Get scroll position in a list activity

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Ivan Soto

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Mar 23, 2009, 9:26:28 PM3/23/09
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Hi,

I'm trying to get the scroll position on a ListActivity. Here is what I'm trying to do:
I have a list activity that is populated from an XML file. Another thread downloads all pictures so the user can see the list while the thread is still downloading the files.

What I'm trying to do is when the thread finishes getting all pics it reads the listview scroll position, refresh the listview and scroll to that position again.

ListView main = getListView(); <-- not sure if I'm really getting the ListView this way.

int scY = main.getScrollY();
Log.d("Scroll", scY + " ");  <--- this is printing zero even when at the moment I run this I already scrolled the list.
setListAdapter(listadapter);
main.scrollTo(0, scY);

Any ideas?

Thanks!


Ivan Soto Fernandez
Web Developer
http://ivansotof.com

Mark Murphy

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Mar 23, 2009, 9:30:48 PM3/23/09
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Ivan Soto wrote:
> I'm trying to get the scroll position on a ListActivity. Here is what
> I'm trying to do:
> I have a list activity that is populated from an XML file. Another
> thread downloads all pictures so the user can see the list while the
> thread is still downloading the files.
>
> What I'm trying to do is when the thread finishes getting all pics it
> reads the listview scroll position, refresh the listview and scroll to
> that position again.

Have you considered using placeholder images instead? Replace the
placeholder images with the real ones as they come in. Then, when the
background thread finishes, you do not have to do anything -- the images
are already loaded. This eliminates the need to "refresh the listview"
and any accompanying confusing such a refresh might cause the user.

--
Mark Murphy (a Commons Guy)
http://commonsware.com
Android App Developer Books: http://commonsware.com/books.html

Romain Guy

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Mar 23, 2009, 9:38:59 PM3/23/09
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ListView does not use scrollY. There's no need to save the scroll
anyway because ListView does it for you.
--
Romain Guy
Android framework engineer
roma...@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

Ivan Soto

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Mar 23, 2009, 10:03:15 PM3/23/09
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But when I refresh the listview with setlistadapter(listadapter) it returns to the top. 

Mark:
Do you have any article/tutorial about the placeholder images to share? I'm trying to find one with no luck.

Thanks for helping.


Ivan Soto Fernandez
Web Developer
http://ivansotof.com



Romain Guy

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Mar 23, 2009, 10:48:08 PM3/23/09
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Don't refresh it with setListAdapter! Just modify the content of the adapter instead.

On Mar 23, 2009 7:03 PM, "Ivan Soto" <ivan...@gmail.com> wrote:

But when I refresh the listview with setlistadapter(listadapter) it returns to the top. 

Mark:
Do you have any article/tutorial about the placeholder images to share? I'm trying to find one with no luck.

Thanks for helping.

Ivan Soto Fernandez Web Developer http://ivansotof.com

On Mon, Mar 23, 2009 at 7:38 PM, Romain Guy <roma...@google.com> wrote:

> > > ListView does not use scrollY. There's no need to save the scroll > anyway because ListView do...

--

> Romain Guy > Android framework engineer > roma...@android.com > > Note: please don't send priva...




--~--~---------~--~----~------------~-------~--~----~ You received this message because you are sub...

Mark Murphy

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Mar 24, 2009, 8:06:33 AM3/24/09
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Ivan Soto wrote:
> Do you have any article/tutorial about the placeholder images to share?
> I'm trying to find one with no luck.

I have used the technique, but not in code I'm allowed to share. I do
need to more formally write this up at some point, but I do not have
anything immediately handy.

The gist of it is that you create your adapter and set it up, in
getView() or newView()/bindView() (depending on adapter choice), to see
if the thumbnail has been downloaded. If so, use it for the list row
being inflated/updated; if not, leave the ImageView in the row pointing
to some placeholder Drawable resource. This means as the user scrolls,
she will pick up the thumbnails. Also, at the end, you can quickly
iterate over the rows (ListView is a ViewGroup, IIRC, so there are
methods to iterate its children) and ensure each of those rows'
ImageViews are using their associated thumbnails.

