Send and receive string values via bluetooth android

Question

I'm developing an app where I need to send 3 seekbar's values to a PCB via bluetooth. I've done all the bluetooth code based on the bluetoothchat example. I first modified it to send a string with these 3 values. But now, I need to do something more dificult and i don't know how to do it.

First of all, in the app i modify the seekbars and then i click on the send button. In the code, I need to set a string for each seekbar's value, because I need to access to the MCU variables and set each variable address, value, CRC etc...

So, I need to know the correct way to do this. Here is the code where i define the send function:

/**
 * SEND THREAD
 */
/**[Start Thread + Send command + Nº bytes thread + Nº bytes variable + Address + Variable value + CRC]*/

public void sendValues() {

    /**Set the seekbars values into a string*/
    send_value1 = Integer.toString(savedProgress1);
    send_value2 = Integer.toString(savedProgress2);
    send_value3 = Integer.toString(savedProgress3);



    String message1 = start_thread+" "+send_command+" "+num_byte_trama1+ " "+num_byte_variable+" "+pos_reg_1+" "+Value+" "+CRC;
    String message2 = start_thread+" "+send_command+" "+num_byte_trama1+ " "+num_byte_variable+" "+pos_reg_2+" "+Value+" "+CRC;
    String message3 = start_thread+" "+send_command+" "+num_byte_trama1+ " "+num_byte_variable+" "+pos_reg_3+" "+Value+" "+CRC;
    String message4 = start_thread+" "+send_command+" "+num_byte_trama2+ " "+num_byte_variable+" "+pos_reg_save_request+" "+Value+" "+CRC;
    String message5 = start_thread+" "+send_command+" "+num_byte_trama2+ " "+num_byte_variable+" "+pos_reg_save_status+" "+Value+" "+CRC;



    /**Check that we're actually connected before trying anything*/
    if (GlobalVar.mTransmission.getState() != GlobalVar.STATE_CONNECTED) {
        Toast.makeText(this, R.string.not_connected, Toast.LENGTH_SHORT).show();
        return;
    }

    /**Get the message bytes and tell the Transmission to write*/
    byte[] send = message.getBytes();
    GlobalVar.mTransmission.write(send);

    /**Reset out string buffer to zero*/
    GlobalVar.mOutStringBuffer.setLength(0);
}

There are these few things that I ask you to help me:

1- Need to know how to calculate the CRC
2- I need to send these 5 strings together when pressing the send button. 

In the part where i get the bytes to send, I don't know If the right way to do this would be to add these 5 strings on 1 and send this one (maybe it would be to long if I do this), or to create a function to send these 5 separately but at the same time.

This is the edited code to send each message one by one:

    /**Conversion to decimal of the seekbar's % value*/
    send_int1 = ((savedProgress1 * 20480) / 100) * -1;
    send_int2 = ((savedProgress2 * 20480) / 100) * -1;
    send_int3 = ((savedProgress3 * 20480) / 100) * -1;

    /**Conversion to string of the previous values to send in the string message*/
    sendValue1 = Integer.toString(send_int1);
    sendValue2 = Integer.toString(send_int1);
    sendValue3 = Integer.toString(send_int1);


    String message1 = start_thread+" "+send_command+" "+num_byte_trama1+" "+num_byte_variable+" "+pos_reg_1+" "+sendValue1+" " ;
    String message2 = start_thread+" "+send_command+" "+num_byte_trama1+" "+num_byte_variable+" "+pos_reg_2+" "+sendValue2+" " ;
    String message3 = start_thread+" "+send_command+" "+num_byte_trama1+" "+num_byte_variable+" "+pos_reg_3+" "+sendValue3+" " ;
    String message4 = start_thread+" "+send_command+" "+num_byte_trama2+" "+num_byte_variable+" "+pos_reg_save_request+" " ;
    String message5 = start_thread+" "+send_command+" "+num_byte_trama2+" "+num_byte_variable+" "+pos_reg_save_status+" " ;



    /**Check that we're actually connected before trying anything*/
    if (GlobalVar.mTransmission.getState() != GlobalVar.STATE_CONNECTED) {
        Toast.makeText(this, R.string.not_connected, Toast.LENGTH_SHORT).show();
        return;
    }

    /**Get the message bytes and tell the Transmission to write*/
    byte[] send1 = message1.getBytes();
    GlobalVar.mTransmission.write(send1);

    //Wait untill I receive the confirmation from the MCU

    byte[] send2 = message2.getBytes();
    GlobalVar.mTransmission.write(send2);

    byte[] send3 = message3.getBytes();
    GlobalVar.mTransmission.write(send3);

    byte[] send4 = message4.getBytes();
    GlobalVar.mTransmission.write(send4);

    byte[] send5 = message5.getBytes();
    GlobalVar.mTransmission.write(send5);


    /**Reset out string buffer to zero*/
    GlobalVar.mOutStringBuffer.setLength(0);
}

Answer 1

For your frame, I recommand you to use this kind of frame :

final byte[] HEADER = AA11 // For example // When you want to send a message : Strign messageToSend = new String(HEADER) + yourStringMessage

It'll be easier for you to analyze the frame when you receive it.

Then, for the CRC, I can't answer if you don't tell the kind of CRC. In my app, I used

private static char createCRC(byte[] frame)
{ 
    int crc = 0;
    for(byte i : frame)
    {
        crc = crc^i;
    }
   return (char)crc;
}

to create the CRC by "XORing" each byte of my message , and then check a CRC is quite easy

UPDATE : Well, I finally get it.

In the BluetoothChat activity, you get a string version of message, and the byte[] one. If you want to get the first byte of the message, just add byte myByte = readBuf[0] before String readMessage = new String(readBuf, 0, msg.arg1); Then, String readMessage = new String(myByte, 0, msg.arg1);