如何在 Xamarin 表单中为 BLE 拆分和发送 >20 字节的数据？答案

【问题标题】：How to split and send data >20 bytes for BLE in Xamarin forms?如何在 Xamarin 表单中为 BLE 拆分和发送 >20 字节的数据？
【发布时间】：2020-12-19 01:21:30
【问题描述】：

我正在尝试在 Xamarin 表单中实现 BLE。我想发送大于 20 字节的数据。我已经看到使用 java 在本机 android 中的实现。如下图所示

private void sendMessage(BluetoothGattCharacteristic characteristic, String CHARACTERS){
        byte[] initial_packet = new byte[3];
        /**
         * Indicate byte
         */
        initial_packet[0] = BLE.INITIAL_MESSAGE_PACKET;
        if (Long.valueOf(
                String.valueOf(CHARACTERS.length() + initial_packet.length))
                > BLE.DEFAULT_BYTES_VIA_BLE) {
            sendingContinuePacket(characteristic, initial_packet, CHARACTERS);
        } else {
            sendingLastPacket(characteristic, initial_packet, CHARACTERS);
        }
    }

private void sendingContinuePacket(BluetoothGattCharacteristic characteristic,
            byte[] initial_packet, String CHARACTERS){
        /**
         * TODO If data length > Default data can sent via BLE : 20 bytes
         */
        // Check the data length is large how many times with Default Data (BLE)
        int times = Byte.valueOf(String.valueOf(
                CHARACTERS.length() / BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET));

        Log.i(TAG, "CHARACTERS.length() " + CHARACTERS.length());
        Log.i(TAG, "times " + times);

        // TODO
        // 100 : Success
        // 101 : Error
        byte[] sending_continue_hex = new byte[BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET];
        for (int time = 0; time <= times; time++) {
            /**
             * Wait second before sending continue packet 
             */
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            if (time == times) {
                Log.i(TAG, "LAST PACKET ");

                /**
                 * If you do not have enough characters to send continue packet,
                 * This is the last packet that will be sent to the band
                 */

                /**
                 * Packet length byte :
                 */
                /**
                 * Length of last packet
                 */
                int character_length = CHARACTERS.length()
                        - BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET*times;

                initial_packet[1] = Byte.valueOf(String.valueOf(character_length
                        + BLE.INITIAL_MESSAGE_PACKET_LENGTH));
                initial_packet[2] = BLE.SENDING_LAST_PACKET;

                Log.i(TAG, "character_length " + character_length);

                /**
                 * Message
                 */
                // Hex file
                byte[] sending_last_hex = new byte[character_length];

                // Hex file : Get next bytes
                for (int i = 0; i < sending_last_hex.length; i++) {
                    sending_last_hex[i] = 
                            CHARACTERS.getBytes()[sending_continue_hex.length*time + i];
                }

                // Merge byte[]
                byte[] last_packet = 
                        new byte[character_length + BLE.INITIAL_MESSAGE_PACKET_LENGTH];
                System.arraycopy(initial_packet, 0, last_packet,
                        0, initial_packet.length);
                System.arraycopy(sending_last_hex, 0, last_packet, 
                        initial_packet.length, sending_last_hex.length);

                // Set value for characteristic
                characteristic.setValue(last_packet);
            } else {
                Log.i(TAG, "CONTINUE PACKET ");
                /**
                 * If you have enough characters to send continue packet,
                 * This is the continue packet that will be sent to the band
                 */
                /**
                 * Packet length byte
                 */
                int character_length = sending_continue_hex.length;

                /**
                 * TODO Default Length : 20 Bytes
                 */
                initial_packet[1] = Byte.valueOf(String.valueOf(
                        character_length + BLE.INITIAL_MESSAGE_PACKET_LENGTH));

                /**
                 * If sent data length > 20 bytes (Default : BLE allow send 20 bytes one time)
                 * -> set 01 : continue sending next packet
                 * else or if after sent until data length < 20 bytes
                 * -> set 00 : last packet
                 */
                initial_packet[2] = BLE.SENDING_CONTINUE_PACKET;
                /**
                 * Message
                 */
                // Hex file : Get first 17 bytes
                for (int i = 0; i < sending_continue_hex.length; i++) {
                    Log.i(TAG, "Send stt : " 
                            + (sending_continue_hex.length*time + i));

                    // Get next bytes
                    sending_continue_hex[i] = 
                            CHARACTERS.getBytes()[sending_continue_hex.length*time + i];
                }

                // Merge byte[]
                byte[] sending_continue_packet = 
                        new byte[character_length + BLE.INITIAL_MESSAGE_PACKET_LENGTH];
                System.arraycopy(initial_packet, 0, sending_continue_packet, 
                        0, initial_packet.length);
                System.arraycopy(sending_continue_hex, 0, sending_continue_packet, 
                        initial_packet.length, sending_continue_hex.length);

                // Set value for characteristic
                characteristic.setValue(sending_continue_packet);
            }