Streets Of Boston

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Mar 24, 2009, 10:13:22 AM3/24/09
to Android Developers
I've done the same in my apps for ListView (whether they be in
ListActivity or in a plain Activity) with good success.

I use the java.util.concurrent's ExecutorService to obtain images:

1. Your getView(..) (or bindView/newView/etc) needs to assign an image/
thumbnail (bitmap) to an ImageView. But you don't have the bitmap yet.
If you do have it (store in a limited size cache), just set it .
2. If not, obtain a FutureTask from the ExecutorService and this
FutureTask then will download the image, create a thumbnail from it
and creates a Bitmap from this thumbnail. Remember the id of the image
(can be an id, Uri, URL, whatever, as long as it is unique) and assign
it to the ImageView (setTag()).
3. When ready, the FutureTask will 'post' back to the main-thread that
it has an new thumbnail.
4. On the 'post'-back, loop through the children of ListView, get the
appropriate ImageView, the one whose tag (getTag()) is equal to the
one that FutureTask you got the image for, assign the Bitmap to this
ImageView. This is it.

For myself I created a sub-system of ExecutorService and FutureTask,
called 'Cancelable' tasks, which make it easier to cancel queued up
tasks when they're no longer necessary. But this is an optimization.
> Mark Murphy (a Commons Guy)http://commonsware.com

Streets Of Boston

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Mar 24, 2009, 10:37:18 AM3/24/09
to Android Developers
Here is a code-snippet. I may not compile, but i think you'll get the
idea :-)

[code]
ExecutorService EXECUTOR = Executors.newFixedThreadPool(3); // max 3
worker threads.
...
public View getView(final AbsListView listView, View convertView, int
pos, long id) {

...
...

final String imgUrl = ...
...
final ImageView imgView = ...
...
Bitmap bm = mBitmapCache.get(imgUrl);
if (bm != null)
imgView.setImageBitmap(bm);
else {
imgView.setTag(imgUrl);
FutureTask<?> task = EXECUTOR.submit(new Runnable() {
public void run() {
final Bitmap newBM = getImageFrom(imgUrl); // you have to
write this method
if (newBM == null)
return;

// be sure that mBitmapCache is thread-safe.
mBitmapCache.put(imgUrl, newBM);

// instead of 'listView', you could use a new Handler
instance.
listView.post(new Runnable() {
public void run() {
String checkUrl = (String)imgView.getTag();
if (checkUrl != null && !checkUrl.equals(imgUrl))
return;

imgView.setImageBitmap(newBM);
}
});
}

private Bitmap getImageFrom(String url) {
// download the data from imgUrl.
// create a bitmap from it.
return bitMap;
}
});
}

// if you want, you can hold on to 'task' and call 'cancel' on it if
necessary, e.g.
// when this convertView is about to be re-used and to be assigned
to a different image.
return convertView;
}
[/code]

On Mar 24, 10:13 am, Streets Of Boston <flyingdutc...@gmail.com>
wrote:
> > Android App Developer Books:http://commonsware.com/books.html- Hide quoted text -
>
> - Show quoted text -

Ivan Soto

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Mar 24, 2009, 10:38:26 AM3/24/09
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Thanks for explaining.
Right now I'm using the efficient list method used on the ApiDemos so when the user scrolls it will refreshes the images that didn't load before. But if you don't move anything, even if the image is downloaded it won't redraw in the imageview.

I will start reading about that FutureTask/ExecutorService


Ivan Soto Fernandez
Web Developer
http://ivansotof.com



Ivan Soto

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Mar 24, 2009, 10:47:55 AM3/24/09
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I think that's what I'm looking for. Can't wait to give it a try after work. Although, I have another thread in my main Activity downloading all pictures. 

Thanks, I will give it a try.


Ivan Soto Fernandez
Web Developer
http://ivansotof.com



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