第二十八章 后台服务
有关IntentService
IntentService服务能够按照顺序执行命令队列里面的命令，这些命令就是IntentService服务需要响应（onHandleIntent）的intent。需要声明这个元素节点：
<service android:name=".PollService"/>
有关后台网络连接安全
在后台连接网络时，需要使用ConnectivityManager确认网络连接是否可用，首先获取权限：
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
然后根据网络服务是否可用(connectivityManager.getActiveNetworkInfo())、网络是否连得上(connectivityManager.getActiveNetworkInfo().isConnected())来确定是否应该启用服务：
@Override
protected void onHandleIntent(@Nullable Intent intent) {
if (!isNetworkAvailableAndConnected())
{
return;
}
...
}
private boolean isNetworkAvailableAndConnected()
{
ConnectivityManager connectivityManager=(ConnectivityManager)getSystemService(CONNECTIVITY_SERVICE);
boolean isNetWorkAvailable=connectivityManager.getActiveNetworkInfo()!=null;
boolean isNetworkConnected=isNetWorkAvailable&&connectivityManager.getActiveNetworkInfo().isConnected();
return isNetworkConnected;
}
有关使用AlarmManager延迟运行服务
PendingIntent打包
PendingIntent.getService()方法打包了一个Context.startService(Intent)方法的调用：
Intent i=PollService.newIntent(context);
PendingIntent pendingIntent=PendingIntent.getService(context,0,i,0);
这个方法有四个参数，第一个是用来发送intent的context，第二个是区分PendingIntent来源的请求码，第三个是待发送的Intent对象，第四个是如何创建PendingIntent的标识符。第四个参数的一个例子如下：
Intent intent=PollService.newIntent(context);
PendingIntent pendingIntent=PendingIntent.getService(context,0,intent, PendingIntent.FLAG_NO_CREATE);
PendingIntent.FLAG_NO_CREATE实际上并不做的发送这个任务，它只是用以判断定时器的启停状态。
AlarmManager定时
AlarmManager.setRepeating()方法是设置定时器，这个方法也有四个参数，第一个是描述定时器事件基准的常量，第二个是定时器启动的时间，第三个是定时器循环的时间间隔，第四个是要发送的PendingIntent。
AlarmManager alarmManager=(AlarmManager)context.getSystemService(Context.ALARM_SERVICE); alarmManager.setRepeating(AlarmManager.ELAPSED_REALTIME, SystemClock.elapsedRealtime(),POLL_INTERVAL_MS,pendingIntent);
取消定时器是AlarmManager.cancel()，参数就是要取消的PendingIntent：
alarmManager.cancel(pendingIntent);
有关通知信息
想要发送通知消息，首先要创建Notification对象，可以用构造器来创建，完整的Notification需要有：tick text、图标、通知信息本身，待触发的PendingIntent。
Notification notification=new NotificationCompat.Builder(this)
.setTicker(resources.getString(R.string.new_pictures_titles))
.setSmallIcon(android.R.drawable.ic_menu_report_image)
.setContentTitle(resources.getString(R.string.new_pictures_titles))
.setContentText(resources.getString(R.string.new_picture_text))
.setContentIntent(pi)
.setAutoCancel(true)
.build();
发送通知信息需要调用NotificationManager的notify方法，构造NotificationManager的方式是从当前的context中取出有关NotificationManagerCompat实例：
Notification notification=new NotificationCompat.Builder(this)
.setTicker(resources.getString(R.string.new_pictures_titles))
.setSmallIcon(android.R.drawable.ic_menu_report_image)
.setContentTitle(resources.getString(R.string.new_pictures_titles))
.setContentText(resources.getString(R.string.new_picture_text))
.setContentIntent(pi)
.setAutoCancel(true)
.build();
NotificationManagerCompat notificationManager= NotificationManagerCompat.from(this);
notificationManager.notify(0,notification);
有关挑战练习 在Lollipop设备上使用JobService
我原先误解了这个题设的意思，我以为是在onStartJob方法中选择启动的方式，导致我卡了很长的时间都没有解决方案。参考了网上的解法之后，我发现挑战练习之前的介绍就是实现的这一题的过程。
public class PollJobService extends JobService：
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
public class PollJobService extends JobService {
private PollTask mCurrentTask;
private static final String TAG="PollJobService";
@Override
public boolean onStartJob(JobParameters jobParameters) {
mCurrentTask=new PollTask();
mCurrentTask.execute(jobParameters);
return true;
}
private class PollTask extends AsyncTask<JobParameters,Void,Void>
{
@Override
protected Void doInBackground(JobParameters... jobParameters) {
JobParameters jobParams=jobParameters[0];
PollFilckr();
jobFinished(jobParams,false);
return null;
}
}
@Override
public boolean onStopJob(JobParameters jobParameters) {
if (mCurrentTask!=null)
{
mCurrentTask.cancel(true);
}
return true;
}
public static void setServiceAlarm(Context context, boolean shouldStartAlarm)
{
final int JOB_ID = 1;
JobScheduler scheduler = (JobScheduler) context.getSystemService(Context.JOB_SCHEDULER_SERVICE);
