ListView getChildAt returning null for visible children

I'm getting some strange behavior from a listview/the getChildAt method.

I have a HashSet, iconsToUpdate, of icons that have been changed in the database. I want to iterate over the visible rows to see if any of their icons need to be updated to reflect the new icons. I don't need to test the icons not currently in view as they will be drawn properly when rendered.

My problem is that getChildAt is returning null when it seems like it shouldn't. I know that getChildAt can only return views that are currently visible, but it is returning null for some of the visible rows.

Here is my code that iterates over the visible rows:

Logger.debug("First visible index: " + f_listView.getFirstVisiblePosition());
Logger.debug("Last visible index: " + f_listView.getLastVisiblePosition());
for (int i = f_listView.getFirstVisiblePosition(); i <= f_listView.getLastVisiblePosition(); i++) {
    String tag = "asdf"; // Remove when bug is fixed.
    if (f_listView == null) {
        Logger.debug("f_listView is null");
    } else if (f_listView.getChildAt(i) == null) {
        Logger.debug("Child at index " + i + " is null");
    } else {
        tag = (String) f_listView.getChildAt(i).getTag();
        Logger.debug("Successful at index " + i + ", tag is: " + tag);
    }
    if (iconsToUpdate.contains(tag)) {
        setIcon(i, f_aim.getInHouseIcon(tag));
    }
}

Here is the log corresponding to a run of this loop:

D/...: First visible index: 3
D/...: Last visible index: 8
D/...: Successful at index 3, tag is: ...
D/...: Successful at index 4, tag is: ...
D/...: Successful at index 5, tag is: ...
D/...: Child at index 6 is null
D/...: Child at index 7 is null
D/...: Child at index 8 is null

It should be noted that the first and last visible indexes are being correctly reported, as I am viewing rows 3-8 when I run this. Rows 6, 7, 8 are being rendered properly. How are they being displayed if they are null?

Also, I do not know if this is important, but row 5 is the last visible row when I am at the top of the listview.

Any info as to why these rows are being returned as null would be greatly appreciated.

Thanks!

Solution 1:

listView.getChildAt(i) works where 0 is the very first visible row and (n-1) is the last visible row (where n is the number of visible views you see).

The get last/first visible return the position in the dataAdapter you have. So you since you start at position 3, with what looks like 6 visible views, that's when get for positions 6-8 you get null.

In your example getChildAt(0) would return position 3. What I usually do is store the position on my views so I can lookup on my dataAdapter later if I need values.

I think your for loop should look like this:

for (int i = 0; i <= f_listView.getLastVisiblePosition() - f_listView.getFirstVisiblePosition(); i++) 

Hope this helps.

Solution 2:

Try

f_listView.getChildAt(positionOfChildYouWantGet - f_listView.getFirstVisiblePosition());

Solution 3:

When you call listView1.getLastVisiblePosition() and listView1.getFirstVisiblePosition(), the listview returns the positions of the items that are partially visible in the listview. For example, the first item may be half visible and the last item may be half visible. In this case, even though you can see part of the item in the listview, the adapter has not yet called the getView() function for the item and therefore the item is still considered null.

Solution 4:

In my case i have a listView and the first item is full visible but when I do getFirstVisiblePosition() the result is 1 !

It gets more weird when I scroll and make the first item half visible then my getView show that getFirstVisiblePosition() is 0.

this my code :

public View getView(final int position, View convertView, final ViewGroup parent) {
        row = convertView;
        final AtomPaymentHolder holder = new AtomPaymentHolder();

        LayoutInflater inflater = ((Activity) context).getLayoutInflater();
        row = inflater.inflate(layoutResourceId, parent, false);
        row.setTag(items.get(position));
        holder.songitem = items.get(position);
        final int firstPosition = MainActivity.listviewSong.getFirstVisiblePosition() - MainActivity.listviewSong.getHeaderViewsCount(); 
        final int lastPosition = MainActivity.listviewSong.getLastVisiblePosition();

        if(MusicService.indexService>= firstPosition && MusicService.indexService<= lastPosition){
            MainActivity.listviewSong.getChildAt(MusicService.indexService-firstPosition).setBackgroundColor(getContext().getResources().getColor(R.color.pressed_color));
        }

(when I selected the next item and scroll, it works good)