Get scroll position in a list activity

by Ivan Soto » Tue, 24 Mar 2009 09:26:45 GMT


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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 

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Get scroll position in a list activity

by Mark Murphy » Tue, 24 Mar 2009 09:31:03 GMT


 


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 

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Get scroll position in a list activity

by Romain Guy » Tue, 24 Mar 2009 09:39:42 GMT


 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
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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Get scroll position in a list activity

by Ivan Soto » Tue, 24 Mar 2009 10:03:22 GMT


 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 








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Get scroll position in a list activity

by Romain Guy » Tue, 24 Mar 2009 10:48:18 GMT


 Don't refresh it with setListAdapter! Just modify the content of the adapter
instead.




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 




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Get scroll position in a list activity

by Mark Murphy » Tue, 24 Mar 2009 20:06:49 GMT


 


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.

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

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Get scroll position in a list activity

by Streets Of Boston » Tue, 24 Mar 2009 22:13:29 GMT


 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.





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Get scroll position in a list activity

by Streets Of Boston » Tue, 24 Mar 2009 22:37:34 GMT


 ere 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, 10:13am, Streets Of Boston <flyingdutc...@gmail.com>
wrote:
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Get scroll position in a list activity

by Ivan Soto » Tue, 24 Mar 2009 22:38:45 GMT


 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 



On Tue, Mar 24, 2009 at 8:13 AM, Streets Of Boston







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Get scroll position in a list activity

by Ivan Soto » Tue, 24 Mar 2009 22:48:03 GMT


  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



On Tue, Mar 24, 2009 at 8:37 AM, Streets Of Boston
<flyingdutc...@gmail.com>wrote:


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