Android Service学习之本地服务

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。

 

    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。

 

生命周期

    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。

 

    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。
    如果调用startService()方法前服务已经被创建，多次调用startService()方法并不会导致多次创建服务，但会导致多次调用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。

 

看看官方给出的比较流程示意图：

 

    官方文档告诉我们，一个service可以同时start并且bind，在这样的情况，系统会一直保持service的运行状态如果service已经start了或者BIND_AUTO_CREATE标志被设置。如果没有一个条件满足，那么系统将会调用onDestory方法来终止service.所有的清理工作（终止线程，反注册接收器）都在onDestory中完成。

 

拥有service的进程具有较高的优先级

    官方文档告诉我们，Android系统会尽量保持拥有service的进程运行，只要在该service已经被启动(start)或者客户端连接(bindService)到它。当内存不足时，需要保持，拥有service的进程具有较高的优先级。

 

1． 如果service正在调用onCreate,onStartCommand或者onDestory方法，那么用于当前service的进程则变为前台进程以避免被killed。

2． 如果当前service已经被启动(start)，拥有它的进程则比那些用户可见的进程优先级低一些，但是比那些不可见的进程更重要，这就意味着service一般不会被killed.

3． 如果客户端已经连接到service (bindService),那么拥有Service的进程则拥有最高的优先级，可以认为service是可见的。

4． 如果service可以使用startForeground(int, Notification)方法来将service设置为前台状态，那么系统就认为是对用户可见的，并不会在内存不足时killed。

 

如果有其他的应用组件作为Service,Activity等运行在相同的进程中，那么将会增加该进程的重要性。

 

本地service

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);
        }
}

 

Activity:

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"));
                                        }
                                });
        }

}

 

在AndroidManifest.xml添加：

<service android:name=".LocalService">
        <intent-filter>
                <action android:name="com.demo.SERVICE_DEMO" />
                <category android:name="android.intent.category.default" />
        </intent-filter>
</service>

否则启动服务时会提示new Intent找不到"com.demo.SERVICE_DEMO"。

 

    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。

 

2.本地服务和Activity交互

    对于这种case，官方的sample(APIDemoapp.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，

1. 添加了一个public内部类继承Binder，并添加getService方法来返回当前的Service对象；

2. 新建一个IBinder对象——new那个Binder内部类；

3. onBind方法返还那个IBinder对象。

 

Activity:

/**
* <p>Example of binding and unbinding to the {@link LocalService}.
* This demonstrates the implementation of a service which the client will
* bind to, receiving an object through which it can communicate with the service.</p>
*/
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)。

    而bindService和unbindService方法都是操作这个ServiceConnection对象的。

 

AndroidManifest.xml里添加:

<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>

这里没什么特别的，因为service没有需要什么特别的action，所以只是声明service而已，而activity和普通的没差别。

 

运行时，发现调用次序是这样的：

bindService:

1.LocalService : onCreate
2.LocalService : onBind
3.Activity: onServiceConnected

unbindService: 只是调用onDestroy

 

可见，onStart是不会被调用的，而onServiceDisconnected没有调用的原因在上面代码的注释有说明。