Android  Service 谷歌官方文档

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http://blog.csdn.net/jaysong2012/article/details/40919627

 

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.

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Service是一个没有用户界面在后台长期运行的组件，其他组件可以启动（start）Service同时即使用户切换到其他应用Service也可以在后台运行。另外，一个组件可以绑定（bind）Service与之交互，甚至执行跨进程通信（IPC）.例如，Service可以从后台执行网络任务、播放音乐、执行IO操作等，或者也可以和上下文提供者（activitty/services等）交互

A service can essentially take two forms:

Started

A service is "started" when an application component (such as an activity) starts it by callingstartService(). 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.

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Service 本质上来讲采用两种方式（运行）

Started

          当一个应用程序组件（例如一个Activity）通过调用startService()启动Service时，service将会通过 "started”方式启动。一旦启动，即使启动Service的应用程序组件被销毁，Service也会在后台持续运行。通常，一个通过"started"方式启动的service 执行 一个单一的操作并且不会返回结果给调用者（启动者）。例如，它（Service）可以通过网络上传或者下载文件。当执行的操作结束的时候。Service会自动终止。

Bound

        当一个应用程序组件（例如一个Activity）通过调用bindService()启动Service时，service将会通过 "bound”方式启动。一个绑定的Service甚至可以在跨进程通信的情形下通过提供发送request 获取result的客户端—服务器式的接口允许组件与之通信。bound Service只有在其他应用程序组件绑定它的时候才会运行。多个组件可以同时绑定Service，但是只有在所有的组件解除绑定之后，service 才会销毁。

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 anIntent. 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 aboutDeclaring the service in the manifest.

Caution: A service runs in the main thread of its hosting process—the service doesnot create its own thread and doesnot 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.

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           纵使上文总体上来讲分别讨论Service两种方式，Service可以同时通过两种方式运行实现，他即可以直接运行（started）也可以绑定（bound）.而这也只是执行onStartCommand()允许组件started或者onBind()绑定来会回调的问题。

          不管你的应用程序（Service）started 还是Bound或是两者启动，任何组件可以使用这个服务（甚至是其他应用）。好比，任何组件可以通过Intent启动activity。然而，你可以把自己的service申明为私有，并在manifest 文件锁住其他应用的使用权限。更多查看Declaring the service in the manifest.

         注意：Service只能运行在它的宿主进程的主进程，它不能创造自己的进程或者运行在一个单独的进程（除非你指定）。这就意味着。如果你的Service要执行任何耗CPU的任务或者阻塞的操作，你应该另起一个包含Service的进程去执行它。通过使用单独进程，你可以减少应用程序未响应或者错误的风险。同时应用程序的主线程可以继续致力于与用户交互活动。

The Basics

Should you use a service or a thread?（是用进程还是Service）

A service is simply a component that can run in the background even when the user is not interacting with your application. Thus, you should create a service only if that is what you need.

If you need to perform work outside your main thread, but only while the user is interacting with your application, then you should probably instead create a new thread and not a service. For example, if you want to play some music, but only while your activity is running, you might create a thread in onCreate(), start running it in onStart(), then stop it inonStop(). Also consider usingAsyncTask orHandlerThread, instead of the traditionalThread class. See theProcesses and Threading document for more information about threads.

Remember that if you do use a service, it still runs in your application's main thread by default, so you should still create a new thread within the service if it performs intensive or blocking operations.

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:

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        Service 是一个即使用户不在与应用程序交互时也在后台运行的基本组件。因此，如果这是你需要的你可以创建自己的服务（Service）

       如果你需要执行一个主线程之外的操作但他只是在用户与你的应用程序交互的情况下，那么。毫无疑问，你需要的只是一个新的线程而不是一个服务。例如，如果你想只在activity运行的时候播放一些音乐。那么你只需要在activity的onCreate()创建一个线程，在onStart()中启动它, 然后在onStop()停止它，   当然你也可以考虑使用AsyncTask 或者HandlerThread替代传统的Thread.在Processes and Threading查看更多

   记住，如果你确定需要使用服务，它默认情况下运行在您的应用程序的主线程，所以你在执行密集或阻塞的操作时还是应该要在服务内创建一个新线程。

   创建一个服务，您必须创建服务类（或它的一个现有的子类）。在你的实现过程中，你需要重写一些处理的服务生命周期的关键方面和提供绑定服务于组件的回调方法，如果合适的话。你应该忽略最重要的回调方法：

onStartCommand()

The system calls this method when another component, such as an activity, requests that the service be started, by callingstartService(). 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 callingstopSelf() orstopService(). (If you only want to provide binding, you don't need to implement this method.)

