20-b Local Service

20.2 Local Service

Serviceis a fundamental component in Android. Often, applications will need to run processes for a long time without any intervention from the user, or very rare interventions. These background processes need to keep running even when the phone is being used for other activities.

To accommodate for such a requirement, android has introduced theServicecomponent. It is a long lived component and does not implement any user interface of its own.

Typically a user interacts with a service through an activity that has a UI. The service by itselfnotifiesthe user in case of any need for user intervention.

In this article, we'll make a very simple service which runs in the background but does not have any notification features.

In order to start and stop the service, an activity called the service provider will be created.

In this example, when the service starts, it toasts a message that the service has started. When the service ends, it toasts a message that the service has ended. This is not of much use in the real sense of services. However, it simplifies the introduction of service creation and destruction.

Ideally, one of the ways could be: a service that is running should notify itself as running by putting up a status bar notification as discussed in the previous section. And when the service has completed running the notification can be removed. Through the notification, the service can give a user interface for binding to the service or viewing the status of the service or any other similar interaction with the service. The combination of service with notification will be an example I will given in the next section.

Let's talk a little bit more about services.

Binding to a service
Once a service has begun and is running in the background, any number of activities can ï¿½bindï¿½ to the service. In fact, if we want to bind to a service that has not started, calling to bind could initiate the service. Such a service would shut down as soon as the last user detaches from the service.

Remote service
The services defined above are those that run in the same process as the application that started it. However, we can have services that run in its own process. This is particularly useful when the service has to run for a long time, typical example being a background music player. For two processes to communicate, the object being passed needs to be marshaled.

For this, Android provides aAIDLtool (Android Interface Definition Language) to handle all marshalling and communication. The remote service example will be taken up in a subsequent article.

Mostconfusionabout the Service class actually revolves around what it isnotrather than what it is:

A Service isnota separate process. The Service object itself does not imply it is running in its own process; unless otherwise specified, it runs in the same process as the application it is part of.
A Service isnota thread. It is not a means itself to do work off of the main thread (to avoid Application Not Responding errors).

Thus a Service itself is actually very simple, providing two main features:

A facility for the application to tell the systemaboutsomething it wants to be doing in the background (even when the user is not directly interacting with the application). This corresponds to calls toContext.startService(), which ask the system to schedule work for the service, to be run until the service or someone else explicitly stop it.
A facility for an application to expose some of its functionality to other applications. This corresponds to calls toContext.bindService(), which allows a long-standing connection to be made to the service in order to interact with it.

When a Service component is actually created, for either of these reasons, all that the system actually does is instantiate the component and call itsonCreate()and any other appropriate callbacks on the main thread. It is up to the Service to implement these with the appropriate behavior, such as creating a secondary thread in which it does its work.

Finally the Service Example.

Let us start with creating a service class that extendsandroid.app.Service. This service just displays a message when started and again displays a message when stopped. Hence theonStart()andonDestroy()methods are implemented. Here is the code.

package com.bogotobogo.serviceprovider;import android.app.Service;import android.content.Intent;import android.os.IBinder;import android.widget.Toast;public class SimpleService extends Service {@Overridepublic IBinder onBind(Intent arg0) {return null;}@Overridepublic void onCreate() {super.onCreate();Toast.makeText(this,"Service created", Toast.LENGTH_LONG).show();}@Overridepublic void onDestroy() {super.onDestroy();Toast.makeText(this,"Service destroyed", Toast.LENGTH_LONG).show();}@Overridepublic void onStart(Intent intent, int startId) {super.onCreate();Toast.makeText(this,"Service started", Toast.LENGTH_LONG).show();}}

An entry for this service needs to be made in the AndroidManifest.xml file. Here it is:
```
<service android:name=".SimpleService"></service>
```

Now, we need to be able to invoke this service. i.e. start the service and stop the service. We choose to write an activity that can start and stop the service. This activity is calledServiceProvider. Here is what it does:
To start/stop the service ï¿½

