ContentObserver onChange

Question

The documentation of the ContentObserver is not clear to me. On which thread is the onChange of a ContentObserver called?

I checked and it is not the thread where you created the observer. It looks like it is the thread that sends the notification but I didn't find a documentation about it.

Answer 1

The Thread that the ContentObserver.onChange() method is executed on is the ContentObserver constructor's Handler's Looper's Thead.

For example, to have it run on the main UI thread, the code may look like this:

// returns the applications main looper (which runs on the application's 
// main UI thread)
Looper looper = Looper.getMainLooper();

// creates the handler using the passed looper
Handler handler = new Handler(looper);

// creates the content observer which handles onChange on the UI thread
ContentObserver observer = new MyContentObserver(handler);

Alternatively, to have it run on a new worker thread, the code may look like this:

// creates and starts a new thread set up as a looper
HandlerThread thread = new HandlerThread("MyHandlerThread");
thread.start();

// creates the handler using the passed looper
Handler handler = new Handler(thread.getLooper());

// creates the content observer which handles onChange on a worker thread
ContentObserver observer = new MyContentObserver(handler);

Or even to have it run on the current thread, the code may look like this. Generally this is not what you want because a Thread that is looping can't do much else because Looper.loop() is a blocking call. Nevertheless:

// prepares the looper of the current thread
Looper.prepare();

// creates a handler for the current thread's looper.
Handler handler = new Handler();

// creates the content observer which handles onChange on this thread
ContentObserver observer = new MyContentObserver(handler);

// starts the current thread's looper (blocking call because it's 
// looping, and handling messages forever). the content observer will
// only execute the onChange method while the thread is looping; 
// interrupting Looper.loop() would "break" the content observer.
Looper.loop();

Answer 2

In order to assure that the onChange is called on the UI thread, use the correct handler when registering to it:

Handler handler = new Handler(Looper.getMainLooper());
ContentObserver observer = new MyContentObserver(handler);
...

Answer 3

This old question seems to touch on your issue:

How to observe contentprovider change? android

Looks like you should create a new thread, running in its own Service, where the onChange method will be called.