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Looper/Handler API - Part 2

Android  #android  #handlers 

This is part two of looper/handler API series

In previous article, we have learnt about why we need handler API, classes used in this API and their responsibilities and in the last we have written a small app for demonstration. In this article, I will explain the example code.

We started off by writing a worker thread. Let’s see what’s in Worker Thread's source code and how it works.

class WorkerThread extends Thread implements Handler.Callback{

    private static final boolean LOG = true;

    private Handler mHandler;

    WorkerThread() {
        super("WorkerThread");
    } // WorkerThread

    @Override
    public void run() {
        Looper.prepare(); // attached looper to current thread
        mHandler = new Handler(this); // attach this handler to looper of current thread
        Looper.loop(); // starts to process queue, this is a blocking call.

        if(LOG){
            log("run -> %s", "terminating worker thread.");
        }

    } // run


    @Override
    public boolean handleMessage(Message msg) {
        try {
            int sleepTime = new Random().nextInt(2000);
            if(LOG){
                log("handleMessage -> sleeping for [%s ms]", sleepTime);
            }
            Thread.sleep(sleepTime);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        if(LOG){
            log("handleMessage -> what[%s], thread[%s]", msg.what, Thread.currentThread().getName());
        }
        return true;
    } // handleMessage

    Handler getHandler(){
        return mHandler;
    } // getHandler

    private void log(String format, Object... args){
        Utils.log("class[WorkerThread]", String.format(format, args));
    } // log

} // WorkerThread

Ok. I will explain each and every thing at very basic level here.

We extend our WorkerThread from java.lang.Thread class because it’s a thread and implement Handler.Callback which is the callback for Handler (Looper will dispatch messages through this callback).

A Handler is declared as a private field. It is the interface used by Main Thread to add Message in the Looper’s queue.

In run method, we call Looper.prepare() to attach a Looper with current thread (WorkerThread). After calling this method, our thread has a looper attached to it, we can initialize ( mHandler = new Handler(this)). The default constructor of Handler binds the handler to the Looper of current thread. Note that we have also passed this to constructor, it is for Handler.Callback. By contract Handler.Callback enforces us to override handleMessage(Message msg). All the messages passed to Looper's queue will be dispatched to Handler through this method.

Once we have set up Looper and Handler, we can start the loop and wait for message. Looper.loop() will block the current thread and whenever a message is sent to the MessageQueue it will be dispatched to the handleMessage(Message msg) method of WorkerThread.

Now let’s go to MainActivity ans see what’s happening there!

public class MainActivity extends AppCompatActivity {

    private static final boolean LOG = true;
    private WorkerThread workerThread;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ActivityUtils.setToolbar(this, getString(R.string.app_name));
    } // onCreate

    @Override
    protected void onStart() {
        super.onStart();
        if(LOG){
            log("onStart()");
        }

        workerThread = new WorkerThread();
        workerThread.start();
    }

    @Override
    protected void onStop() {
        super.onStop();
        if(LOG){
            log("onStop()");
        }
        if(workerThread != null){
            if(LOG){
                log("onStop -> %s", "quit looper.");
            }

            workerThread.getHandler().getLooper().quit();
            workerThread = null;
        }
    } // onStop

    private void log(String format, Object... args){
        Utils.log("class[MainActivity]", String.format(format, args));
    } // log

    public void onDoWork(View view) {
        if(workerThread.getHandler() != null){
            int what = new Random().nextInt(100);
            if(LOG){
                log("onDoWork -> seding [%d]", what);
            }

            workerThread.getHandler().sendMessage(workerThread.getHandler().obtainMessage(what));
        }
    } // onDoWork

} // MainActivity

In MainActivity we start off by initializing the worker Thread. Next we override onStop method, here we are quiting the loop so that WorkerThread can terminate.

onDoWork methods is registered as onClick listener for Button in layout. Whenever this method is called, we get the handler from WorkerThread and insert a Message in queue using sendMessage method.

sendMessage adds the method in the queue which is eventually dispatched to handleMessage(Message msg) in WorkerThread.

In upcoming articles, I will write in detail about messages and other callbacks related to handler. Read part 3 here

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