android系列8.Service学习
Service的启动有两种方式：context.startService() 和 context.bindService()。
startService();调用者和服务之间没有联系，即使调用者退出了，服务仍然进行
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
如果Service还没有运行，则android先调用onCreate()然后调用onStart()；如果Service已经运行，则只调用onStart()，所以一个Service的onStart方法可能会重复调用多次。
stopService的时候直接onDestroy，如果是调用者自己直接退出而没有调用stopService的话，Service会一直在后台运行。该Service的调用者再启动起来后可以通过stopService关闭Service。
所以调用startService的生命周期为：onCreate --> onStart(可多次调用) --> onDestroy
1.编写类继承Service
2.重写onCreate()，onStart(Intent intent, int startId)，onStartCommand(Intent intent, int flags, int startId)，onDestroy()
3.manifest文件中声明服务
<service android:name=".Service"/>
4.启动服务
5.关闭服务
控制service运行的主要方式有两种，主要是根据onStartCommand方法返回的数值。方法：
2、START_NOT_STICKY or START_REDELIVER_INTENT
这里主要解释这三个变量的意义：
1、 START_STICKY
在运行onStartCommand后service进程被kill后，那将保留在开始状态，但是不保留那些传入的intent。不久后service就会再次尝试重新创建，因为保留在开始状态，在创建 service后将保证调用onstartCommand。如果没有传递任何开始命令给service，那将获取到null的intent
2、 START_NOT_STICKY
在运行onStartCommand后service进程被kill后，并且没有新的intent传递给它。Service将移出开始状态，并且直到新的明显的方法（startService）调用才重新创建。因为如果没有传递任何未决定的intent那么service是不会启动，也就是期间onstartCommand不会接收到任何null的intent。
3、 START_REDELIVER_INTENT
在运行onStartCommand后service进程被kill后，系统将会再次启动service，并传入最后一个intent给onstartCommand。直到调用stopSelf(int)才停止传递intent。如果在被kill后还有未处理好的intent，那被kill后服务还是会自动启动。因此onstartCommand不会接收到任何null的intent。
客户端也可以使用bindService来保持跟service持久关联。谨记：如果使用这种方法，那么将不会调用onstartCommand（跟startService不一样，下面例子注释也有解析，大家可试试）。客户端将会在onBind回调中接收到IBinder接口返回的对象。通常IBinder作为一个复杂的接口通常是返回aidl数据。
2.BoudService()
bindService():调用者和服务绑在一起，调用者一旦退出服务也就终止
context.bindService()->onCreate()->onBind()->Service running
onUnbind() -> onDestroy() ->Service stop
onBind将返回给客户端一个IBinder接口实例，IBind允许客户端回调服务的方法，比如得到Service运行的状态或其他操作。这个时候把调用者（Context，例如Activity）会和Service绑定在一起，Context退出了，Srevice就会调用onUnbind->onDestroy相应退出。
所以调用bindService的生命周期为：onCreate --> onBind(只一次，不可多次绑定) --> onUnbind --> onDestory。
在Service每一次的开启关闭过程中，只有onStart可被多次调用(通过多次startService调用)，其他onCreate，onBind，onUnbind，onDestory在一个生命周期中只能被调用一次。
BoundService三种方式
1.扩展Binder类
Extending the Binder class
If your service is private to your own application and runs in the same process as the client (which is common), you should create your interface by extending the Binder class and returning an instance of it from onBind(). The client receives the Binder and can use it to directly access public methods available in either the Binder implementation or even the Service.
This is the preferred technique when your service is merely a background worker for your own application. The only reason you would not create your interface this way is because your service is used by other applications or across separate processes.
public class LocalService extends Service { // Binder given to clients private final IBinder mBinder = new LocalBinder(); // Random number generator private final Random mGenerator = new Random(); /** * Class used for the client Binder. 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 this instance of LocalService so clients can call public methods return LocalService.this; } } @Override public IBinder onBind(Intent intent) { return mBinder; } /** method for clients */ public int getRandomNumber() { return mGenerator.nextInt(100); } }
public class BindingActivity extends Activity { LocalService mService; boolean mBound = false; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); } @Override protected void onStart() { super.onStart(); // Bind to LocalService Intent intent = new Intent(this, LocalService.class); bindService(intent, mConnection, Context.BIND_AUTO_CREATE); } @Override protected void onStop() { super.onStop(); // Unbind from the service if (mBound) { unbindService(mConnection); mBound = false; } } /** Called when a button is clicked (the button in the layout file attaches to * this method with the android:onClick attribute) */ public void onButtonClick(View v) { if (mBound) { // Call a method from the LocalService. // However, if this call were something that might hang, then this request should // occur in a separate thread to avoid slowing down the activity performance. int num = mService.getRandomNumber(); Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show(); } } /** Defines callbacks for service binding, passed to bindService() */ private ServiceConnection mConnection = new ServiceConnection() { @Override public void onServiceConnected(ComponentName className, IBinder service) { // We've bound to LocalService, cast the IBinder and get LocalService instance LocalBinder binder = (LocalBinder) service; mService = binder.getService(); mBound = true; } @Override public void onServiceDisconnected(ComponentName arg0) { mBound = false; } }; }
2.使用Messenger
3.使用AIDL(Android Interface Definition Language)
