Android四大组件之Service

转自http://developer.android.com/guide/components/services.html 自己胡乱翻译了一部分有时间再补上吧，不当之处，请指正。

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.
Service组件能在后台执行需要长时间运行的操作而且不会提供用户界面。其他组件能启动service，而且service会保持运行即使是用户切换到了另一个应用程序。另外，一个组件可以通过绑定service来与service交互，甚至可以进行进程间通信（IPC）。例，service可以在后台处理网络操作，播放音乐，执行文件的输入输出，或者同content provider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 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.
Service有两种启动方式
一种方式是应用组件通过调用startService()来启动一个service。service一旦启动，便在后台无限期地运行，即使是用来启动它的组件已经销毁。通常，以这种方式启动的service只执行一个单一的操作并且不会返回结果给调用者。例，通过service在网络上上传或者下载一个文件。所以，当操作结束时，service应该自己停止自己。
另一种方式是应用组件通过调用bindService()来绑定一个service。一个绑定的service提供一个允许组件与service交互的如同client-server的接口，组件能够发送请求，接收返回值，甚至可以通过IPC在进程之间这样做。只要有其他组件绑定到service，service就会在运行。多个组个可以同时绑定到一个service，此时，只有当所有的组件接触绑定，service才会停止。

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.
不管你以何种方式启动service，在任何应用组件中都可以使用service（甚至是其他应用的组件），就好像在任意组件中可以通过Intent来启动一个activity一样。然而，你也可以在manifest声明service为私有的，来屏蔽来自其他应用的访问，此内容在下面的Declaring the service in the manifest仔细介绍。

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.
注意：service运行在它的宿主进程的主线程中——service并不会自动创建自己的线程也不会运行在其他进程中（除非你事先定义过，如在manifest文件中）。这意味着，如果你的service将要运行CPU密集型工作或者阻塞性操作（如MP3回放或者网络访问），你应该在service中创建一个新的线程来执行。通过新建一个线程，就减少出现ANR的风险，这样主线程就能更专注地服务于与用户的交互中。

The Basics

Should you use a service or a thread?

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.
service是一个即使用户不与你的应用交互时仍然在后台运行的组件。如果这恰是你的需要，你应该创建一个service。

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 in onStop(). Also consider using AsyncTask or HandlerThread, instead of the traditional Thread class. See the Processes and Threading document for more information about threads.
如果你需要在主线程之外或仅当用户与你的应用交互时才执行工作，你或许应该创建一个新线程而不是service。例如，你想播放音乐但仅仅是你的应用运行时，你也许应该在onCreate()中创建线程，在onStart()中启动线程，再在onStop()中停止。也可以考虑吧用AsyncTask或者HandlerThread来代替这传统的Thread类。

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.
记住，如果你确定使用service，service默认运行在你应用的主线程中，所以当你需要service来执行密集型或阻塞性操作时最好在service内创建一个新线程。

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:
要创建一个service，你必须继承Service类(或Service的子类)，在编写过程中，你需要重载一些回调函数来处理service生命周期中的活动和提供一种供其他组件绑定到service的机制，一般来说，你应该重载以下函数：

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.); 当其他组件，例如activity，通过调用startService()来启动一个service时，系统会调用此方法。一旦此方法执行，service启动并在后台无限期运行，在service的工作完成后需要你自己调用stopSelf()或stopService()来停止它。（如果你只是想提供绑定，你不必实现此方法）
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.; 当其他组件通过调用bindService()来绑定到你的service时，比如执行RPC，系统会调用此方法。在你实现此方法的过程中，你必须返回一个IBinder来提供一个供用户与service交互的接口，此方法必须重载，如果你的service不允许绑定，该方法返回null。
onCreate(): The system calls this method when the service is first created, to perform one-time setup procedures (before it calls either onStartCommand() oronBind()). If the service is already running, this method is not called.; 当service首次运行时，系统会调用此方法来执行一个一次性的配置操作（在系统调用onStartCommand()或onBind()之前）。如果service已经运行，此方法不会被调用。
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不再需要运行将要销毁时，系统会调用此方法。你的service应该实现此方法来清理不再使用的资源如线程，注册的监听器，接收器等。这是service接收的最后一个调用。

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().
如果一个组件通过startService()来启动服务（最终会调用onStartCommand函数），则service会一直运行知道service自己调用stopSelf()或其他组件调用stopService()来停止它。

If a component callsbindService() 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()来启动service，则service保持运行只要有组件绑定在其上，当组件不再绑定时，系统会自动销毁此service。

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.
仅当内存不足，系统需要回收资源来保证有用户焦点的activity运行时，才会强制停止service。如果service绑定的activity拥有用户焦点，那么它不太可能被杀掉；如果service声明为在前景运行时，那么他基本永远不会被停止。

