Android Num: June 2016

A Service is an application component that can perform long-running operations in the background and does not provide a user interface. Another application component can start a service and it will continue to run in the background even if the user switches to another application. Additionally, a component can bind to a service to interact with it and even perform interprocess communication (IPC). For example, a service might handle network transactions, play music, perform file I/O, or interact with a content provider, all from the background.

A service can essentially take two forms:

Started
A service is "started" when an application component (such as an activity) starts it by calling startService(). Once started, a service can run in the background indefinitely, even if the component that started it is destroyed. Usually, a started service performs a single operation and does not return a result to the caller. For example, it might download or upload a file over the network. When the operation is done, the service should stop itself.

Bound
A service is "bound" when an application component binds to it by calling bindService(). A bound service offers a client-server interface that allows components to interact with the service, send requests, get results, and even do so across processes with interprocess communication (IPC). A bound service runs only as long as another application component is bound to it. Multiple components can bind to the service at once, but when all of them unbind, the service is destroyed.

Although this documentation generally discusses these two types of services separately, your service can work both ways—it can be started (to run indefinitely) and also allow binding. It's simply a matter of whether you implement a couple callback methods: onStartCommand() to allow components to start it and onBind() to allow binding.

Regardless of whether your application is started, bound, or both, any application component can use the service (even from a separate application), in the same way that any component can use an activity—by starting it with an Intent. However, you can declare the service as private, in the manifest file, and block access from other applications. This is discussed more in the section about Declaring the service in the manifest.

Caution: A service runs in the main thread of its hosting process—the service does not create its own thread and does not run in a separate process (unless you specify otherwise). This means that, if your service is going to do any CPU intensive work or blocking operations (such as MP3 playback or networking), you should create a new thread within the service to do that work. By using a separate thread, you will reduce the risk of Application Not Responding (ANR) errors and the application's main thread can remain dedicated to user interaction with your activities.

The Basics

To create a service, you must create a subclass of Service (or one of its existing subclasses). In your implementation, you need to override some callback methods that handle key aspects of the service lifecycle and provide a mechanism for components to bind to the service, if appropriate. The most important callback methods you should override are:

onStartCommand()

The system calls this method when another component, such as an activity, requests that the service be started, by calling startService(). Once this method executes, the service is started and can run in the background indefinitely. If you implement this, it is your responsibility to stop the service when its work is done, by calling stopSelf() or stopService(). (If you only want to provide binding, you don't need to implement this method.)

onBind()

The system calls this method when another component wants to bind with the service (such as to perform RPC), by calling bindService(). In your implementation of this method, you must provide an interface that clients use to communicate with the service, by returning an IBinder. You must always implement this method, but if you don't want to allow binding, then you should return null.

onCreate()

The system calls this method when the service is first created, to perform one-time setup procedures (before it calls either onStartCommand() or onBind()). If the service is already running, this method is not called.

onDestroy()

The system calls this method when the service is no longer used and is being destroyed. Your service should implement this to clean up any resources such as threads, registered listeners, receivers, etc. This is the last call the service receives.

If a component starts the service by calling startService() (which results in a call to onStartCommand()), then the service remains running until it stops itself with stopSelf() or another component stops it by calling stopService().

If a component calls bindService() to create the service (and onStartCommand() is not called), then the service runs only as long as the component is bound to it. Once the service is unbound from all clients, the system destroys it.

The Android system will force-stop a service only when memory is low and it must recover system resources for the activity that has user focus. If the service is bound to an activity that has user focus, then it's less likely to be killed, and if the service is declared to run in the foreground (discussed later), then it will almost never be killed. Otherwise, if the service was started and is long-running, then the system will lower its position in the list of background tasks over time and the service will become highly susceptible to killing—if your service is started, then you must design it to gracefully handle restarts by the system. If the system kills your service, it restarts it as soon as resources become available again (though this also depends on the value you return from onStartCommand(), as discussed later). For more information about when the system might destroy a service, see the Processes and Threading document.

In the following sections, you'll see how you can create each type of service and how to use it from other application components.

Declaring a service in the manifest

Like activities (and other components), you must declare all services in your application's manifest file.

To declare your service, add a <service> element as a child of the <application> element. For example:

<manifest ... >

...

<application ... >

<service android:name=".ExampleService" />

...

</application>

</manifest>

See the <service> element reference for more information about declaring your service in the manifest.

There are other attributes you can include in the <service> element to define properties such as permissions required to start the service and the process in which the service should run. The android:name attribute is the only required attribute—it specifies the class name of the service. Once you publish your application, you should not change this name, because if you do, you risk breaking code due to dependence on explicit intents to start or bind the service (read the blog post, Things That Cannot Change).

To ensure your app is secure, always use an explicit intent when starting or binding your Service and do not declare intent filters for the service. If it's critical that you allow for some amount of ambiguity as to which service starts, you can supply intent filters for your services and exclude the component name from the Intent, but you then must set the package for the intent with setPackage(), which provides sufficient disambiguation for the target service.