        当其他组件（如Activity）通过调用startService()启动Service时，系统会自动调用该方法（onStartCommand()）。一旦这个方法被执行，Service会自动在后台运行。如果你实现了该方法。你就要记得在任务完成时调用stopSelf() orstopService()停止Service。（如果你只想提供绑定就不需要实现该方法）

onBind()

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

       当其他组件想通过调用bindService()绑定服务去执行比如RPC的服务时，系统会自动调用该方法。在你是实现该方法时，你必须提供一个返回IBinder的，客户端能够与Service交互的接口。如果你不允许绑定，你可以 return  null

onCreate()

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

       当服务第一次创建的时候系统会自动调用该方法去执行一系列建立Service操作（它一定会在onStartCommand() oronBind()).前调用）。如果你的Service已经在运行。这个方法不会被调用。

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.

当Service不再使用或者被销毁时系统会自动调用该方法。你的服务应该实现这个去清理如线程，注册的监听器，接收器，等资源。这是最后一次调用服务接收。

 

If a component starts the service by callingstartService() (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().

注意：如果任何组件通过调用startService()（执行onStartCommand()）启动Service，Service将会一直运行直到他自己调用stopSelf()或者其他组件调用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.

          如果一个组件通过bindService()启动服务（你执行onStartCommand()），这个服务只会在组件绑定他的时候运行，一旦这个服务被所有组件取消绑定，系统会自动销毁它

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 theProcesses and Threading document.

        当系统可用内存不足的时候，Android系统将会强行停止Service为回复用户聚焦的核心Activity需要的资源。如果你的Service被绑定到用户聚焦的Activity，Service将很少几率被杀掉，如果你的Service被申明run in the foreground（稍后讨论），那么他将几乎不能可能被杀掉。相应的，如果Service启动了并且长期运行，那么随着时间推移，系统会自动降低他在后台任务队列中的地位，那么它将极有可能被杀掉。如果你的服务极有可能被杀掉，那么你应该巧妙的设计让它能够被系统重新启动。如果系统最终还是杀掉了你的service，那么在资源可用（同样，者就取决于你onStartCommand()返回值了，稍后讨论）时，他就会重新启动了。更多信息，查看Processes and Threading

     

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

     下面就告诉你如何创建不同的service和从其他应用组件如何使用它

Declaring a service in the manifest（在manifest中申明）

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. Theandroid: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 yourService 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 theIntent, but you then must set the package for the intent withsetPackage(), which provides sufficient disambiguation for the target service.

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

              <service>中可以包含一些启动服务所需的权限和服务应该运行在的进程的属性  。

         android:name是唯一一个必须的属性。它指定了服务的类名称。一旦你发布你的应用，你不能改变这个名称，因为如果你做，你可能打破代码由于对启动或绑定服务的依赖的Intent（阅读博客文章，Things That Cannot Change）。

          为了确保你的应用是安全的，记住用明确的Intent去启动或者绑定你的Service，并且不要使用启动service的intent filters。如果你必须允许一定量的模糊性去启动服务，您可以为您的服务提供intent filters 和排除component name的Intent，但同时你也必须为你的intent用setPackage() 设置能够消除歧义的包名。

         同时，你可是设置android:exported="false"去确保你的Service只对你的app可用，这有效地停止从其他应用程序启动你的服务，甚至使用一个明确的Intent时

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 callingstopService().

An application component such as an activity can start the service by calling startService() and passing anIntent that specifies the service and includes any data for the service to use. The service receives thisIntent in theonStartCommand() 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 tostartService(). 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 services 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 implementonHandleIntent(), which receives the intent for each start request so you can do the background work.

             started service是通过调用startService()执行service'sonStartCommand()的方法一类组件

      当一个service 通过started启动，它的生命周期独立于启动他的组件，纵使启动它的组件被销毁，它也可以在后台运行，正因为如此，终止服务需要自己调用stopSelf()或者其他组件调用stopService()

     好比activity的组件可以调用startService()启动Service并传递一个intent指定服务并包含全部服务使用的数据，Service在onStartCommand()接收Intent

     例如，假设一个Activity需要保存一些数据到网络数据库。Activity可以启动一个服务通过一个Intent启动startService()传递数据去保存，服务在onstartcommand()接收Intent，连接到互联网并执行数据库事务。当交易完成后，该服务停止并摧毁。

     注意： Service 默认情况下运行在它所在的应用的主线程，如果你的服务执行密集或阻塞的操作而用户又在于应用程序交互，那么service将拖慢你的程序效率，为了提高你的用户体验，你应该在Service内拧起一个线程

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 theIntentService class.

