As soon as you change the count you must notifyDatasetChanged().
On Fri, Nov 20, 2009 at 10:59 PM, Robert Woodruff <woodman...@gmail.com> wrote:
> Hi Romain,
>
>
>
> Thanks for the info. I think you are saying that the small example below,
> all of which runs on the UI thread, will cause the exception to be thrown?
> Right?
>
>
>
>> 1. Change the count in the base adapter
>
>> 2. notifyDataSetChanged() // Which captures a count and will react some
>> time in the future
>
>> 3. Immediately change the count in the base adapter, again
>
>
>
> Even though the count is only ever changed in the UI thread? It may explain
> why the exception is being thrown in one of my apps by ListView even though
> the adapter count is only ever changed in the UI thread.
>
>
>
> Say there is a thread running, in an AsyncTask, gathering data to be placed
> in the adapter. It reaches a point to report that data is to be added to the
> Adapter. The AsyncTask.doInBackground() calls publishProgress() which
> queues a Handler on the UI thread to eventually call onProgressUpdate().
>
>
>
> Eventually the onProgressUpdate() Handler runs on the UI thread and causes
> the adapter count to change. But notifyDataSetChanged() was called before
> the publishProgress () Handler runs and the Handler runs before the ListView
> Handler comes around to update the list. So ListView throws an exception
> because it sees a different count than when notifyDataSetChanged() was
> called. But the Adapter count was changed always on the UI thread.
>
>
>
> This brings back the question: When is it safe to change the count in a
> BaseAdapter()? It seems like there should be a semaphore or listener or
> something that says "Now it is OK to change the count/Now it is NOT OK to
> change the count" Or maybe notifyDataSetChanged() could tell the ListView it
> to suspend operations, update the count and continue operations.
>
>
>
> If I am not too far off base with this it would be good to know when it is
> safe to change the adapter count.
>
>
>
> Thank you,
>
> On Fri, Nov 20, 2009 at 4:14 PM, Romain Guy <romain...@google.com> wrote:
>>
>> > 1. Change the count in the base adapter
>> > 2. notifyDataSetChanged() // Which will react some time in the future
>> > 3. immediately change the count in the base adapter, again
>>
>> > Or another way: when does the ListView grab the BaseAdapter count? Does
>> > it
>> > grab it on the call to notifyDataSetChanged() or whenever it later
>> > starts
>> > the update operation?
>>
>> It grabs it in notifyDatasetChanged(). Also, the count change must
>> happen on the UI thread.
>>
>> >
>> > Thank you,
>> >
>> > On Thu, Nov 19, 2009 at 2:24 PM, Dianne Hackborn <hack...@android.com>
>> > wrote:
>> >>
>> >> As the exception says:
>> >>
>> >> throw new IllegalStateException("The content of the
>> >> adapter has changed but "
>> >> + "ListView did not receive a notification.
>> >> Make
>> >> sure the content of "
>> >> + "your adapter is not modified from a
>> >> background
>> >> thread, but only "
>> >> + "from the UI thread. [in ListView(" + getId()
>> >> +
>> >> ", " + getClass()
>> >> + ") with Adapter(" + mAdapter.getClass() +
>> >> ")]");
>> >>
>> >> You can't modify an adapter from a background thread; this must be done
>> >> on
>> >> the main thread.
>> >>
>> >> This exception is not adding a restriction; it is making it more
>> >> obvious
>> >> when you do something that would always break in weird, subtle, and/or
>> >> horrible ways.
>> >>
>> >> On Thu, Nov 19, 2009 at 5:56 AM, WoodManEXP <woodman...@gmail.com>
>> >> wrote:
>> >>>
>> >>> For Google about BaseAdapter class
>> >>>
>> >>> In Android 1.6 and 2.0 the BaseAdapter class has apparently been
>> >>> modified to throw an exception when it sees the Adapter.getCount()
>> >>> method return a number different than what it picked up when
>> >>> BaseAdapter.notifyDataSetChanged() was called (at least I did not
>> >>> observe this behavior in pre 1.6).
>> >>>
>> >>> So now the question is, when should one call notifyDataSetChanged() or
>> >>> more importantly, when can the count in the Adapter be safely changed?
>> >>>
>> >>> Consider the case of an app with a background thread delivering data
>> >>> to a BaseAdapter at a rate faster than the ListView responds to
>> >>> notifyDataSetChanged(). Because notifyDataSetChanged() is a
>> >>> synchronous call and the ListView updates are taking place sometime in
>> >>> the indeterminate future on the UI thread, how can an app know when it
>> >>> is safe to change the adapter count?
>> >>>
>> >>> If the app changes the count while the ListView is updating itself the
>> >>> exception will be thrown (and there is no way to catch the exception).
>> >>>
>> >>> Is there a way to know when it is safe to change the count in the
>> >>> BaseAdapter?
>> >>>
>> >>> --
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>> >>
>> >>
>> >>
>> >> --
>> >> Dianne Hackborn
>> >> Android framework engineer
>> >> hack...@android.com
>> >>
>> >> Note: please don't send private questions to me, as I don't have time
>> >> to
>> >> provide private support, and so won't reply to such e-mails. All such
>> >> questions should be posted on public forums, where I and others can see
>> >> and
>> >> answer them.
>> >>
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>>
>>
>> --
>> Romain Guy
>> Android framework engineer
>> romain...@android.com
>>
>> Note: please don't send private questions to me, as I don't have time
>> to provide private support. All such questions should be posted on
>> public forums, where I and others can see and answer them
>>
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>
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--
Romain Guy
Android framework engineer
romain...@android.com
Note: please don't send private questions to me, as I don't have time
to provide private support. All such questions should be posted on
public forums, where I and others can see and answer them
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