Additionally, you can ensure that your service is available to only your app by including the android:exported attribute and setting it to "false". This effectively stops other apps from starting your service, even when using an explicit intent.

Creating a Started Service

A started service is one that another component starts by calling startService(), resulting in a call to the service's onStartCommand() method.

When a service is started, it has a lifecycle that's independent of the component that started it and the service can run in the background indefinitely, even if the component that started it is destroyed. As such, the service should stop itself when its job is done by calling stopSelf(), or another component can stop it by calling stopService().

An application component such as an activity can start the service by calling startService() and passing an Intent that specifies the service and includes any data for the service to use. The service receives this Intent in the onStartCommand() method.

For instance, suppose an activity needs to save some data to an online database. The activity can start a companion service and deliver it the data to save by passing an intent to startService(). The service receives the intent in onStartCommand(), connects to the Internet and performs the database transaction. When the transaction is done, the service stops itself and it is destroyed.

Caution: A service runs in the same process as the application in which it is declared and in the main thread of that application, by default. So, if your service performs intensive or blocking operations while the user interacts with an activity from the same application, the service will slow down activity performance. To avoid impacting application performance, you should start a new thread inside the service.

Traditionally, there are two classes you can extend to create a started service:

Service

This is the base class for all services. When you extend this class, it's important that you create a new thread in which to do all the service's work, because the service uses your application's main thread, by default, which could slow the performance of any activity your application is running.

IntentService

This is a subclass of Service that uses a worker thread to handle all start requests, one at a time. This is the best option if you don't require that your service handle multiple requests simultaneously. All you need to do is implement onHandleIntent(), which receives the intent for each start request so you can do the background work.

The following sections describe how you can implement your service using either one for these classes.

Extending the IntentService class

Because most started services don't need to handle multiple requests simultaneously (which can actually be a dangerous multi-threading scenario), it's probably best if you implement your service using the IntentService class.

The IntentService does the following:

Creates a default worker thread that executes all intents delivered to onStartCommand() separate from your application's main thread.

Creates a work queue that passes one intent at a time to your onHandleIntent() implementation, so you never have to worry about multi-threading.

Stops the service after all start requests have been handled, so you never have to call stopSelf().

Provides default implementation of onBind() that returns null.

Provides a default implementation of onStartCommand() that sends the intent to the work queue and then to your onHandleIntent() implementation.

All this adds up to the fact that all you need to do is implement onHandleIntent() to do the work provided by the client. (Though, you also need to provide a small constructor for the service.)

Here's an example implementation of IntentService:

 public class HelloIntentService extends IntentService {  
   
  /**  
   * A constructor is required, and must call the super IntentService(String)  
   * constructor with a name for the worker thread.  
   */  
  public HelloIntentService() {  
    super("HelloIntentService");  
  }  
   
  /**  
   * The IntentService calls this method from the default worker thread with  
   * the intent that started the service. When this method returns, IntentService  
   * stops the service, as appropriate.  
   */  
  @Override  
  protected void onHandleIntent(Intent intent) {  
    // Normally we would do some work here, like download a file.  
    // For our sample, we just sleep for 5 seconds.  
    try {  
      Thread.sleep(5000);  
    } catch (InterruptedException e) {  
      // Restore interrupt status.  
      Thread.currentThread().interrupt();  
    }  
  }  
 }

That's all you need: a constructor and an implementation of onHandleIntent().

If you decide to also override other callback methods, such as onCreate(), onStartCommand(), or onDestroy(), be sure to call the super implementation, so that the IntentService can properly handle the life of the worker thread.

For example, onStartCommand() must return the default implementation (which is how the intent gets delivered to onHandleIntent()):

 @Override  
 public int onStartCommand(Intent intent, int flags, int startId) {  
   Toast.makeText(this, "service starting", Toast.LENGTH_SHORT).show();  
   return super.onStartCommand(intent,flags,startId);  
 }

Besides onHandleIntent(), the only method from which you don't need to call the super class is onBind() (but you only need to implement that if your service allows binding).

In the next section, you'll see how the same kind of service is implemented when extending the base Service class, which is a lot more code, but which might be appropriate if you need to handle simultaneous start requests.

Extending the Service class
As you saw in the previous section, using IntentService makes your implementation of a started service very simple. If, however, you require your service to perform multi-threading (instead of processing start requests through a work queue), then you can extend the Service class to handle each intent.

For comparison, the following example code is an implementation of the Service class that performs the exact same work as the example above using IntentService. That is, for each start request, it uses a worker thread to perform the job and processes only one request at a time.

 public class HelloService extends Service {  
  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) {  
      // Normally we would do some work here, like download a file.  
      // For our sample, we just sleep for 5 seconds.  
      try {  
        Thread.sleep(5000);  
      } catch (InterruptedException e) {  
        // Restore interrupt status.  
        Thread.currentThread().interrupt();  
      }  
      // 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",  
       Process.THREAD_PRIORITY_BACKGROUND);  
   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, "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;  
  }  
   
  @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();  
  }  
 }

As you can see, it's a lot more work than using IntentService.

