This document describes how we can break a long running background task in an Android service into different states and how we can notify the UI about these states as they proceed.,

1. Implementation of a State machine for a long
running background task in an Android service
by
Somenath Mukhopadhyay
som.mukhopadhyay@gmail.com
In the below example i will show how we can break a long running background task running in a
service into different states of a state machine and notify the front end UI about each and every
stage as they occur in the service. Here i have used a service called LongRunningService which
actually (theoretically) does the task of downloading a big file from a network server (however, for
simplicity i have just stubbed out the actual download code with a thread having delay of 1000
ms). This background task has been splitted into different states according to the state machine
like “Start Connection”, “Connection Completed”, “Start Downloading” and “Stop Downloading”.
This application also showcases the concept of communicating from a background service to
the frontend UI through Android messenger framework.
So lets start digging into the source code of the application.
First of all the main Activity class.
As it is clear from the code that the main activity has a messenger whose message handling
part has been defined by a class called MessageHandler (derived from Handler). This is the
messenger object through which the background service notifies the UI thread.
The UI has a button. Upon clicking it, it starts the service and as soon as it starts the service the
service starts notifying about the different states of the Service through the messenger.
This is pretty simple. Right!!!
The class MainActivity.Java
package com.somitsolutions.android.example.statepatterninservice;
import
import
import
import
android.app.Activity;
android.content.Context;
android.content.Intent;
android.os.Bundle;

2. import
import
import
import
import
import
import
import
android.os.Handler;
android.os.Message;
android.os.Messenger;
android.view.Menu;
android.view.View;
android.view.View.OnClickListener;
android.widget.Button;
android.widget.Toast;
public class MainActivity extends Activity implements OnClickListener{
private static final int CONNECTING = 1;
private static final int CONNECTED = 2;
private static final int DOWNLOADSTARTED = 3;
private static final int DOWNLOADFINISHED = 4;
Button startButton;
private MessageHandler handler;
private static MainActivity mMainActivity;
public Messenger mMessenger = new Messenger(new MessageHandler(this));
private class MessageHandler extends Handler{
private Context c;
MessageHandler(Context c){
this.c = c;
}
@Override
public void handleMessage(Message msg) {
switch(msg.what){
case CONNECTING:
Toast.makeText(getApplicationContext(),
Toast.LENGTH_LONG).show();
break;
case CONNECTED:
Toast.makeText(getApplicationContext(),
Toast.LENGTH_LONG).show();
break;
case DOWNLOADSTARTED:
Toast.makeText(getApplicationContext(),
Toast.LENGTH_LONG).show();
break;
case DOWNLOADFINISHED:
Toast.makeText(getApplicationContext(),
Toast.LENGTH_LONG).show();
break;
default:
super.handleMessage(msg);
"Connecting",
"Connected",
"Download Started",
"Download Finished",

3. }
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mMainActivity = this;
startButton = (Button)findViewById(R.id.button1);
startButton.setOnClickListener(this);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
public static MainActivity getMainActivity(){
return mMainActivity;
}
@Override
public void onClick(View arg0) {
// TODO Auto-generated method stub
Intent serv = new Intent(MainActivity.this, LongRunningService.class);
startService(serv);
}
}
Now lets start digging the LongrunningServivce class.
As we know that a service usually runs in the main thread. Hence the UI thread may seem to be
frozen in case of a long background service. To overcome that a background thread is being
created the moment one starts the service and the task is executed in that thread. This is clear
from the following piece of code.
@Override
public void onCreate() {
// Start up the thread running the service. Note that we create a

4. // separate thread because the service normally runs in the process's
// main thread, which we don't want to block. We also make it
// background priority so CPU-intensive work will not disrupt our UI.
HandlerThread thread = new HandlerThread("ServiceStartArguments",
Thread.NORM_PRIORITY);
thread.start();
// Get the HandlerThread's Looper and use it for our Handler
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
The service class also has a Handler class called ServiceHandler through which we send
messages from the service to the thread’s message loop. Inside the message loop, we actually
accomplish the long running task. Lets have a look at this ServiceHandler class
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
Messenger messenger= MainActivity.getMainActivity().mMessenger;
try {
messenger.send(Message.obtain(null, CONNECTING, "Connecting"));
// Normally we would do some work here, like download a file.
// For our sample, we just sleep for 10 seconds.
Thread.sleep(1000);
// Normally we would do some work here, like download a
file.
// For our sample, we just sleep for 10 seconds.
messenger.send(Message.obtain(null, CONNECTED,
"Connected"));
// Normally we would do some work here, like download a
file.
// For our sample, we just sleep for 10 seconds.
Thread.sleep(1000);
messenger.send(Message.obtain(null,

