什么时候需要Service呢?比如播放多媒体的时候用户启动了其他Activity这个时候程序要在后台继续播放,比如检测SD卡上文件的变化,再或者在后台记录你地理信息位置的改变等等,总之服务嘛,总是藏在后头的。
Android开发中,当需要创建在后台运行的程序的时候,就要使用到Service。
每个服务都继承Service基类。
Service 特别需要注意的是Service跟Activities是不同的(简单来说可以理解为后台与前台的区别)。 需要注意的是,如果Service已经启动了,当我们再次启动Service时,不会在执行onCreate()方法,而是直接执行onStart()方法。
启动一个Service的过程如下 :
context.startService() ->onCreate()- >onStart()->Service running
其中onCreate()可以进行一些服务的初始化工作,onStart()则启动服务。
停止一个Service的过程如下 :
context.stopService() | ->onDestroy() ->Service stop
Service是在一段不定的时间运行在后台,不和用户交互应用组件。每个Service必须在manifest中 通过<service>来声明。
可以通过contect.startservice和contect.bindserverice来启动 。
Service和其他的应用组件一样,运行在进程的主线程中。这就是说如果service需要很多耗时或者阻塞的操作,需要在其子线程中实现。
调用方式:
//1、 startService(new Intent(this, MyService.class)); bindService(new Intent(this, MyService.class), mConnection, Context.BIND_AUTO_CREATE); <service android:enabled="true" android:name=".MyService" /> //2、 startService(new Intent("com.demo.SERVICE_DEMO")); bindService(new Intent("com.demo.SERVICE_DEMO"),this.serviceConnection, BIND_AUTO_CREATE); <service android:name=".LocalService"> <intent-filter> <action android:name="com.demo.SERVICE_DEMO" /> <category android:name="android.intent.category.default" /> </intent-filter> </service>
service的两种模式(startService()/bindService()不是完全分离的):
本地服务 Local Service 用于应用程序内部:
它可以启动并运行,直至有人停止了它或它自己停止。在这种方式下,它以调用Context.startService()启动,而以调用Context.stopService()结束。它可以调用Service.stopSelf() 或 Service.stopSelfResult()来自己停止。不论调用了多少次startService()方法,你只需要调用一次stopService()来停止服务。
用于实现应用程序自己的一些耗时任务,比如查询升级信息,并不占用应用程序比如Activity所属线程,而是单开线程后台执行,这样用户体验比较好。
远程服务 Remote Service 用于android系统内部的应用程序之间:
它可以通过自己定义并暴露出来的接口进行程序操作。客户端建立一个到服务对象的连接,并通过那个连接来调用服务。连接以调用Context.bindService()方法建立,以调用 Context.unbindService()关闭。多个客户端可以绑定至同一个服务。如果服务此时还没有加载,bindService()会先加载它。
可被其他应用程序复用,比如天气预报服务,其他应用程序不需要再写这样的服务,调用已有的即可。
两种启动Service的方式有所不同。这里要说明一下的是如果你在Service的onCreate或者onStart做一些很耗时间的事情,最好在 Service里启动一个线程来完成,因为Service是跑在主线程中,他也不是一个独立的线程,而是和main用一个线程。会影响到你的UI操作或者阻塞主线程中的其他事情。
但要注意,如果直接在service中直接启动另一个Activity则会出错。原因是没有设置flag,即在声明Intent的时候要设置flag:intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK)。
服务不能自己运行 ,需要通过调用Context.startService()或Context.bindService()方法启动服务。这两个方法都可以启动Service,但是它们的使用场合有所不同。
1. 使用startService()方法启用服务,调用者与服务之间没有关连,即使调用者退出了,服务仍然运行。
如果打算采用Context.startService()方法启动服务,在服务未被创建时,系统会先调用服务的onCreate()方法,接着调用onStart()方法。
采用startService()方法启动的服务,只能调用Context.stopService()方法结束服务,服务结束时会调用onDestroy()方法。
2. 使用bindService()方法启用服务,调用者与服务绑定在了一起,调用者一旦退出,服务也就终止 ,大有“不求同时生,必须同时死”的特点。
onBind()只有采用Context.bindService()方法启动服务时才会回调该方法。该方法在调用者与服务绑定时被调用,当调用者与服务已经绑定,多次调用Context.bindService()方法并不会导致该方法被多次调用。
采用Context.bindService()方法启动服务时只能调用onUnbind()方法解除调用者与服务解除,服务结束时会调用onDestroy()方法。
Service的生命周期并不像Activity那么复杂,它只继承了onCreate(),onStart(),onDestroy()三个方法,当我们第一次启动Service时,先后调用了onCreate(),onStart()这两个方法,当停止Service时,则执行onDestroy()方法,这里需要注意的是,如果Service已经启动了,当我们再次启动Service时,不会在执行onCreate()方法,而是直接执行onStart()方法。
而启动service,根据onStartCommand的返回值不同,有两个附加的模式:
1. START_STICKY 用于显示启动和停止service。
2. START_NOT_STICKY或START_REDELIVER_INTENT用于有命令需要处理时才运行的模式。
1.不需和Activity交互的本地服务
public class LocalService extends Service { private static final String TAG = "LocalService"; @Override public IBinder onBind(Intent intent) { Log.i(TAG, "onBind"); return null; } @Override public void onCreate() { Log.i(TAG, "onCreate"); super.onCreate(); } @Override public void onDestroy() { Log.i(TAG, "onDestroy"); super.onDestroy(); } @Override public void onStart(Intent intent, int startId) { Log.i(TAG, "onStart"); super.onStart(intent, startId); } }
public class ServiceActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.servicedemo); ((Button) findViewById(R.id.startLocalService)).setOnClickListener( new View.OnClickListener(){ @Override public void onClick(View view) { // TODO Auto-generated method stub startService(new Intent("com.demo.SERVICE_DEMO")); } }); ((Button) findViewById(R.id.stopLocalService)).setOnClickListener( new View.OnClickListener(){ @Override public void onClick(View view) { // TODO Auto-generated method stub stopService(new Intent("com.demo.SERVICE_DEMO")); } }); } }
