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  1. Android 13: Services and Content Providers Kirk Scott

  2. 13.1 Introduction • 13.2 Services

  3. 13.1 Introduction • This unit belongs to the third third of the course • It covers topics that might be of some interest when developing your final project

  4. 13.2 Services

  5. Services • 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.

  6. 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:

  7. [1] 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.

  8. [2] 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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:

  18. [1] 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.)

  19. [2] 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.

  20. [3] 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.

  21. [4] 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.

  22. 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().

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. 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:

  28. <manifest ... >  ...  <application ... >      <service android:name=".ExampleService" />      ...  </application></manifest>

  29. 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.

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

  31. 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().

  32. 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.

  33. Additionally, you can ensure that your service is private to your application only if you include the android:exported attribute and set it to "false". • This is effective even if your service supplies intent filters. • For more information about creating intent filters for your service, see the Intents and Intent Filters document.

  34. 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.

  35. 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().

  36. 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.

  37. 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.

  38. 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.

  39. Traditionally, there are two classes you can extend to create a started service: • [1] 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.

  40. [2] 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.

  41. 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.

  42. 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.

  43. 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.

  44. 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: