Service官方教程(1)Started与Bound的区别、要实现的函数、声明service

Services 简介和分类

　　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.

　started类型的service一但启动，一直在后台运行，直到自己停止或被停止，生命周期与启动者无关，不与启动者通信。

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.

　Bound类型的service生命随绑定者存在，当有多个时，直到最后一个解绑才销毁。它是c-s中的s,通常与c通信。

　　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 重要函数

　　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 (声明一个service)

　　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.

　两种类型的Service都要以精确型Intent启动。

　　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.