   由于大多数的started Service 不需要处理多路请求(实际上是一个多线程的危险的情况下),你最好还是使用IntentService

The IntentService does the following:

• Creates a default worker thread that executes all intents delivered to onStartCommand() separate from your application's main thread.
• 创建一个默认的工作线程去把Intent传递给onStartCommand()从而和你应用的主线程分开
• Creates a work queue that passes one intent at a time to your onHandleIntent() implementation, so you never have to worry about multi-threading.
• 创建一个工作队列去初次传递你的Intent给你实现的onHandleIntent()从而让你不用担心多线程的情况
• Stops the service after all start requests have been handled, so you never have to callstopSelf().
• 在处理完所有的request被处理后自动终止Service，你不需要调用stopSelf().
• Provides default implementation of onBind() that returns null.
• 提供默认返回null的onBind()实现
• Provides a default implementation of onStartCommand() that sends the intent to the work queue and then to your onHandleIntent() implementation.
• 提供默认的onStartCommand()传递Intent到工作队列和你实现的onHandleIntent()

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.)

以上所增减的特性你只需要在onHandleIntent()做出处理工作（同时，你需要提供一个简单构造类）

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.      long endTime = System.currentTimeMillis() + 5*1000;      while (System.currentTimeMillis() < endTime) {          synchronized (this) {              try {                  wait(endTime - System.currentTimeMillis());              } catch (Exception e) {              }          }      }  }}

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(), oronDestroy(), be sure to call the super implementation, so that theIntentService can properly handle the life of the worker thread.

记得重写onCreate(),onStartCommand(), oronDestroy()一定要super.   从而IntentService可以自动处理

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

@Overridepublic 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 baseService class, which is a lot more code, but which might be appropriate if you need to handle simultaneous start requests.

//今天先翻译到这把，下次有时间补完它

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 theService 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.          long endTime = System.currentTimeMillis() + 5*1000;          while (System.currentTimeMillis() < endTime) {              synchronized (this) {                  try {                      wait(endTime - System.currentTimeMillis());                  } catch (Exception e) {                  }              }          }          // 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 forIntentService 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 anIntent (specifying the service to start) tostartService(). The Android system calls the service'sonStartCommand() method and passes it theIntent. (You should never callonStartCommand() directly.)

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

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 aPendingIntent for a broadcast (withgetBroadcast()) 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'sonStartCommand(). However, only one request to stop the service (with stopSelf() orstopService()) 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 afteronStartCommand() returns. So, the service must stop itself by calling stopSelf() or another component can stop it by callingstopService().

Once requested to stop with stopSelf() orstopService(), 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 callstopSelf(int), you pass the ID of the start request (thestartId delivered to onStartCommand()) to which your stop request corresponds. Then if the service received a new start request before you were able to callstopSelf(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 callingstopService(). Even if you enable binding for the service, you must always stop the service yourself if it ever received a call toonStartCommand().

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

Creating a Bound Service

A bound service is one that allows application components to bind to it by callingbindService() in order to create a long-standing connection (and generally does not allow components tostart it by callingstartService()).

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 donot need to stop a bound service in the way you must when the service is started throughonStartCommand()).

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 ofIBinder and is what your service must return from theonBind() callback method. Once the client receives theIBinder, 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 callsunbindService() to unbind. Once there are no clients bound to the service, the system destroys the service.

There are multiple ways to implement a bound service and the implementation is more complicated than a started service, so the bound service discussion appears in a separate document aboutBound Services.

Sending Notifications to the User

Once running, a service can notify the user of events using Toast Notifications or Status Bar Notifications.

A toast notification is a message that appears on the surface of the current window for a moment then disappears, while a status bar notification provides an icon in the status bar with a message, which the user can select in order to take an action (such as start an activity).

Usually, a status bar notification is the best technique when some background work has completed (such as a file completed downloading) and the user can now act on it. When the user selects the notification from the expanded view, the notification can start an activity (such as to view the downloaded file).

See the Toast Notifications or Status Bar Notifications developer guides for more information.

Running a Service in the Foreground

A foreground service is a service that's considered to be something the user is actively aware of and thus not a candidate for the system to kill when low on memory. A foreground service must provide a notification for the status bar, which is placed under the "Ongoing" heading, which means that the notification cannot be dismissed unless the service is either stopped or removed from the foreground.

For example, a music player that plays music from a service should be set to run in the foreground, because the user is explicitly aware of its operation. The notification in the status bar might indicate the current song and allow the user to launch an activity to interact with the music player.

