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() {
				@Override
				public void run() {
					// start downloading
				}
			}).start();
			Log.d("MyService", "startDownload executed");
		}

		public int getProgress() {
			Log.d("MyService", "getProgress executed");
			return 0;
		}

	}

	@Override
	public IBinder onBind(Intent intent) {
		Log.d("MyService", "onBind executed");
		return mBinder;
	}

	@Override
	public 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");
	}

	@Override
	public 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);
	}

	@Override
	public void onDestroy() {
		super.onDestroy();
		Log.d("MyService", "onDestroy executed");
	}

}

@Override
	public 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;
	
	@Override
	protected 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 {

	@Override
	public IBinder onBind(Intent intent) {
		return null;
	}

	@Override
	public int onStartCommand(Intent intent, int flags, int startId) {
		new Thread(new Runnable() {
			@Override
			public 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 {

	@Override
	public void onReceive(Context context, Intent intent) {
		Log.d("LongRunningService", "1");
		Intent i = new Intent(context, LongRunningService.class);
		context.startService(i);
		
	}

}