if(shouldStartAlarm){
scheduler.cancel(JOB_ID);
}else{
JobInfo jobInfo = new JobInfo.Builder(JOB_ID, new ComponentName(context, JobService.class))
.setRequiredNetworkType(JobInfo.NETWORK_TYPE_UNMETERED)
.setPeriodic(1000 * 60*15)
.setPersisted(true)
.build();
scheduler.schedule(jobInfo);
}
}
public static boolean isHasBeenScheduled(Context context){
final int JOB_ID = 1;
JobScheduler scheduler = (JobScheduler)context.getSystemService(JOB_SCHEDULER_SERVICE);
boolean hasBeenScheduled = false;
for (JobInfo jobInfo : scheduler.getAllPendingJobs()) {
if (jobInfo.getId() == JOB_ID) {
hasBeenScheduled = true;
}
}
return hasBeenScheduled;
}
private void PollFilckr()
{
String query=QueryPreferences.getStoredQuery(PollJobService.this);
String lastResultId=QueryPreferences.getPrefLastResultId(PollJobService.this);
List<GalleryItem> items;
if (query==null)
{
items=new FlickrFetchr().fetchRecentPhotos(1);
}
else
{
items=new FlickrFetchr().searchPhotos(query);
}
if (items.size()==0)
{
return;
}
String resultId=items.get(0).getID();
if (resultId.equals(lastResultId))
{
Log.i(TAG,"Got an old result: "+resultId);
}
else {
Log.i(TAG,"Got a new result: "+resultId);
Resources resources=getResources();
Intent i=PhototGalleryActivity.newIntent(PollJobService.this);//就是一个context对象
PendingIntent pi=PendingIntent.getActivity(PollJobService.this,0,i,0);
Notification notification=new NotificationCompat.Builder(PollJobService.this)
.setTicker(resources.getString(R.string.new_pictures_titles))
.setSmallIcon(android.R.drawable.ic_menu_report_image)
.setContentTitle(resources.getString(R.string.new_pictures_titles))
.setContentText(resources.getString(R.string.new_picture_text))
.setContentIntent(pi)
.setAutoCancel(true)
.build();
NotificationManagerCompat notificationManager= NotificationManagerCompat.from(PollJobService.this);
notificationManager.notify(0,notification);
}
QueryPreferences.setPrefLastResultId(PollJobService.this,resultId);
}
}
在fragment中的菜单项目中选择使用的服务：
@Override
public void onCreateOptionsMenu(@NonNull Menu menu, @NonNull MenuInflater inflater) {
super.onCreateOptionsMenu(menu, inflater);
inflater.inflate(R.menu.fragment_photo_gallery,menu);
MenuItem searchItem=menu.findItem(R.id.menu_item_search);
final SearchView searchView=(SearchView)searchItem.getActionView();
searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
@Override
public boolean onQueryTextSubmit(String s) {
Log.i(TAG,"QueryTextSumbit: "+s);
QueryPreferences.setStoredQuery(getActivity(),s);
updateItems();
searchView.onActionViewCollapsed();
return true;
}
@Override
public boolean onQueryTextChange(String s) {
Log.i(TAG,"QueryTextChange: "+s);
return false;
}
});
searchView.setOnSearchClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
String query=QueryPreferences.getStoredQuery(getActivity());
searchView.setQuery(query,false);
}
});
MenuItem toggleItem =menu.findItem(R.id.menu_item_toggle_polling);
if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP){
if(PollJobService.isHasBeenScheduled(getActivity())){
toggleItem.setTitle(R.string.stop_polling);
}else {
toggleItem.setTitle(R.string.start_polling);
}
}else{
if (PollService.isServiceAlarmOn(getActivity())) {
toggleItem.setTitle(R.string.stop_polling);
} else {
toggleItem.setTitle(R.string.start_polling);
}
}
}
@Override
public boolean onOptionsItemSelected(@NonNull MenuItem item) {
switch (item.getItemId())
{
case R.id.menu_item_clear:
QueryPreferences.setStoredQuery(getActivity(),null);
updateItems();
return true;
case R.id.menu_item_toggle_polling:
if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP){
boolean shouldStartAlarm = PollJobService.isHasBeenScheduled(getActivity()); //启动服务
PollJobService.setServiceAlarm(getActivity(),shouldStartAlarm);
getActivity().invalidateOptionsMenu();
}else{
boolean shouldStartAlarm = !PollService.isServiceAlarmOn(getActivity());
PollService.setServiceAlarm(getActivity(),shouldStartAlarm);
getActivity().invalidateOptionsMenu(); //刷新菜单项
}
return true;
default:
return super.onOptionsItemSelected(item);
}
}