Passing a handler from a background Handler Thread, to background thread

Question

Can anyone point me in the right direction here please ?

I have an activity which spawns two threads, a thread for handling messages, using a Looper

    public static class MiddleThread extends Handler{
    static public Handler handler;

    public void run() {
        Looper.prepare();
        Log.d("MiddleThread", "Looper is prepared !");
        handler = new Handler() {

            public void handleMessage(Message msg)
            {
                Bundle bundle = msg.getData();
                String exitString = bundle.getString("endmessage");
                if(exitString.equals(("ExitOK")))
                {
                    boolean searchFinished = true;
                    Looper looper = Looper.myLooper();
                    looper.quit();
                } else
                {
                    int fileCount = bundle.getInt("filecount");
                    String fileName = bundle.getString("filename");
                    Log.d("MiddleThread", "File Number " + fileCount + " is " + fileName);
                }
            }
        };
        Log.d("MiddleThread", "nandler should be initialised");
        Looper.loop();
    }

... then it spawns the main Worker Thread, which is passed a handler from the UI Thread, and the handler from the above thread.

public class BasicSearch {
public Handler handlerUi, handlerMiddleThread;
public Message messageUi, messageMiddleThread;
public int fileCount = 0;
public BasicSearch(Handler ui, Handler mt) {
    handlerUi = ui;
    handlerMiddleThread = mt;
}

public void listFiles()
{
    File searchPath = Environment.getExternalStorageDirectory();
    messageUi = handlerUi.obtainMessage();
    messageMiddleThread = handlerMiddleThread.obtainMessage();
    walk(searchPath);
    Bundle b = new Bundle();
    b.putString("endmessage", "ExitOK");
    messageMiddleThread.setData(b);
    handlerMiddleThread.dispatchMessage(messageMiddleThread);
}

private void walk(File path) {
    File[] list = path.listFiles();
    for(File f : list)
    {
        if(f.isDirectory())
        {               
            walk(new File(f.getAbsolutePath()));
        } else {
            processFile(f);
        }
    }
}

private void processFile(File f) {
    Bundle b = new Bundle();
    fileCount++;
    b.putString("filename", f.getName());
    b.putInt("filecount", fileCount);
    messageMiddleThread.setData(b);
    Log.d("BasicSearch", "Data is set, to send to MiddleThread");
    handlerMiddleThread.dispatchMessage(messageMiddleThread);
    Log.d("BasicSearch", "Message sent");

}

    }

Whatever happens, when it tries to dispatchMessage, handlerMiddleThread reverts to being null. I even have the following code in my activity, to try and ensure that it isn't null, but it still ends up being null when I get to send the message.

      startMiddleThread();
    while(true)
    {
        if(MiddleThread.handler != null)
            break;
    }   
    startSearchThread();

This is a test project, as I wanted to be able to get the Handler/Looper concept properly understood before continuing on with my project.

I have successfully managed to use a Handler in my UI Threads before, but my current project has too much processing going on in the UI, and I want to have a secondary thread handling the output from the searchThread, and just receive a message in UI thread when the thread is complete.

post your activity code with the code where you call `run` and `new BasicSearch` — njzk2, Jul 17 '13 at 14:26

score 2 · Answer 1 · answered Jul 17 '13 at 14:35

So I think I see what you're trying to do and let me suggest a slightly easier way:

To start your background thread and get a handler to it:

HandlerThread bgThread = new HandlerThread();
bgThread.start();
Handler bgHandler = new Handler(bgThread.getLooper());

Then you can send whatever messages you want to your bgHandler. Note that you need to call start on a HandlerThread before creating the bgThread (otherwise getLooper() will return null).

That being said I think I know whats wrong with your code as you posted it. First, MiddleThread extends Handler (which doesn't have a run() method!) not Thread. Second, the run() method on MiddleThread is never called, so Handler is never instantiated. Even if your just mistyped Handler in your code above and you're actually extending Thread, you still need to call start on MiddleThread in order for anything in run() to be executed. Really though, what you're doing is waaay more complicated that it needs to be, and you almost certainly want to just do what I mentioned above.

Thanks Monkeyless. I put that back to Thread, and it does in fact not need the start() method. The code within run() is executed without, it is confirmed by the log.d output, in LogCat. I'll test your suggestion later, but you are right, I am perhaps overcomplicating things. What I want to do could all be done in the one thread, with UI just being told - via a handler - that it has finished. I just want to get the hang of using handlers purely for background processing though. — Douglas Brett, Jul 17 '13 at 17:49
Ah no using handlers for background processing is totally fine and that is exactly what my example does. Any runnable passed via bgHandler.post(new Runnable()...) will be executed on the background thread. What I meant was that in order to create and use a background thread, you do not need to extend Thread or create/call a Looper yourself. Instead you can use HandlerThread as I showed, which handles all of that for you. — Sam Judd, Jul 17 '13 at 18:44

Passing a handler from a background Handler Thread, to background thread

1 Answers1