服务—Android

服务部依赖于任何用户界面，即使程序被切换到后台，或者用户打开了另外一个应用程序。服务仍然能够保持运行。

当某个应用程序进程被杀死后，所依赖与该进程的服务也会停止运行。

服务并不会自动开启线程，所有的代码都是默认运行在主线程当中，也就是说我们需要在服务内部手动创建子线程，并在这里执行具体任务。

Android的ui是线程不安全的。也就是说，如果想要更新应用程序里的ui元素，必须在主线程中进行，否则会出现异常。

这是一个基本的服务。BInder对象是服务和活动通讯的桥梁。

一旦调用了startService方法，相应的服务就会起来，并回调onStartCommond（）方法。如果这个服务之前还没有被创建过，oncreate方法会先于onStartCommand()方法执行，服务启动之后一直处与运行的状态，直到调用一次stopService或者stopself方法。每个服务都只存在一个实例，不管你调用多少次startService，只需调用一次stopService方法或者stopSelf方法。服务就会停止。

另外，如果调用bindService方法来获取一个服务的持久连接，这时就会回调onBInd方法。如果这个服务还没有被创建过，onCreate方法会吸纳与onBInd方法执行。调用方法后可返回实例。

调用了bindService方法后用调用startService方法，那么要同时调用stopService方法和unbindService方法。onDestroy方法才能执行。

package com.example.servicetest;import android.app.Notification;import android.app.PendingIntent;import android.app.Service;import android.content.Intent;import android.os.Binder;import android.os.IBinder;import android.util.Log;public class MyService extends Service {private DownloadBinder mBinder = new DownloadBinder();class DownloadBinder extends Binder {public void startDownload() {new Thread(new Runnable() {@Overridepublic void run() {// start downloading}}).start();Log.d("MyService", "startDownload executed");}public int getProgress() {Log.d("MyService", "getProgress executed");return 0;}}@Overridepublic IBinder onBind(Intent intent) {Log.d("MyService", "onBind executed");return mBinder;}@Overridepublic void onCreate() {super.onCreate();Notification notification = new Notification(R.drawable.ic_launcher,"Notification comes", System.currentTimeMillis());Intent notificationIntent = new Intent(this, MainActivity.class);PendingIntent pendingIntent = PendingIntent.getActivity(this, 0,notificationIntent, 0);notification.setLatestEventInfo(this, "This is title", "This is content",pendingIntent);startForeground(1, notification);Log.d("MyService", "onCreate executed");}@Overridepublic int onStartCommand(Intent intent, int flags, int startId) {Log.d("MyService", "onStartCommand executed");new Thread(new Runnable() {          @Override          public void run() {              // do something here        }      }).start();return super.onStartCommand(intent, flags, startId);}@Overridepublic void onDestroy() {super.onDestroy();Log.d("MyService", "onDestroy executed");}}

@Overridepublic void onClick(View v) {switch (v.getId()) {case R.id.start_service:Intent startIntent = new Intent(this, MyService.class);startService(startIntent);break;case R.id.stop_service:Intent stopIntent = new Intent(this, MyService.class);stopService(stopIntent);break;case R.id.bind_service:Intent bindIntent = new Intent(this, MyService.class);bindService(bindIntent, connection, BIND_AUTO_CREATE);break;case R.id.unbind_service:unbindService(connection);break;case R.id.start_intent_service:Log.d("MainActivity", "Thread id is " + Thread.currentThread().getId());Intent intentService = new Intent(this, MyIntentService.class);startService(intentService);break;default:break;}}

后台执行的定时任务，这里执行一个打印时间的例子。

MainActivity.java  在这里启动服务

package com.example.servicebestpractice;import android.app.Activity;import android.content.Intent;import android.os.Bundle;import android.view.View;public class MainActivity extends Activity {View view;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);Intent intent = new Intent(this, LongRunningService.class);startService(intent);}}

LongRunningService.java  设置定时器，和广播类型。系统会在下个溢出时间广播pi。

package com.example.servicebestpractice;import java.util.Date;import android.app.AlarmManager;import android.app.PendingIntent;import android.app.Service;import android.content.Intent;import android.os.IBinder;import android.os.SystemClock;import android.util.Log;public class LongRunningService extends Service {@Overridepublic IBinder onBind(Intent intent) {return null;}@Overridepublic int onStartCommand(Intent intent, int flags, int startId) {new Thread(new Runnable() {@Overridepublic void run() {Log.d("LongRunningService", "executed at " + new Date().toString());}}).start();AlarmManager manager = (AlarmManager) getSystemService(ALARM_SERVICE);int anHour = 60 * 1000;long triggerAtTime = SystemClock.elapsedRealtime() + anHour;Intent i = new Intent(this, AlarmReceiver.class);PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, 0);    manager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, triggerAtTime, pi);Log.d("LongRunningService", "2");return super.onStartCommand(intent, flags, startId);}}

AlarmReceiver.java  广播接收器，接收到广播信号后，会启动服务。

package com.example.servicebestpractice;import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;import android.util.Log;public class AlarmReceiver extends BroadcastReceiver {@Overridepublic void onReceive(Context context, Intent intent) {Log.d("LongRunningService", "1");Intent i = new Intent(context, LongRunningService.class);context.startService(i);}}