一直是听过其大名，现在终于是学到了。
1、什么是Service
作为android四大组件之一，它一直默默付出，不像Activity，可以被人所见。Service一般进行长时间的后台操作，没有界面，不是进程也不是线程，比avtivity有更高的优先级。使用Service，有两步。
- 定义一个继承Service的子类
- 在AndroidManifest中注册该Service
Service有一些可以重写的方法：
IBinder onBind(Intent intent)：这是必须重写的方法，当有一个客户端和Service绑定时，返回的对象可以作为communication channel（我不知道怎么翻译）和Service通信，如果没有绑定，返回null。IBinder是一个接口，但是一般不直接实现，而是继承IBinder的实现类Binder。void onCreate()：第一次创建Service后会被调用。onStartCommand(Intent intent, int flags, int startId)：客户端调用startService(Intent intent)时会被调用。取代了原本的方法onStart(Intent intent, int startId)void onDestroy()：Service关闭之前会调用的方法
Service的生命周期
说完了service的一些基本知识，下面看下它的生命周期：
从图中可以看出Service有两个运行方式，一个是startService()，另一个是bindService()。所以下面我用两种方式运行Service来看一下它的运行过程
2、启动和停止service
写好Service的子类之后，然后写了两个按钮，为其添加点击事件如下
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.start:
startService(new Intent(MusicButtonActivity.this, MusicService.class));
break;
case R.id.stop:
stopService(new Intent(MusicButtonActivity.this, MusicService.class));
break;
}
}然后Service的代码如下，使用MediaPlayer播放了一段放在res\raw下的音乐
public class MusicService extends Service{
private static final String TAG = MusicService.class.getSimpleName();
//private MyBinder mMyBinder = new MyBinder();
@Override
public void onCreate() {
super.onCreate();
Log.i(TAG, "onCreate");
mMediaPlayer = MediaPlayer.create(this, R.raw.my);
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Log.i(TAG, "onStartCommand");
mMediaPlayer.start();
return START_NOT_STICKY;
}
@Override
public void onDestroy() {
super.onDestroy();
Log.i(TAG, "onDestroy");
mMediaPlayer.stop();
}
@Nullable
@Override
public IBinder onBind(Intent intent) {
Log.i(TAG, "onBind");
return mMyBinder;
}
@Override
//客户端和Service解绑时回调的方法
public boolean onUnbind(Intent intent) {
Log.i(TAG, "onUnbind");
return super.onUnbind(intent);
}
}
按下start，然后stop，可以看到后台打印
I/MusicService: onCreate
I/MusicService: onStartCommand
I/MusicService: onDestroy按下start，然后再按start，后台打印如下：
I/MusicService: onCreate
I/MusicService: onStartCommand
I/MusicService: onStartCommand可见onCreate()只会调用一次，当Service已经启动后，之后就只会调用onStartCommand()方法。
以上使用Service的方法，Activity和Service基本上没有什么关联，两者之间无法进行通信，交换数据什么的。所以想实现通信应该用bindService()和unBindService()
3、绑定Service
bindService()里面有三个参数boolean bindService(Intent service, ServiceConnection conn, int flags)
- 第一个就是通过Intent启动指定的Service。
- 第二个是ServiceConnection对象，监听访问者(clients)和Service的连接情况，我们可以看一下它的代码
private ServiceConnection mServiceConnection = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
System.out.println("connected");
}
@Override
public void onServiceDisconnected(ComponentName name) {
System.out.println("disconnected");
}
};当访问者和Servive连接成功时会回调onServiceConnected(ComponentName name, IBinder service)其中service就是onBinder(Intent intent)返回的IBinder对象，通过这个IBinder对象与Service就可以进行通信。另外onServiceDisconnected(ComponentName name)只会在异常断开和Service连接时，才会调用该方法，正常断开不会回调
- 最后一个指定绑定的时候如果还没有创建Service，是否自动创建Service，0为不创建，BIND_AUTO_CREATE为自动创建。
在上个代码上修改，在activity里得到歌曲的总时长（毫秒）。
public class MusicService extends Service{
private static final String TAG = MusicService.class.getSimpleName();
private MyBinder mMyBinder = new MyBinder();
private MediaPlayer mMediaPlayer;
@Override
public void onCreate() {
super.onCreate();
Log.i(TAG, "onCreate");
mMediaPlayer = MediaPlayer.create(this, R.raw.my);
mMediaPlayer.start();
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Log.i(TAG, "onStartCommand");
// mMediaPlayer.start();
return START_NOT_STICKY;
}
@Override
public void onDestroy() {
super.onDestroy();
Log.i(TAG, "onDestroy");
mMediaPlayer.stop();
}
@Nullable
@Override
public IBinder onBind(Intent intent) {
Log.i(TAG, "onBind");
return mMyBinder;
}
@Override
public boolean onUnbind(Intent intent) {
Log.i(TAG, "onUnbind");
return super.onUnbind(intent);
}
public class MyBinder extends Binder{
public int getProgress(){
return mMediaPlayer.getDuration();
}
}
}上面onBind()方法返回了一个可以得到歌曲总时长的Binder对象，该对象将会传给访问该Service的访问者。
在activity里的代码：
public class MusicButtonActivity extends AppCompatActivity implements View.OnClickListener{
private Button mStop_button;
private Button mStart_button;
MusicService.MyBinder mBinder;
private ServiceConnection mServiceConnection = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
System.out.println("connected");
//得到传入的对象
mBinder = (MusicService.MyBinder) service;
//得到了歌曲的时长
int a = mBinder.getProgress();
}
@Override