Button start = (Button)findViewById(R.id.startButton);Button stop = (Button)findViewById(R.id.stopButton);        start.setOnClickListener(this);stop.setOnClickListener(this);}public void onClick(View src) {    switch (src.getId()) {    case R.id.startButton:        startService(new Intent(this, SimpleService.class));      break;    case R.id.stopButton:          stopService(new Intent(this, SimpleService.class));      break;    }  }

The entry for this class in theAndroidManifest.xmlfile is as usual:

<activity android:name=".ServiceProvider"android:label="@string/app_name"><intent-filter><action android:name="android.intent.action.MAIN" /><category android:name="android.intent.category.LAUNCHER" /></intent-filter></activity>

Here is our Java files,ServiceProvider.java:

package com.bogotobogo.serviceprovider;import android.app.Activity;import android.content.Intent;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class ServiceProvider extends Activity implements OnClickListener{@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.main);        Button start = (Button)findViewById(R.id.startButton);Button stop = (Button)findViewById(R.id.stopButton);        start.setOnClickListener(this);stop.setOnClickListener(this);}public void onClick(View src) {    switch (src.getId()) {    case R.id.startButton:        startService(new Intent(this, SimpleService.class));      break;    case R.id.stopButton:          stopService(new Intent(this, SimpleService.class));      break;    }  }}

andSimpleService.java.

package com.bogotobogo.serviceprovider;import android.app.Service;import android.content.Intent;import android.os.IBinder;import android.widget.Toast;public class SimpleService extends Service {@Overridepublic IBinder onBind(Intent arg0) {return null;}@Overridepublic void onCreate() {super.onCreate();Toast.makeText(this,"Service created", Toast.LENGTH_LONG).show();}@Overridepublic void onDestroy() {super.onDestroy();Toast.makeText(this,"Service destroyed", Toast.LENGTH_LONG).show();}@Overridepublic void onStart(Intent intent, int startId) {super.onCreate();Toast.makeText(this,"Service started", Toast.LENGTH_LONG).show();}}

Or we can have a slightly differentServiceProvideB.javafile depending on the way how theListenerinterface is implemented and another fileSimpleService.javahas been modified slightly so that it can play audio, synchronized with the start/stop button.

ServiceProvideB.java:

package com.bogotobogo.serviceproviderb;import android.app.Activity;import android.content.Intent;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class ServiceProviderB extends Activity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.main);        Button start = (Button)findViewById(R.id.startButton);Button stop = (Button)findViewById(R.id.stopButton);                start.setOnClickListener(startListener);        stop.setOnClickListener(stopListener); }   private OnClickListener startListener = new OnClickListener() {public void onClick(View v){startService(new Intent(ServiceProviderB.this,SimpleService.class));}        };   private OnClickListener stopListener = new OnClickListener() {public void onClick(View v){      stopService(new Intent(ServiceProviderB.this,SimpleService.class));}        };}

SimpleService.java

package com.bogotobogo.serviceproviderb;import android.app.Service;import android.content.Intent;import android.media.MediaPlayer;import android.os.IBinder;import android.widget.Toast;public class SimpleService extends Service {MediaPlayer player;@Overridepublic IBinder onBind(Intent arg0) {return null;}@Overridepublic void onCreate() {super.onCreate();Toast.makeText(this,"Service created", Toast.LENGTH_LONG).show();player = MediaPlayer.create(this, R.raw.amanda);}@Overridepublic void onDestroy() {super.onDestroy();Toast.makeText(this,"Service destroyed", Toast.LENGTH_LONG).show();player.stop();}@Overridepublic void onStart(Intent intent, int startId) {super.onCreate();Toast.makeText(this,"Service started", Toast.LENGTH_LONG).show();player.start();}}

Now we can launch this application and we can start and stop a service.

Files used in this Service example,ServiceProvider.zip
orServiceProviderB.zip

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