_build setType:Message_MessageTypeLoginReq];
[msg_tmp_build setLoginRequest:msg_tmp_login_build.build];

// send the message to the socket: we need to code the stream first
Message *msg=[msg_tmp_build build];
NSMutableData *msg_data=[[NSMutableData alloc] initWithCapacity:[[msg data] length]];
PBCodedOutputStream *coded_output_stream=[PBCodedOutputStream streamWithData:msg_data];
[msg writeToCodedOutputStream:coded_output_stream];
[socket writeData:msg_data withTimeout:-1 tag:Message_MessageTypeLoginReq];

但是，代码总是在 writeToCodedOutputStream 中收到“out of space”错误，详细的跟踪信息是：writeToCodedOutputStream/writeEnum/writeTag/writeRawVariant32/writeRawByte/flush。

有什么帮助吗？谢谢！

Answer 1

我自己已经找到了答案，通过阅读 protobuf 的 netty 编解码器的代码，可以在 netty/codec/src/main/java/io/netty/handler/codec/protobuf/ 中找到该代码

在netty protobuf中，必须在消息体之前插入一个描述protobuf消息长度的消息头（后面参考消息体）。消息体的长度必须通过 RawVariant32 类型编码。更多细节请参考 ProtobufVariant32FrameDecoder.java 中的示例，下面也引用了该示例：

/**
 * A decoder that splits the received {@link ByteBuf}s dynamically by the
 * value of the Google Protocol Buffers
 * <a href="http://code.google.com/apis/protocolbuffers/docs/encoding.html#varints">Base
 * 128 Varints</a> integer length field in the message.  For example:
 * <pre>
 * BEFORE DECODE (302 bytes)       AFTER DECODE (300 bytes)
 * +--------+---------------+      +---------------+
 * | Length | Protobuf Data |----->| Protobuf Data |
 * | 0xAC02 |  (300 bytes)  |      |  (300 bytes)  |
 * +--------+---------------+      +---------------+
 * </pre>
 *
 * @see CodedInputStream
 */
public class ProtobufVarint32FrameDecoder extends ByteToMessageDecoder {

    // TODO maxFrameLength + safe skip + fail-fast option
    //      (just like LengthFieldBasedFrameDecoder)

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        in.markReaderIndex();
        final byte[] buf = new byte[5];
        for (int i = 0; i < buf.length; i ++) {
            if (!in.isReadable()) {
                in.resetReaderIndex();
                return;
            }

            buf[i] = in.readByte();
            if (buf[i] >= 0) {
                int length = CodedInputStream.newInstance(buf, 0, i + 1).readRawVarint32();
                if (length < 0) {
                    throw new CorruptedFrameException("negative length: " + length);
                }

                if (in.readableBytes() < length) {
                    in.resetReaderIndex();
                    return;
                } else {
                    out.add(in.readBytes(length));
                    return;
                }
            }
        }

        // Couldn't find the byte whose MSB is off.
        throw new CorruptedFrameException("length wider than 32-bit");
    }
}

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