Updating Android UI using threads

Question

I am writing an Android Chat App. I am listening for connections and I receive data and I can see it in the Log.d, but whenever I try to update my UI the application crashes.

Code snippet:

private class chatReceiver implements Runnable {     @Override     public void run() {         try {             skt = new DatagramSocket(Integer.parseInt(Main.prefs.getString("port_number", "5432")));             DatagramPacket rcvPkt = new DatagramPacket(rcvBuf,rcvBuf.length);             String ack = "Hello from our SimpleUDPServer";             byte[] sndBuf = ack.getBytes();             while (true) {                 Log.d("Server received: " ,"entered loop");                 skt.receive(rcvPkt);                 String rcvMsg = new String(rcvBuf, 0, rcvPkt.getLength(), "UTF-8");                 Log.d("Server received: " ,"receiving" + rcvMsg);                 if (rcvMsg != null) {                     Log.d("Server received: " ,"not equal null");                     // I want to update my UI here                 }                 DatagramPacket k = new DatagramPacket(sndBuf, sndBuf.length,                         rcvPkt.getAddress(), rcvPkt.getPort());                 skt.send(k);                 Log.d("Server sent" ,ack);              }         } catch (IOException ex) {             Log.d("ThreadStart", "Error Starting thread" + ex.getStackTrace());         }      } }  

and to update the UI I do use :

public static void updateUI(Bubble b, View itemView) {      TextView txt_display_name = (TextView) itemView             .findViewById(R.id.display_name);     txt_display_name.setText(b.getDisplay_name());     TextView txt_chat_body = (TextView) itemView             .findViewById(R.id.chat_body);     txt_chat_body.setText(b.getChat_body());     TextView txt_creation_date = (TextView) itemView             .findViewById(R.id.creation_date);     txt_creation_date.setText(b.getCreation_time()); } 

The application keeps crashing.

Answer 1

You cannot change UI elements from a non-UI thread. Try using runOnUiThread.

runOnUiThread(new Runnable(){     @Override     public void run(){         // change UI elements here     } }); 

Answer 2

You cannot touch anything in the UI thread from a background thread, to do that use Handlers, initialize your background thread passing it a Handler object. When data arrives to use the handler to send a message to the UI. In the UI when the message from the background thread comes, just update the Views.

Example Code Snippet :

in the background thread:

if(dataArrives){     Message msg = handler.obtainMessage();     msg.what = UPDATE_IMAGE;     msg.obj = bitmap;     msg.arg1 = index;     handler.sendMessage(msg); } 

in the UI thread:

final Handler handler = new Handler(){   @Override   public void handleMessage(Message msg) {     if(msg.what==UPDATE_IMAGE){       images.get(msg.arg1).setImageBitmap((Bitmap) msg.obj);     }     super.handleMessage(msg);   } }; 

and pass the handler to the background thread.