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>

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:nameattribute 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 might break some functionality where explicit intents are used to reference your service (read the blog post, Things That Cannot Change).
<service>元素还可以定义其他性质，例如启动服务所需的许可(permissions)和service运行的进程(process)。但android:name属性是必须定义的，它指定了service的类名。一旦你发布你的应用，就不应再更改。

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

Just like an activity, a service can define intent filters that allow other components to invoke the service using implicit intents. By declaring intent filters, components from any application installed on the user's device can potentially start your service if your service declares an intent filter that matches the intent another application passes to startService().
service也可以像activity一样定义intent filters允许其他组件激活service，声明了intent filter以后，组件安装在用户设备上的其他应用的组件也能够启动你的service，只要其他应用传递给startService()的intent与service声明的intent filter匹配。

If you plan on using your service only locally (other applications do not use it), then you don't need to (and should not) supply any intent filters. Without any intent filters, you must start the service using an intent that explicitly names the service class. More information about starting a service is discussed below.
如果你只想在本应用内使用service，你便不用提供任何intent filters。未声明intent filter的话，你必须使用一个显式给出service类名的intent来启动服务。

Additionally, you can ensure that your service is private to your application only if you include the android:exportedattribute and set it to "false". This is effective even if your service supplies intent filters.
另外，你还可以通过包含android:exported属性并将其设置为false来确保你的service是应用私有的，该属性即使在你的service声明了intent filters时仍然有效。

For more information about creating intent filters for your service, see the Intents and Intent Filtersdocument.

Creating a Started Service

Targeting Android 1.6 or lower

If you're building an application for Android 1.6 or lower, you need to implement onStart(), instead of onStartCommand() (in Android 2.0,onStart() was deprecated in favor of onStartCommand()).
在Android 1.6 或更低的版本，你需要实现onStart()方法而不是onStartCommand()方法（在 Android 2.0 以后，不推荐使用onStart()）。

For more information about providing compatibility with versions of Android older than 2.0, see the onStartCommand() documentation.

A started service is one that another component starts by calling startService(), resulting in a call to the service'sonStartCommand() method.
通过调用startService()启动的service，最终会调用service中定义的onStartCommand()方法。

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().
以如此方式启动的service拥有一个独立于启动它组件的生命周期，会在后台无限期运行，即使是启动它的组件已经被销毁。这样的话，在停止service时，就需要service在完成任务后调用stopSelf()或者通过其他组件调用stopService()。

An application component such as an activity can start the service by calling startService() and passing an Intentthat specifies the service and includes any data for the service to use. The service receives this Intent in the onStartCommand() method.
一个应用组件，如activity，可以通过调用startService()并传递一个指明了service类和包含了service需要的数据intent来启动一个service。service在onStartCommand()中接收该intent。

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.
假设，一个activity需要保存一些数据到在线数据库。activity就通过传递一个包含了要保存数据的intent到startService()方法来启动一个service。service在onStartCommandCommand方法中接收该intent，然后执行连接Internet和保存数据到在线数据库的操作。当数据库操作完成后，service调用stopSelf()方法来停止自己。

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.
注意：service与声明它的应用运行在同一进程中并且默认地service运行在该进程的主线程中，因此，如果当用户与activity在交互而你的service执行密集型或阻塞性的操作时，service将会降低activity的性能(响应速度)。为避免此种情况的发生，你应该在service中新建一个线程来执行这些操作。

Traditionally, there are two classes you can extend to create a started service:
一般地，你可以扩展实现下面两个类来创建你自己的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.; 这各类是所有services的基类。当扩展实现此类时，最好在其中新建一个线程来执行service的工作，以免影响性能。
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.; IntentService是Service的子类，它用一个工作者线程来处理所有的启动请求，一次一个地。如果你不需要同时处理多个请求，这是最好的选择。扩展此类时，你只需实现onHandleIntent()方法，该方法接收没个启动请求发来的intent并在后台运行定义的工作。

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 callstopSelf().
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.
         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().
你需要实现的：一个构造器和onHandleIntent()方法。

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.
如果你决定要重载其他回调方法，如onCreate()，onStartCommand()，onDestroy()等，确保最后调用它们的超类实现，如此IntentService才能恰当的处理工作者线程的生命周期。

For example, onStartCommand() must return the default implementation (which is how the intent gets delivered to onHandleIntent()):
例如，onStartCommand必须返回其默认实现（调用父类的onStartCommand，如此intent才能传递给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.
使用IntentService使实现一个service非常简单。不过，当你需要你的service执行多线程操作时（而不是通过一个工作队列的形式来处理启动请求），就需要扩展service类来处理每个intent了。

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.
作为对比，下面代码与上面代码完成相同的做工，不同的是下面的代码使用Service类，对于每个启动请求，它使用一个工作者线程来执行工作，一次一个地。