            // Write characteristic via BLE
            mBluetoothGatt.writeCharacteristic(characteristic);
        }
    }

public boolean writeCharacteristic(BluetoothGattCharacteristic characteristic,
            String data) {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return false;
        }

        if (ActivityBLEController.IS_FIRST_TIME) {
            /**
             * In the first time, 
             * should send the Title
             */
            byte[] merge_title = sendTitle(data);

            // Set value for characteristic
            characteristic.setValue(merge_title);

            // Write characteristic via BLE
            mBluetoothGatt.writeCharacteristic(characteristic);

            // Reset
            ActivityBLEController.IS_FIRST_TIME = false;

            return true;
        } else {
            /**
             * In the second time, 
             * should send the Message
             */
            if (data.length() <= BLE.LIMIT_CHARACTERS) {
                sendMessage(characteristic, data);

                // Reset
                ActivityBLEController.IS_FIRST_TIME = true; 

                return true;
            } else {
                // Typed character
                typed_character = data.length();

                return false;
            }
        }
    }

这是我的 Xamarin 代码，我在其中编写操作

 private async Task<string> ReadAndWriteCharacterisicsValue(ICharacteristic _characteristics)
        {
            if (_characteristics != null)
            {
                var sbnew = new StringBuilder("BLE Characteristics\n");
                byte[] senddata = Encoding.UTF8.GetBytes(string.IsNullOrEmpty("") ? "Hi1290004847846767627723676" : "");

                if (MainThread.IsMainThread)
                {
                    string writeTypes = _characteristics.WriteType.ToString();
                    await _characteristics.WriteAsync(senddata);

                }

                //_characteristics.ReadAsync();


                var charVal = _characteristics.Value;

                var str = Encoding.UTF8.GetString(charVal);
                sbnew.AppendLine($"Characteristics found on this device: {string.Join(", ", str.ToString())}");

                return sbnew.ToString();


            }
            return null;
        }

当我尝试发送字符串 Hi1290004847846767627723676(27 字节) 时，我在外围设备没有崩溃得到 Hi129000484784676762(20 字节)。我正在使用 plugin.ble 最新版本，而且我的蓝牙设备版本是 5.0。 我什至尝试请求 Mtu。如下代码所示

 if (_characteristics != null)
            {
                try
                {

                    var sbnew = new StringBuilder("BLE Characteristics\n");
                     byte[] senddata = Encoding.UTF8.GetBytes(string.IsNullOrEmpty("") ? "Start{'command':'UnSelectEnhancement','data':[{'UnSelectEnhancement':'VitaminC'},{'UnSelectEnhancement':'CitricAcid'},  {'UnSelectEnhancement':'Electolytes'},{'UnSelectEnhancement':'Sweetener'}]}End":"");
                   
                    await _device.RequestMtuAsync(2000);
                  

                    
                    if (MainThread.IsMainThread)
                    {
                        string writeTypes = _characteristics.WriteType.ToString();
                         await _characteristics.WriteAsync(senddata);

                    }

我想发送最大 200 字节的数据。我还尝试使用此代码拆分字符串。

IEnumerable<string> s = str.Split();
                    IEnumerable<string> Split( )
                    {
                        while (!string.IsNullOrWhiteSpace(str))
                        {
                            var chunk = str.Take(size).ToArray();
                           str = str.Substring(chunk.Length);
                           yield return new string(chunk);

                       }

                    }

                    
                    Console.WriteLine("Error of split");
                    
                    Console.WriteLine(String.Join(Environment.NewLine, str));
                    Console.WriteLine(String.Join(Environment.NewLine, str));

但它不起作用。对我来说它看起来很复杂。 有什么简单的方法可以在 xamarin 表单中使用 c# 对大于 20 字节的数据进行写入操作？

【问题讨论】：

您使用的是哪个版本的 BLE？ 4.2+ 允许在一条消息中发送超过 20 个字节
插件应该至少支持 4.2 的所有功能，如果不是更多的话。 BLE 版本取决于您设备的 BLE 版本。所有现代智能手机都至少支持 v4.2。要使用 v4.2 的功能，外设也需要支持 4.2。如果是这种情况，您应该能够要求更高的 MTU 来发送更大的消息
这可能是 MTU 设置为 20 造成的。我快速浏览了一下，但在 Plugin.BLE 中没有发现任何允许请求更高 MTU 的内容。
我明白了。将 MTU 设置为更高的值可以在不拆分数据的情况下解决该问题。
我只能假设 2000 太大了。尝试一些现实的东西，比如 251，BLE 4.2 的最大值。如果这不起作用，我就没有更多线索了

标签： java c# android xamarin.forms bluetooth-lowenergy

【解决方案1】：

您可以尝试以多个块的形式发送数据，每个块的最大大小为 20。此示例发送第一个 20 个字节的块，然后发送另一个带有剩余 7 个字节的块。

            byte[] senddata = Encoding.ASCII.GetBytes("Hi1290004847846767627723676");
            int start = 0;
            while (start < senddata.Length)
            {
                int chunkLength = Math.Min(20, senddata.Length - start);
                byte[] chunk = new byte[chunkLength];
                Array.Copy(senddata, start, chunk, 0, chunkLength);
                await writeBuf.WriteAsync(chunk);
                start += 20;
            }

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