public void onServiceDisconnected(ComponentName name) {
System.out.println("disconnected");
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_music_button);
mStart_button = (Button) findViewById(R.id.start);
mStop_button = (Button) findViewById(R.id.stop);
mStart_button.setOnClickListener(this);
mStop_button.setOnClickListener(this);
}
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.start:
//startService(new Intent(MusicButtonActivity.this, MusicService.class));
bindService(new Intent(MusicButtonActivity.this, MusicService.class), mServiceConnection, Service.BIND_AUTO_CREATE);
break;
case R.id.stop:
unbindService(mServiceConnection);
//stopService(new Intent(MusicButtonActivity.this, MusicService.class));
break;
}
}
}点击start，后台打印
MusicService: onCreate
MusicService: onBind点击stop，后台打印
08-07 15:59:01.430 14412-14412/com.example.myactionbardemo I/MusicService: onCreate
08-07 15:59:01.485 14412-14412/com.example.myactionbardemo I/MusicService: onBind
08-07 15:59:04.333 14412-14412/com.example.myactionbardemo I/MusicService: onUnbind
08-07 15:59:04.334 14412-14412/com.example.myactionbardemo I/MusicService: onDestroy
通过这种方法可以对Service的数据进行我们需要的操作。
除了以上的两种情况，如果在Service已经启动的情况下，再去绑定的话，这个时候如果只是先解绑，在stop才能把Service销毁
08-07 16:01:29.551 17091-17091/com.example.myactionbardemo I/MusicService: onCreate
08-07 16:01:29.596 17091-17091/com.example.myactionbardemo I/MusicService: onStartCommand
08-07 16:01:29.599 17091-17091/com.example.myactionbardemo I/MusicService: onBind
08-07 16:01:40.265 17091-17091/com.example.myactionbardemo I/MusicService: onUnbind
08-07 16:01:40.275 17091-17091/com.example.myactionbardemo I/MusicService: onDestroy4、IntentService
我们开篇说过Service不是一个线程，它其实运行在主线程里，所以它里面不能处理一个耗时操作，当Service处理耗时操作超过20秒时，将会出现ANR(application not responding)异常，这是不能允许的。如果确实想在Service处理耗时操作，建议另开一条线程处理。除此之外，还可以使用Service的子类IntentService。如下面的代码
public class MyIntentService extends IntentService{
/**
* Creates an IntentService. Invoked by your subclass's constructor.
*
* @param name Used to name the worker thread, important only for debugging.
*/
public MyIntentService(String name) {
super(name);
}
@Override
protected void onHandleIntent(Intent intent) {
}
}看了下IntentService的源码
public abstract class IntentService extends Service {
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;
private String mName;
private boolean mRedelivery;
//处理Intent请求
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
onHandleIntent((Intent)msg.obj);
stopSelf(msg.arg1);
}
}
@Override
//创建了一个单独的线程
public void onCreate() {
// TODO: It would be nice to have an option to hold a partial wakelock
// during processing, and to have a static startService(Context, Intent)
// method that would launch the service & hand off a wakelock.
super.onCreate();
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
//发送处理intent请求的消息
@Override
public void onStart(Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
/**
* You should not override this method for your IntentService. Instead,
* override {@link #onHandleIntent}, which the system calls when the IntentService
* receives a start request.
* @see android.app.Service#onStartCommand
*/
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
onStart(intent, startId);
return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}
@Override
public void onDestroy() {
mServiceLooper.quit();
}
/**
* Unless you provide binding for your service, you don't need to implement this
* method, because the default implementation returns null.
* @see android.app.Service#onBind
*/
@Override
public IBinder onBind(Intent intent) {
return null;
}
/**
* This method is invoked on the worker thread with a request to process.
* Only one Intent is processed at a time, but the processing happens on a
* worker thread that runs independently from other application logic.
* So, if this code takes a long time, it will hold up other requests to
* the same IntentService, but it will not hold up anything else.
* When all requests have been handled, the IntentService stops itself,
* so you should not call {@link #stopSelf}.
*
* @param intent The value passed to {@link
* android.content.Context#startService(Intent)}.
*/
@WorkerThread
protected abstract void onHandleIntent(Intent intent);
}
IntentService会创建一个HandlerThread这样的线程来处理所有的Intent请求，而我们只用重写void onHandleIntent(Intent intent)。IntentService就会帮我们处理。