However, because you handle each call to onStartCommand() yourself, you can perform multiple requests simultaneously. That's not what this example does, but if that's what you want, then you can create a new thread for each request and run them right away (instead of waiting for the previous request to finish).

Notice that the onStartCommand() method must return an integer. The integer is a value that describes how the system should continue the service in the event that the system kills it (as discussed above, the default implementation for IntentService handles this for you, though you are able to modify it). The return value from onStartCommand() must be one of the following constants:

START_NOT_STICKY
If the system kills the service after onStartCommand() returns, do not recreate the service, unless there are pending intents to deliver. This is the safest option to avoid running your service when not necessary and when your application can simply restart any unfinished jobs.

START_STICKY
If the system kills the service after onStartCommand() returns, recreate the service and call onStartCommand(), but do not redeliver the last intent. Instead, the system calls onStartCommand() with a null intent, unless there were pending intents to start the service, in which case, those intents are delivered. This is suitable for media players (or similar services) that are not executing commands, but running indefinitely and waiting for a job.

START_REDELIVER_INTENT
If the system kills the service after onStartCommand() returns, recreate the service and call onStartCommand() with the last intent that was delivered to the service. Any pending intents are delivered in turn. This is suitable for services that are actively performing a job that should be immediately resumed, such as downloading a file.
For more details about these return values, see the linked reference documentation for each constant.

Starting a Service
You can start a service from an activity or other application component by passing an Intent (specifying the service to start) to startService(). The Android system calls the service's onStartCommand() method and passes it the Intent. (You should never call onStartCommand() directly.)

For example, an activity can start the example service in the previous section (HelloSevice) using an explicit intent with startService():

 Intent intent = new Intent(this, HelloService.class);  
 startService(intent);

The startService() method returns immediately and the Android system calls the service's onStartCommand() method. If the service is not already running, the system first calls onCreate(), then calls onStartCommand().

If the service does not also provide binding, the intent delivered with startService() is the only mode of communication between the application component and the service. However, if you want the service to send a result back, then the client that starts the service can create a PendingIntent for a broadcast (with getBroadcast()) and deliver it to the service in the Intent that starts the service. The service can then use the broadcast to deliver a result.

Multiple requests to start the service result in multiple corresponding calls to the service's onStartCommand(). However, only one request to stop the service (with stopSelf() or stopService()) is required to stop it.

Stopping a service
A started service must manage its own lifecycle. That is, the system does not stop or destroy the service unless it must recover system memory and the service continues to run after onStartCommand() returns. So, the service must stop itself by calling stopSelf() or another component can stop it by calling stopService().

Once requested to stop with stopSelf() or stopService(), the system destroys the service as soon as possible.

However, if your service handles multiple requests to onStartCommand() concurrently, then you shouldn't stop the service when you're done processing a start request, because you might have since received a new start request (stopping at the end of the first request would terminate the second one). To avoid this problem, you can use stopSelf(int) to ensure that your request to stop the service is always based on the most recent start request. That is, when you call stopSelf(int), you pass the ID of the start request (the startId delivered to onStartCommand()) to which your stop request corresponds. Then if the service received a new start request before you were able to call stopSelf(int), then the ID will not match and the service will not stop.

Caution: It's important that your application stops its services when it's done working, to avoid wasting system resources and consuming battery power. If necessary, other components can stop the service by calling stopService(). Even if you enable binding for the service, you must always stop the service yourself if it ever received a call to onStartCommand().

For more information about the lifecycle of a service, see the section below about Managing the Lifecycle of a Service.

Creating a Bound Service
A bound service is one that allows application components to bind to it by calling bindService() in order to create a long-standing connection (and generally does not allow components to start it by calling startService()).

You should create a bound service when you want to interact with the service from activities and other components in your application or to expose some of your application's functionality to other applications, through interprocess communication (IPC).

To create a bound service, you must implement the onBind() callback method to return an IBinder that defines the interface for communication with the service. Other application components can then call bindService() to retrieve the interface and begin calling methods on the service. The service lives only to serve the application component that is bound to it, so when there are no components bound to the service, the system destroys it (you do not need to stop a bound service in the way you must when the service is started through onStartCommand()).

To create a bound service, the first thing you must do is define the interface that specifies how a client can communicate with the service. This interface between the service and a client must be an implementation of IBinder and is what your service must return from the onBind() callback method. Once the client receives the IBinder, it can begin interacting with the service through that interface.

Multiple clients can bind to the service at once. When a client is done interacting with the service, it calls unbindService() to unbind. Once there are no clients bound to the service, the system destroys the service.

For more detail:
Source : https://developer.android.com/guide/components/services.html

Android Num

Saturday, 11 June 2016

Android Preferences

Preference Demo Activity Class

Services

The Basics