5. DOWNLOADSTARTED, "Download Started"));
// Normally we would do some work here, like download a
file.
// For our sample, we just sleep for 10 seconds.
Thread.sleep(1000);
messenger.send(Message.obtain(null,
DOWNLOADFINISHED, "Download Finished"));
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (RemoteException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// Stop the service using the startId, so that we don't stop
// the service in the middle of handling another job
stopSelf(msg.arg1);
}
}
As it becomes clear from the above code that in this overridden HandleMessage function of the
service handler, we acquire a reference to the messenger of the main activity and it falls through
different states like “Connecting”, “Connected”, “Start Downloading” and “Finish Downloading”. In
each state a different integer constant is being passed to the UI thread through the messenger.
In the main UI thread the handler function of the messenger handles these messages from the
service and displays the status of the each state.
After that the service stops itself.
package com.somitsolutions.android.example.statepatterninservice;
import
import
import
import
import
import
import
import
import
android.app.Service;
android.content.Intent;
android.os.Handler;
android.os.HandlerThread;
android.os.IBinder;
android.os.Looper;
android.os.Message;
android.os.Messenger;
android.os.RemoteException;

6. import android.widget.Toast;
public class LongRunningService extends Service {
private
private
private
private
static
static
static
static
final
final
final
final
int
int
int
int
CONNECTING = 1;
CONNECTED = 2;
DOWNLOADSTARTED = 3;
DOWNLOADFINISHED = 4;
private Looper mServiceLooper;
private ServiceHandler mServiceHandler; // Handler that receives messages from
the thread
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
Messenger messenger= MainActivity.getMainActivity().mMessenger;
try {
messenger.send(Message.obtain(null, CONNECTING, "Connecting"));
// Normally we would do some work here, like download a file.
// For our sample, we just sleep for 10 seconds.
Thread.sleep(1000);
// Normally we would do some work here, like download a
file.
// For our sample, we just sleep for 10 seconds.
messenger.send(Message.obtain(null, CONNECTED,
"Connected"));
// Normally we would do some work here, like download a
file.
// For our sample, we just sleep for 10 seconds.
Thread.sleep(1000);
messenger.send(Message.obtain(null,
DOWNLOADSTARTED, "Download Started"));
// Normally we would do some work here, like download a
file.
// For our sample, we just sleep for 10 seconds.
Thread.sleep(1000);

7. messenger.send(Message.obtain(null,
DOWNLOADFINISHED, "Download Finished"));
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (RemoteException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// Stop the service using the startId, so that we don't stop
// the service in the middle of handling another job
stopSelf(msg.arg1);
}
}
@Override
public void onCreate() {
// Start up the thread running the service. Note that we create a
// separate thread because the service normally runs in the process's
// main thread, which we don't want to block. We also make it
// background priority so CPU-intensive work will not disrupt our UI.
HandlerThread thread = new HandlerThread("ServiceStartArguments",
Thread.NORM_PRIORITY);
thread.start();
// Get the HandlerThread's Looper and use it for our Handler
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Toast.makeText(this, "download service starting",
Toast.LENGTH_SHORT).show();
// For each start request, send a message to start a job and deliver the
// start ID so we know which request we're stopping when we finish the job
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
mServiceHandler.sendMessage(msg);
// If we get killed, after returning from here, restart
return START_STICKY;
}

8. @Override
public IBinder onBind(Intent intent) {
// We don't provide binding, so return null
return null;
}
@Override
public void onDestroy() {
Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show();
}
}
The main.xml layout file looks like the following:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="165dp"
android:text="Start Service" />
</RelativeLayout>
And the manifest file of this application is as follows:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.somitsolutions.android.example.statepatterninservice"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />

9. <application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.somitsolutions.android.example.statepatterninservice.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<service android:name=".LongRunningService"></service>
</application>
</manifest>
Hope it clears some of the ideas behind Android messenger and how we can use it for
notification from a background service to the frontend UI.