杂记：解决Android扫描BLE设备名称不刷新问题

背景

个人开发过一种BLE设备有这样一种需求：当设备处于状态A时，广播设备名称A；处于状态B时，广播设备名称B。

问题

我们发现，当Android在进行Ble扫描的时候，扫描回调函数onScanResult中获取的设备名称并未随设备实时改变。

但是当使用nrfConnect进行扫描时，却发现设备广播名称确实是实时改变的。

原因

我们使用如下代码获取BLE设备名称，然而此时device.getName()中的名称是系统缓存的数据，而非实时的广播数据，要想获取实时的设备名称，必须自行解码实时广播数据。

   private ScanCallback mScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            
            BluetoothDevice device = result.getDevice();
            if(null != device && null != device.getName()) {
                 Log.d("test", device.getName()); // not the real name
            }
        }


        @Override
        public void onScanFailed(int errorCode) {
              super.onScanFailed(errorCode);
        }

    };

解决

通过分析BLE广播包协议，自行解码出设备名称。

     private ScanCallback mScanCallback = new ScanCallback() {
          @Override
          public void onScanResult(int callbackType, ScanResult result) {
              
              BluetoothDevice device = result.getDevice();
              if(null != device && null != device.getScanRecord().getBytes()) {
                   byte[] scanRecord = device.getScanRecord().getBytes(); // advertised data
                   String realName = parseDeviceName(scanRecord);
              }
          }
 
 
         @Override
         public void onScanFailed(int errorCode) {
               super.onScanFailed(errorCode);
         }
 
     };

    // return name String(successful) or null(failed)
      public static String parseDeviceName(byte[] scanRecord) {
        String ret = null;
        if(null == scanRecord) {
            return ret;
        }

        ByteBuffer buffer = ByteBuffer.wrap(scanRecord).order(ByteOrder.LITTLE_ENDIAN);
        while (buffer.remaining() > 2) {
            byte length = buffer.get();
            if (length == 0)
                break;

            byte type = buffer.get();
            length -= 1;
            switch (type) {
                case 0x01: // Flags
                    buffer.get(); // flags
                    length--;
                    break;
                case 0x02: // Partial list of 16-bit UUIDs
                case 0x03: // Complete list of 16-bit UUIDs
                case 0x14: // List of 16-bit Service Solicitation UUIDs
                    while (length >= 2) {
                        buffer.getShort();
                        length -= 2;
                    }
                    break;
                case 0x04: // Partial list of 32 bit service UUIDs
                case 0x05: // Complete list of 32 bit service UUIDs
                    while (length >= 4) {
                        buffer.getInt();
                        length -= 4;
                    }
                    break;
                case 0x06: // Partial list of 128-bit UUIDs
                case 0x07: // Complete list of 128-bit UUIDs
                case 0x15: // List of 128-bit Service Solicitation UUIDs
                    while (length >= 16) {
                        long lsb = buffer.getLong();
                        long msb = buffer.getLong();
                        length -= 16;
                    }
                    break;
                case 0x08: // Short local device name
                case 0x09: // Complete local device name
                    byte sb[] = new byte[length];
                    buffer.get(sb, 0, length);
                    length = 0;
                    ret = new String(sb).trim();
                    return ret;
                case (byte) 0xFF: // Manufacturer Specific Data
                    buffer.getShort();
                    length -= 2;
                    break;
                default: // skip
                    break;
            }
            if (length > 0) {
                buffer.position(buffer.position() + length);
            }
        }
        return ret;
    }

参考资料：

https://www.race604.com/ble-advertising/