<service android:name=".LocalService"> <intent-filter> <action android:name="com.demo.SERVICE_DEMO" /> <category android:name="android.intent.category.default" /> </intent-filter> </service>
对于这类不需和Activity交互的本地服务,是使用startService/stopService的最好例子。
运行时可以发现第一次startService时,会调用onCreate和onStart,在没有stopService前,无论点击多少次startService,都只会调用onStart。而stopService时调用onDestroy。再次点击stopService,会发现不会进入service的生命周期的,即不会再调用onCreate,onStart和onDestroy。
而onBind在startService/stopService中没有调用。
对于这种case,官方的sample(APIDemo\app.LocalService)是最好的例子:
/** * This is an example of implementing an application service that runs locally * in the same process as the application. The {@link LocalServiceController} * and {@link LocalServiceBinding} classes show how to interact with the * service. * * <p>Notice the use of the {@link NotificationManager} when interesting things * happen in the service. This is generally how background services should * interact with the user, rather than doing something more disruptive such as * calling startActivity(). */ public class LocalService extends Service { private NotificationManager mNM; /** * Class for clients to access. Because we know this service always * runs in the same process as its clients, we don't need to deal with * IPC. */ public class LocalBinder extends Binder { LocalService getService() { return LocalService.this; } } @Override public void onCreate() { mNM = (NotificationManager)getSystemService(NOTIFICATION_SERVICE); // Display a notification about us starting. We put an icon in the status bar. showNotification(); } @Override public int onStartCommand(Intent intent, int flags, int startId) { Log.i("LocalService", "Received start id " + startId + ": " + intent); // We want this service to continue running until it is explicitly // stopped, so return sticky. return START_STICKY; } @Override public void onDestroy() { // Cancel the persistent notification. mNM.cancel(R.string.local_service_started); // Tell the user we stopped. Toast.makeText(this, R.string.local_service_stopped, Toast.LENGTH_SHORT).show(); } @Override public IBinder onBind(Intent intent) { return mBinder; } // This is the object that receives interactions from clients. See // RemoteService for a more complete example. private final IBinder mBinder = new LocalBinder(); /** * Show a notification while this service is running. */ private void showNotification() { // In this sample, we'll use the same text for the ticker and the expanded notification CharSequence text = getText(R.string.local_service_started); // Set the icon, scrolling text and timestamp Notification notification = new Notification(R.drawable.stat_sample, text, System.currentTimeMillis()); // The PendingIntent to launch our activity if the user selects this notification PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, LocalServiceController.class), 0); // Set the info for the views that show in the notification panel. notification.setLatestEventInfo(this, getText(R.string.local_service_label), text, contentIntent); // Send the notification. // We use a layout id because it is a unique number. We use it later to cancel. mNM.notify(R.string.local_service_started, notification); } }这里可以发现onBind需要返回一个IBinder对象。也就是说和上一例子LocalService不同的是,
