Inflate a view in a background thread

Question

I have a very simple question:

Is or is it not possible to inflate a view (not add it to layout) in a background thread (ex: in the doInBackground of an AsyncTask)?

I know that it is possible, because I have implemented most Activities in my application this way and never had a problem, until I had this problem on a Galaxy S: Android: android.view.InflateException: Binary XML file line #13: Error inflating class <unknown> in SAMSUNG Galaxy S

I've been told that I should not inflate Views in background threads, but what are the specific reasons and why does my aproach work in most devices but not in Galaxy S?

Answer 1

The LayoutInflater does not make any assumptions about what thread it runs on. And nothing is mentioned about this in its documentation. Also its code seams to be thread-agnostic.

On the other hand, Views that are created by LayoutInflater might instantiate Handlers in their constructors. Well, they probably shouldn't do that, but there is no requirement for them to not create/use Handlers in their constructors.

My guess is that Samsung Galaxy S had some modifications in its EditText that somehow triggers creation of Handler (according to crash log from your other question instance of GestureDetector was instantiated which in turn created new Handler). While default implementation doesn't do this.

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Overall, I'd say that because there is no explicit requirement for Views to not use Handlers and Loopers in their constructors you can't assume inflating Views from non-UI thread is safe.

You can actually create HandlerThread and try inflating Views inside it. But I'd say this is very risky, as in Samsung Galaxy S example the view assumes that this thread will be alive during View lifetime and will process all messages using its Looper. Which might result in crash later on.

Answer 2

With latest support lib you can use android.support.v4.view.AsyncLayoutInflater to inflate views asynchronously. Be careful though that it can fallback to inflating on UI thread if specific requirements are not met:

For a layout to be inflated asynchronously it needs to have a parent whose generateLayoutParams(AttributeSet) is thread-safe and all the Views being constructed as part of inflation must not create any Handlers or otherwise call myLooper(). If the layout that is trying to be inflated cannot be constructed asynchronously for whatever reason, AsyncLayoutInflater will automatically fall back to inflating on the UI thread.