To request that your service run in the foreground, call startForeground(). This method takes two parameters: an integer that uniquely identifies the notification and theNotification for the status bar. For example:

Notification notification = new Notification(R.drawable.icon, getText(R.string.ticker_text),        System.currentTimeMillis());Intent notificationIntent = new Intent(this, ExampleActivity.class);PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);notification.setLatestEventInfo(this, getText(R.string.notification_title),        getText(R.string.notification_message), pendingIntent);startForeground(ONGOING_NOTIFICATION_ID, notification);

Caution: The integer ID you give to startForeground() must not be 0.

To remove the service from the foreground, call stopForeground(). This method takes a boolean, indicating whether to remove the status bar notification as well. This method doesnot stop the service. However, if you stop the service while it's still running in the foreground, then the notification is also removed.

For more information about notifications, see Creating Status Bar Notifications.

Managing the Lifecycle of a Service

The lifecycle of a service is much simpler than that of an activity. However, it's even more important that you pay close attention to how your service is created and destroyed, because a service can run in the background without the user being aware.

The service lifecycle—from when it's created to when it's destroyed—can follow two different paths:

• A started service

  The service is created when another component calls startService(). The service then runs indefinitely and must stop itself by calling stopSelf(). Another component can also stop the service by calling stopService(). When the service is stopped, the system destroys it..

• A bound service

  The service is created when another component (a client) calls bindService(). The client then communicates with the service through an IBinder interface. The client can close the connection by calling unbindService(). Multiple clients can bind to the same service and when all of them unbind, the system destroys the service. (The service doesnot need to stop itself.)

These two paths are not entirely separate. That is, you can bind to a service that was already started withstartService(). For example, a background music service could be started by calling startService() with an Intent that identifies the music to play. Later, possibly when the user wants to exercise some control over the player or get information about the current song, an activity can bind to the service by callingbindService(). In cases like this, stopService() orstopSelf() does not actually stop the service until all clients unbind.

Implementing the lifecycle callbacks

Like an activity, a service has lifecycle callback methods that you can implement to monitor changes in the service's state and perform work at the appropriate times. The following skeleton service demonstrates each of the lifecycle methods:

public class ExampleService extends Service {    int mStartMode;       // indicates how to behave if the service is killed    IBinder mBinder;      // interface for clients that bind    boolean mAllowRebind; // indicates whether onRebind should be used    @Override    public void onCreate() {        // The service is being created    }    @Override    public int onStartCommand(Intent intent, int flags, int startId) {        // The service is starting, due to a call to startService()        return mStartMode;    }    @Override    public IBinder onBind(Intent intent) {        // A client is binding to the service with bindService()        return mBinder;    }    @Override    public boolean onUnbind(Intent intent) {        // All clients have unbound with unbindService()        return mAllowRebind;    }    @Override    public void onRebind(Intent intent) {        // A client is binding to the service with bindService(),        // after onUnbind() has already been called    }    @Override    public void onDestroy() {        // The service is no longer used and is being destroyed    }}

Note: Unlike the activity lifecycle callback methods, you arenot required to call the superclass implementation of these callback methods.

Figure 2. The service lifecycle. The diagram on the left shows the lifecycle when the service is created withstartService() and the diagram on the right shows the lifecycle when the service is created withbindService().

By implementing these methods, you can monitor two nested loops of the service's lifecycle:

• The entire lifetime of a service happens between the time onCreate() is called and the timeonDestroy() returns. Like an activity, a service does its initial setup inonCreate() and releases all remaining resources inonDestroy(). For example, a music playback service could create the thread where the music will be played inonCreate(), then stop the thread inonDestroy().

  The onCreate() andonDestroy() methods are called for all services, whether they're created bystartService() orbindService().

• The active lifetime of a service begins with a call to eitheronStartCommand() oronBind(). Each method is handed theIntent that was passed to eitherstartService() orbindService(), respectively.

  If the service is started, the active lifetime ends the same time that the entire lifetime ends (the service is still active even afteronStartCommand() returns). If the service is bound, the active lifetime ends when onUnbind() returns.

Note: Although a started service is stopped by a call to eitherstopSelf() orstopService(), there is not a respective callback for the service (there's no onStop() callback). So, unless the service is bound to a client, the system destroys it when the service is stopped—onDestroy() is the only callback received.

Figure 2 illustrates the typical callback methods for a service. Although the figure separates services that are created bystartService() from those created bybindService(), keep in mind that any service, no matter how it's started, can potentially allow clients to bind to it. So, a service that was initially started withonStartCommand() (by a client callingstartService()) can still receive a call toonBind() (when a client callsbindService()).

For more information about creating a service that provides binding, see the Bound Services document, which includes more information about the onRebind() callback method in the section about Managing the Lifecycle of a Bound Service.