AsyncQueryHandler's call to onQueryComplete on ui thread? ( Android )

Question

I am using AsyncQueryHandler and it calls onQueryComplete once the query is complete.

My question: is onQueryComplete called on the UI thread?

I know it does the query in the background.

Does it matter where AsyncQueryHandler is instantiated? (If instantiated in UI thread will mean onQueryComplete will be called on UI thread).

Answer 1

Is onQueryComplete called on UI thread?

It's called on the calling thread... so the thread on which you launched the AsyncQueryHandler.

Does it matter where AsyncQueryHandler is instantiated?

AsyncQueryHandler is a subclass of Handler that performs an asynchronous query (or insert, update, delete) to the ContentResolver and returns the result to the calling thread. Handlers are allowed to be associated with threads other than the main UI thread, and so are queries/transactions to the ContentResolver. Therefore, you should be allowed to call AsyncQueryHandler from a separate thread too.

That being said, doing so is probably not what you want for a couple reasons:

1. When you instantiate the AsyncQueryHandler on a separate thread, all subsequent callbacks will be returned to that calling thread. This is usually not what you want because it doesn't provide an easy means of syncing with the main UI thread, the thread that is responsible for creating your layouts and receiving your touch events.

2. You are already on a separate thread, so calls to the ContentResolver will already be asynchronous with respect to the UI thread.

And by the way, the documentation on AsyncQueryHandler doesn't mention anything about these points, but you can figure most of it out by reading the source code. The documentation for Handler (its immediate base class) explains the theory behind it too.