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I am trying to create an application that contains Content Observers in a remote service but kept getting the error:

"Can’t create handler inside thread that has not called Looper prepare"

I have done much reading on the Android Developers site as well as many Google searches I am still not fully understanding the problem. What I've taken from it so far is that the Content Observer needs a thread to run on (for the Handler). After adding:

Looper.myLooper().prepare();
mObserver = new Observer(new Handler());

a Looper call immediately before creating a new Content Observer makes the application work correctly. However I would like a better understanding of why this is - nothing I have found online has help me grasp why thus far. Also, is adding the Looper line of code the correct and efficient thing to do? And is there any cleanup I need to do with this to prevent memory leaks later down the road?

The simpler the explanation the better. Still in my first month of Android Development and making good progress - StackOverflow has been a tremendous help! Thanks.

UPDATE
After playing around and doing more reading I think have a better grasp on how to make this work and how to clean it up but still missing one small part.

From what I've gathered I can call Looper.prepare() as a static method. I can then create my Content Observer using the new Handler(). When I want to stop this Content Observer I can get the thread of the object with mLooper = Looper.myLooper.getThread() and then I can run a mLooper.quit(). However must I and when would I and why would I call mLooper.loop()?

UPDATE 2
Okay so here the problem I am addressing. The application is in house and is a little complicated to explain but here is my best shot:

We have a remote service running that receives and sends network data. Our message types are defined in the data so we can direct them to the correct database on the phone. There is a Jar interface file for other APK's to import to connect to the service running. These APK's can register to receive broadcasts when a new message they are interested in is received. To do this a content observer is registered with the content uri the APK uses thus when new data is added to the database a the content observer will send out a broadcast to alert the correct APK. The APK can then handle this however it wishes.

So would it be okay to use null as the parameter of a new Content Observer or would it be better to use new Handler() to create a thread for this observer.

I have tried creating a thread but so far it is not working, I think its because I have not started the Looper.loop (this freezes my service). Any advice would be greatly appreciated! Thanks in advance.

jjNford
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1 Answers1

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When you start Looper in a thread, other threads gain the ability to send messages to it and it will receive and process them. This scheme is widely used in Android: looper is initialized on UI thread and other threads are assigned to perform some lenghty operations. Those 'worker' threads can send the results of their hard labor to UI thread by posting messages to it's Handler.

And, yes, you need to call Looper.loop() in order to get things running. Otherwise your handler will never receive any events. And it seems like you'll have to invoke Looper.quit() in order to stop it and avoid leaking resources.

And here's the funny part. Judging by the ContentObserver's source code, it does not exactly require a Handler. If you pass null, it will simply execute onChange() on the same thread on which it runs.

P.S. My dev machine is down so I have no way to verify my statements. Maybe current version of ContentObserver does require Handler to work. Experiment will show.

Ash
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  • Putting null in my code made it work! I think its because I never said Looper.loop() to get everything started. I will explain more in an update to my question. This works... but I would rather have the onChange running in its own thread due to traffic. – jjNford Oct 06 '11 at 15:53
  • Let me rephrase your update#2. You have a database as a hub for storing messages which are then received by different clients in separate APKs, and ContentObserver does the dispatching, correct? – Ash Oct 06 '11 at 16:54
  • Correct. There are actually a few databases, but the incoming message will carry the ID for the DB that it belongs to. I want this to run as efficiently as possible with no log and figured threading as much as possible would be the best thing. Any ideas? – jjNford Oct 06 '11 at 16:58
  • The simpliest option would be to initialize `ContentObserver` in clients' Activities (or Services, they both have Looper by default) with a new `Handler` instance. This way your client classes will receive messages from Observer with minimal overhead. – Ash Oct 06 '11 at 18:36
  • Yea, this is the route I initially went but it will not work for me. When the client apk dies so does the content observer. Due to context of the application I cannot rely on this not happening. – jjNford Oct 11 '11 at 12:40