Can't Understand mHandler.obtainMessage() in Android Bluetooth sample

Question

I'm Working on Bluetooth rfcomm connection. There's a line in Android Sample that I can't understand and unfortunately I couldn't find a good answer in other questions and resources.

Here is the whole code:

public void run() {
        byte[] buffer = new byte[1024];  // buffer store for the stream
        int bytes; // bytes returned from read()

        // Keep listening to the InputStream until an exception occurs
        while (true) {
            try {
                // Read from the InputStream
                bytes = mmInStream.read(buffer);
                // Send the obtained bytes to the UI activity
                mHandler.obtainMessage(MESSAGE_READ, bytes, -1, buffer)
                        .sendToTarget();
            } catch (IOException e) {
                break;
            }
        }
    }

    /* Call this from the main activity to send data to the remote device */
    public void write(byte[] bytes) {
        try {
            mmOutStream.write(bytes);
        } catch (IOException e) { }
    }

I can't Understand this line:

 // Read from the InputStream
                    bytes = mmInStream.read(buffer);    
// Send the obtained bytes to the UI activity
                        mHandler.obtainMessage(MESSAGE_READ, bytes, -1, buffer)
                                .sendToTarget();

mHandler is not defined in this code and also MESSAGE_READ

I can't understand what does bytes do?

I think and as mentioned in comment it sends the received Bytes to the Activity which I set as my Main Activity. Can I make a Static TextView in my main Activity instead of sendToTarget() to show the received message?

Solution

The main goal of Handler is to provide interface between producer and consumer thread, here, between UI thread and worker thread. Implementation of Handler goes into consumer thread.

In your case, you want to communicate MESSAGE_READ between threads.

Without handler you can do nothing out of your main Activity Thread.

Therefore, look for mHandler initiation into main Activity.

The default handler init should be like:

Handler mHandler = new Handler(){
 @Override
    public void handleMessage(Message msg) {
 /**/
  }
};

If you use Eclipse, click on your Project -> Ctrl+H -> File Search -> "Handler".

Or in Notepad++ -> Serch -> Find in files ....

[EDIT]

final int MESSAGE_READ = 9999; // its only identifier to tell to handler what to do with data you passed through.  

// Handler in DataTransferActivity
public Handler mHandler = new Handler() {
public void handleMessage(Message msg) {
  switch (msg.what) {
    case SOCKET_CONNECTED: {
      mBluetoothConnection = (ConnectionThread) msg.obj;
      if (!mServerMode)
        mBluetoothConnection.write("this is a message".getBytes());
      break;
    }
    case DATA_RECEIVED: {
      data = (String) msg.obj;
      tv.setText(data);
      if (mServerMode)
       mBluetoothConnection.write(data.getBytes());
     }
     case MESSAGE_READ:
      // your code goes here

I'm sure you must implement something like:

new ConnectionThread(mBluetoothSocket, mHandler);

sources i found here