Implementation of a State machine for a long
running background task in an Android service
by
Somenath Mukhopadhyay
som.mu...
import
import
import
import
import
import
import
import

android.os.Handler;
android.os.Message;
android.os.Messenger;
and...
}
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R....
// separate thread because the service normally runs in the process's
// main thread, which we don't want to block. We als...
DOWNLOADSTARTED, "Download Started"));
// Normally we would do some work here, like download a
file.
// For our sample, we...
import android.widget.Toast;
public class LongRunningService extends Service {
private
private
private
private

static
sta...
messenger.send(Message.obtain(null,
DOWNLOADFINISHED, "Download Finished"));

} catch (InterruptedException e) {
// TODO A...
@Override
public IBinder onBind(Intent intent) {
// We don't provide binding, so return null
return null;
}
@Override
publ...
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:them...
Upcoming SlideShare
Loading in...5
×

Implementation of a state machine for a longrunning background task in android service

393

Published on

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.,

Published in: Technology
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total Views
393
On Slideshare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
11
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Implementation of a state machine for a longrunning background task in android service

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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.

×