问题
I'm trying to work out if bound service is appropriate for doing background work in my app. The requirements are that various application components can make web requests through it of varying priority. (Thus the service must maintain some sort of queue and be able to cancel it's ongoing requests for others of higher priority). I'd like the service to be relatively unobtrusive to the user such that they don't find it running after they are done with the application - if I want to do something more important that continues while the application is closed I can use startForeground() to push a notification during the process.
Solution the first: bind from the activity
So, for a given application component it should be able to bind to the service to get work done. But there seems to be a well known problem that if an activity is doing the binding, the binding will be lost during configuration change (rotation) as the activity will be closed.
So, I was thinking I could use another context that I create (new Context()) and bind from that to the service, then use a non-UI fragment to maintain this context across config changes until I deem that I am finished with it. I could do this only during the configuration change or as a permanent alternative to binding from the activity. (I should probably point out that this is a standard and recommended way to maintain instances across config changes)
Solution numero 2:
The main alternative I see is that I can use the application context to do the binding - but could this persist too long? and/or could there be some cyclic relationship between the app context and the service thus preventing the service and the app context being destroyed?
Questions:
So the question I'm trying to answer to myself is: should I use the first method (activities with temporary contexts)? Or the second (just bind service to the app context)?
Am I right in thinking the app context can bind to the service multiple times and then unbind from it the same number of times? (I.e. that you can have multiple valid bindings PER context)?
Could using my own context (new Context()) in the first solution cause any issues?
Edit
Found some more information: https://groups.google.com/forum/#!topic/android-developers/Nb58dOQ8Xfw
It also seems that it will difficult to 'create' a context arbitrarily so a combination of solution 1 and 2 seems appropriate where the service connection is maintained across the configurations change but the binding is to the app context. I am still concerned about the possibility of unbinding twice from the app context. Keeping count of the bindings myself seems unnecessary - can anyone confirm/deny that bindings are per connection and not per context?
回答1:
So after doing some digging I think I have come up with an (as yet) untested solution.
Firstly, based on Diane's suggestion here: https://groups.google.com/forum/#!topic/android-developers/Nb58dOQ8Xfw I should be binding to the application context - so my problem of losing the context is gone - I can maintain my ServiceConnection across configuration changed with a Non-UI fragment - great. Then when I am done I can use the app context to hand back the service connection and unbind. I shouldn't receive any leaky service connection warnings. (I should probably point out that this is a standard and recommended way to maintain instances across config changes)
The final crux of this problem was I was unsure of whether I could bind multiple times from the same context - the documentations on bindings imply there is some dependence between the binding and the context's lifecycle and so I was worried I would have to do my own form of reference counting. I had a look at the source code and ended up here: http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.4.2_r1/android/app/LoadedApk.java#LoadedApk.forgetServiceDispatcher%28android.content.Context%2Candroid.content.ServiceConnection%29
Crucially, these lines:
sd = map.get(c);
if (sd != null) {
map.remove(c);
sd.doForget();
if (map.size() == 0) {
mServices.remove(context);
}
Reveal that the map is being used for the reference counting I was worried about.
SO the take home is this:
- Bound service will work fine with the app context and we SHOULD do this to prevent leaking a service connection from one activity to another during a config change
- I can keep my service connection on a non-UI fragment safely and use it to unbind when I am done
I'll try and post some tested code soon.
UPDATE and tested solution: I've made some code to test this and published here: https://github.com/samskiter/BoundServiceTest
It seems to work quite well and the non-ui fragment (data fragment) acts as a nice proxy listener during rotation changes to catch results from the service (the intention of the listeners is to closely bind the requests to the UI in order to guarantee it stays responsive. Obviously any model changes can be propagated to the UI via observers.)
Edit: I thought I should explicitly answer the questions in the OP...
should I use the first method (activities with temporary contexts)? Or the second (just bind service to the app context)? The second
Am I right in thinking the app context can bind to the service multiple times and then unbind from it the same number of times? (I.e. that you can have multiple valid bindings PER context)? Yes
Could using my own context (new Context()) in the first solution cause any issues? This is not even possible
A final summary:
This pattern should be pretty powerful - I can prioritise network IO (or other tasks) coming from a variety of sources across my app. I could have a foreground activity making some small io the user has asked for, simultaneously I could have kicked of a foreground service to sync all my users data. Both the foregrounds service and the activity can be bound to the same Network service to get their requests done.
All this while making sure the service lives only exactly as long as it needs to - i.e. it plays nicely with android.
I'm excited to get this into an app soon.
