Question

I have found this code in two posts here:

String url = "content://sms/"; 
Uri uri = Uri.parse(url); 
getContentResolver().registerContentObserver(uri, true, new MyContentObserver(handler));

But I don’t know what this handler passed in the constructor of MyContentObserver is.

I want to register a ContentObserver in “content://sms//sent” to be notified when Android sends a sms, and in the documentation of ContentObserver says that the method OnChange will be called from the handled passed in the constructor.

Answer 1

A Handler is used in conjunction with a Looper to execute a queue of tasks one by one. In the case of a ContentObserver, its onChange() method might be called multiple times. To queue these calls and execute them sequentially, you need to supply a handler.

If you want onChange() to be executed in the main thread, simply create a new Handler like: Handler handler = new Handler(); and pass it.

But if you want onChange() to be executed in another thread, you first need to create a Looper for that thread using Looper.prepare();

The reason you don’t need to create a Looper for the main thread is that it is automatically created for you.

For a nice explanation of how Handler & Looper work, check this article.