public class LocalServiceBinding extends Activity { private boolean mIsBound; private LocalService mBoundService; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.local_service_binding); // Watch for button clicks. Button button = (Button)findViewById(R.id.bind); button.setOnClickListener(mBindListener); button = (Button)findViewById(R.id.unbind); button.setOnClickListener(mUnbindListener); } private ServiceConnection mConnection = new ServiceConnection() { public void onServiceConnected(ComponentName className, IBinder service) { // This is called when the connection with the service has been // established, giving us the service object we can use to // interact with the service. Because we have bound to a explicit // service that we know is running in our own process, we can // cast its IBinder to a concrete class and directly access it. mBoundService = ((LocalService.LocalBinder)service).getService(); // Tell the user about this for our demo. Toast.makeText(LocalServiceBinding.this, R.string.local_service_connected, Toast.LENGTH_SHORT).show(); } public void onServiceDisconnected(ComponentName className) { // This is called when the connection with the service has been // unexpectedly disconnected -- that is, its process crashed. // Because it is running in our same process, we should never // see this happen. mBoundService = null; Toast.makeText(LocalServiceBinding.this, R.string.local_service_disconnected, Toast.LENGTH_SHORT).show(); } }; private OnClickListener mBindListener = new OnClickListener() { public void onClick(View v) { // Establish a connection with the service. We use an explicit // class name because we want a specific service implementation that // we know will be running in our own process (and thus won't be // supporting component replacement by other applications). bindService(new Intent(LocalServiceBinding.this, LocalService.class), mConnection, Context.BIND_AUTO_CREATE); mIsBound = true; } }; private OnClickListener mUnbindListener = new OnClickListener() { public void onClick(View v) { if (mIsBound) { // Detach our existing connection. unbindService(mConnection); mIsBound = false; } } }; }明显看出这里面添加了一个名为ServiceConnection类,并实现了onServiceConnected(从IBinder获取Service对象)和onServiceDisconnected(set Service to null)。
<service android:name=".app.LocalService" /> <activity android:name=".app.LocalServiceBinding" android:label="@string/activity_local_service_binding"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.SAMPLE_CODE" /> </intent-filter> </activity>
public class MyService extends Service { private static final String TAG = "MyService"; MediaPlayer player; @Override public IBinder onBind(Intent intent) { return null; } @Override public void onCreate() { Toast.makeText(this, "My Service Created", Toast.LENGTH_LONG).show(); Log.d(TAG, "onCreate"); player = MediaPlayer.create(this, R.raw.braincandy);//运行例子是,需要替换音乐的名称 player.setLooping(false); // Set looping } @Override public void onDestroy() { Toast.makeText(this, "My Service Stopped", Toast.LENGTH_LONG).show(); Log.d(TAG, "onDestroy"); player.stop(); } @Override public void onStart(Intent intent, int startid) { Toast.makeText(this, "My Service Started", Toast.LENGTH_LONG).show(); Log.d(TAG, "onStart"); player.start(); } }
public class ServicesDemo extends Activity implements OnClickListener { private static final String TAG = "ServicesDemo"; Button buttonStart, buttonStop; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); buttonStart = (Button) findViewById(R.id.buttonStart); buttonStop = (Button) findViewById(R.id.buttonStop); buttonStart.setOnClickListener(this); buttonStop.setOnClickListener(this); } public void onClick(View src) { switch (src.getId()) { case R.id.buttonStart: Log.d(TAG, "onClick: starting srvice"); startService(new Intent(this, MyService.class)); break; case R.id.buttonStop: Log.d(TAG, "onClick: stopping srvice"); stopService(new Intent(this, MyService.class)); break; } } }
<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example" android:versionCode="1" android:versionName="1.0"> <application android:icon="@drawable/icon" android:label="@string/app_name"> <activity android:name=".ServicesDemo" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <service android:enabled="true" android:name=".MyService" /> </application> <uses-sdk android:minSdkVersion="3" /> </manifest>...