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 =newServiceHandler(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).
上面代码中，你需要自己处理到onStartCommand()的调用，如果你想要同时处理多个请求，你可以为每一个请求创建一个线程并马上运行。

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:
onStartCommand()方法必须返回一个整型值，这个整型值决定了系统结束service后再次启动它时，该如何工作。onStartCommand()的返回值必须是下面的常量之一。

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.; 如果系统在onStartCommand()返回后杀掉service，除非有pending intent要发送，否则service不会重建。这是最安全的选项，避免你的service在不必要时运行同时你的应用也能重启那些未完成的任务。
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.; 如果系统在onStartCommand()返回后杀掉service，重建service并调用onStartCommand()，但不会重新发送最后一个intent，代替的是，除非有pending intent要发送到onStartCommand()，否则系统会以一个空 intent调用onStartCommand()。适合于多媒体播放器（或者类似的服务），无限期运行并等待命令。
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.; 如果系统在onStartCommand()返回后杀掉service，重建服务并传递最后一个发送到service的intent到onStartCommand()。其他pending intent依次发送。适合于执行需要及时恢复的任务，例如下载文件。

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().
startService()立刻返回，然后安卓系统调用service的onStartCommand方法。如果service还没有运行，系统会先调用onCreate()，然后调用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.
如果service不提供绑定，那么以startService()发送的intent就是应用组件与service沟通的唯一方式。不过，如果你想要service返回一个结果，启动服务的client可以创建一个PendingInent并将其作为启动service的intent。service可以使用broadcast返回一个结果。

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() 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().
通过startService启动的service必须自己管理它的生命周期，就是，系统不会停止或者销毁service，除非系统必须回收其资源，否则service将在onStartCommand()返回后继续运行。此时就需service调用stopSelf()或者由其他组件调用stopService()来停止。

Once requested to stop with stopSelf() or stopService(), the system destroys the service as soon as possible.
一旦介绍到来自stopSelf()或stopService()的请求，系统就会尽快地销毁service。

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 startIddelivered 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.
然而，当你的service并发的处理着多个请求时，就不应在处理完一个启动请求后停止service，因为你的service此时可能已经接受了一个新的启动请求并在处理，在第一个请求处理完成后结束service也将终结第二个请求的执行。为避免这个问题，你可以使用stopSelf(int)来确保你请求停止service是基于最近的启动请求。流程如下，当你调用stopSelf(int)时，你将启动请求的ID（即传递给onStartCommand的startId）传递给你对应的停止请求，此时如果一个新的启动请求在你调用stopSelf(int)之前被service接收，此时ID不会匹配那么service也不会停止。

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().
注意：在service完成它的任务记得要停止，避免占用系统资源和消耗电量。如果必要，其他组件也能通过stopService来停止service，即使你允许绑定service，只要它收到一个队onStartCommand()的请求，你就必须自己负责停止service。

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 callbindService() 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.

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 about Bound 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 Notificationsdeveloper 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 the Notification 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, notification);

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 does not stop the service. However, if you stop the service while it's still running in the foreground, then the notification is also removed.

Note: The methods startForeground() and stopForeground() were introduced in Android 2.0 (API Level 5). In order to run your service in the foreground on older versions of the platform, you must use the previous setForeground() method—see the startForeground() documentation for information about how to provide backward compatibility.

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 callingunbindService(). Multiple clients can bind to the same service and when all of them unbind, the system destroys the service. (The service does not need to stop itself.)

These two paths are not entirely separate. That is, you can bind to a service that was already started with startService(). 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 calling bindService(). In cases like this, stopService() or stopSelf() 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 killedIBinder mBinder;
    // interface for clients that bindboolean 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 with startService() and the diagram on the right shows the lifecycle when the service is created with bindService().

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 time onDestroy() returns. Like an activity, a service does its initial setup inonCreate() and releases all remaining resources in onDestroy(). For example, a music playback service could create the thread where the music will be played in onCreate(), then stop the thread in onDestroy().
The onCreate() and onDestroy() methods are called for all services, whether they're created by startService() or bindService().
The active lifetime of a service begins with a call to either onStartCommand() or onBind(). Each method is handed the Intent that was passed to either startService() or bindService(), respectively.
If the service is started, the active lifetime ends the same time that the entire lifetime ends (the service is still active even after onStartCommand() returns). If the service is bound, the active lifetime ends when onUnbind() returns.

Note: Although a started service is stopped by a call to either stopSelf() or stopService(), 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 by startService() from those created by bindService(), 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 with onStartCommand() (by a client calling startService()) can still receive a call to onBind() (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.

posted on 2013-03-02 15:55 夜月升阅读(463) 评论(0) 编辑收藏举报

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