UPDATE: I've tried to write this up and give some context to the wider problem of background work in a blog entry here: http://blog.airsource.co.uk/2014/09/10/android-bound-services/
回答2:
Can you not just select configurations that you would like to handle with the configChanges attribute in your manifest and do the orientation changes in the UI manually? In this case you only need to bind to the service in onCreate and then unBind in onDestroy.
or maybe try something like this ( I have not done proper error checking):
class MyServiceConnection implements ServiceConnection,Parcelable {
public static final Parcelable.Creator CREATOR
= new Parcelable.Creator() {
public MyServiceConnection createFromParcel(Parcel in) {
return new MyServiceConnection(in);
}
public MyServiceConnection[] newArray(int size) {
return new MyServiceConnection[size];
}
};
@Override
public int describeContents() {
return 0;
}
@Override
public void writeToParcel(Parcel dest, int flags) {
}
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
}
@Override
public void onServiceDisconnected(ComponentName name) {
}
}
MyServiceConnection myServiceConnection;
boolean configChange = false;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState != null) {
myServiceConnection = savedInstanceState.getParcelable("serviceConnection");
} else {
myServiceConnection = new MyServiceConnection();
}
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
if (myServiceConnection != null) {
outState.putParcelable("serviceConnection",myServiceConnection);
configChange = true;
}
}
@Override
protected void onDestroy() {
super.onDestroy();
if (!configChange && myServiceConnection != null){
unbindService(myServiceConnection);
}
}
}
回答3:
There is a much easier way to handle this situation called an IntentService which you can read more about here. From the android site:
"The IntentService class provides a straightforward structure for running an operation on a single background thread. This allows it to handle long-running operations without affecting your user interface's responsiveness. Also, an IntentService isn't affected by most user interface lifecycle events, so it continues to run in circumstances that would shut down an AsyncTask"
Rather than binding your service to your activity you can start long-running actions on a background thread by simply using an intent that starts your IntentService
public class RSSPullService extends IntentService {
@Override
protected void onHandleIntent(Intent workIntent) {
// Gets data from the incoming Intent
String dataString = workIntent.getDataString();
...
// Do work here, based on the contents of dataString
...
}
}
This is an example taken from the android docs. You would send an intent with the relevant data then handle that data within the service to do what you want. For example, you could just add a priority flag to your intent so your service knows which requests come before others.
The benefit of an intent service is that it runs on a background thread and is not tied to the lifecycle of the starting activity. That means you configuration changes should not have an effect on the service execution.
When your service is done you can report work status by using a local broadcast - either sending the results directly back to the activity (via broadcast receiver) or possibly even through onNewIntent() (though getting that to work is a bit more clunky.
Edit - answer questions in comment
IntentService is a relatively small class. This makes it easy to modify. The stock code for IntentService calls stopSelf() and dies when it runs out of work to do. This can be easily fixed. Examining the source for the IntentService (see the previous link) you can see that it pretty much works off a queue already, receiving messages in onStart() and then executing them in the order received as described in the comment. Overriding onStart() will allow you to implement a new queue structure to meet your needs. Use the example code there for how to handle the incoming message and get the Intent then just create your own data structure for handling priority. You should be able to start/stop your web requests in the IntentService the same way you would in a Service. Thus by overriding onStart() and onHandleIntent() you should be able to do what you want.
回答4:
I had a similar problem, where I have a Bound Service used in an Activity. Inside the activity I define a ServiceConnection, mConnection, and inside onServiceConnected I set a class field, syncService that's a reference to the Service:
private SynchronizerService<Entity> syncService;
(...)
/** 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
Log.d(debugTag, "on Service Connected");
LocalBinder binder = (LocalBinder) service;
//HERE
syncService = binder.getService();
//HERE
mBound = true;
onPostConnect();
}
@Override
public void onServiceDisconnected(ComponentName arg0) {
Log.d(debugTag, "on Service Disconnected");
syncService = null;
mBound = false;
}
};
Using this method, whenever the orientation changed I would get a NullPointerException when referencing the syncService variable, despite the fact the service was running, and I tried several methods that never worked.
I was about to implement the solution proposed by Sam, using a retained fragment to keep the variable, but first remembered to try a simple thing: setting the syncService variable to static.. and the connection reference is maintained when the orientation changes!
So now I have
private static SynchronizerService<Entity> syncService = null;
...
/** 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
Log.d(debugTag, "on Service Connected");
LocalBinder binder = (LocalBinder) service;
//HERE
if(syncService == null) {
Log.d(debugTag, "Initializing service connection");
syncService = binder.getService();
}
//HERE
mBound = true;
onPostConnect();
}
@Override
public void onServiceDisconnected(ComponentName arg0) {
Log.d(debugTag, "on Service Disconnected");
syncService = null;
mBound = false;
}
};
来源：https://stackoverflow.com/questions/24312016/bind-to-service-from-new-context-for-configuration-changes-or-bind-from-app-cont