GridView scrolling problem on Android

Question

this must be somethng very simple I'm overlooking, but I have the following problem (the post is rather lengthy, but I want to provide as much info as possible :) ).

I have a gridview in my Android application where each cell holds custom view:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  >
    <GridView
    android:id = "@+id/photosGridView"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" 
    android:clickable="true" 
    android:drawSelectorOnTop="true" 
    android:focusable="true" 
    android:focusableInTouchMode="true" 
    android:numColumns="6" 
    android:columnWidth="90dp"
    android:verticalSpacing="5dp"
    android:horizontalSpacing="5dp"
    android:stretchMode="columnWidth"     
    >
</GridView>

</RelativeLayout>

and each cell is

<?xml version="1.0" encoding="utf-8"?>
<com.myapp.widgets.ImageThumbView
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:background = "@android:color/transparent"
  android:paddingLeft = "1dip"
  android:paddingRight = "1dip"
  android:paddingTop = "2dip"
  android:paddingBottom = "2dip"
  >
<ImageView
    android:id="@+id/thumbImage"
    android:layout_width="fill_parent"
    android:layout_height = "fill_parent"
    android:src="@drawable/icon_small"
    /> 

  <RelativeLayout
    android:layout_width="fill_parent"
    android:layout_height = "fill_parent"
    android:background = "@android:color/transparent"
   >

  <ImageView
   android:id="@+id/iconRight"
   android:layout_width="40px"
   android:layout_height = "40px"  
   android:src="@drawable/album_check"
   android:visibility="gone"
   android:layout_alignParentTop = "true"
   android:layout_alignParentRight = "true"     
   />

  <ImageView
   android:id="@+id/iconLeft"
   android:src="@drawable/album_check"
   android:visibility="gone"
   android:layout_width = "40px"
   android:layout_height="40px"
   android:layout_alignParentTop = "true"
   android:layout_alignParentLeft = "true"     
   /> 
</RelativeLayout>

</com.myapp.widgets.ImageThumbView>

My adapter looks like this:

public class ImageAdapter extends BaseAdapter {
    private List<String> mPictures = null;

    public ImageAdapter(List<String> pictures) {
         mPictures = pictures;
    }

    public int getCount() {
          return mPictures != null ? mPictures.size() : 0;
    }
    public Object getItem(int position) {
          return mPictures != null ?  mPictures.get(position) : null;
    }
    public long getItemId(int position) {
          return mPictures != null ? position : -1;
    }
    @Override
    public View getView(int position,View convertView,ViewGroup parent) 
    {
        ImageThumbView i = null;
        try
            {               
            Thread.sleep(100);

            if (convertView == null) 
            {
                String path = mPictures.get(position);
                Log.d(((Integer)position).toString(), path);
                i = addSingleView(_li, path);
                TextView idx = (TextView) i.findViewById(R.id.caption);
                if (idx != null)
                    idx.setText(((Integer)position).toString());
            }
            else 
            {
                Log.d(((Integer)position).toString(), "ALREADY NOT NULL");
                i = (ImageThumbView) convertView;
                                    // These 2 lines were added only in desperate attempt to get it working, but it makes no difference
                String path = mPictures.get(position);
                i.updateView(path);
            }
        }
        catch(InterruptedException ie)
        {
            ie.printStackTrace();
        }
    return i;
        }
}

So initially it works properly, ie it shows first 18 images and few pixels from the next row. But when I start scrolling the gridvew, the images start to appear at random, ie after the last image I see few from the beginning and so on. Out of curiosity, I've tried few samples like this one: http://androidsamples.blogspot.com/2009/06/how-to-display-thumbnails-of-images.html ...and see the same result.

So, am I doing something wrong? why on earth would GridView display more items than it is supposed to do? and why do items appear at the wrong positions?

BR, Alex

Answer 1

Actually the problem here is that you set the content of the "convertView" within the if or else. Or you should always do that after instantiating the view if it is null and set the content only before returning the view.

Consequently, you're sure the content of the view is always the right one, updated using the position and not a false recycled view.

So your should generally speaking do the following:
(based on the tutorial from Android Guide here)

public View getView(int position, View convertView, ViewGroup parent) {
    ImageView imageView;
    if (convertView == null) { 
        imageView = new ImageView(mContext);
        //just creating the view if not already present
    } else {
        imageView = (ImageView) convertView;
        //re-using if already here
    }

    //here is the tricky part : set the content of the view out of the if, else
    //just before returning the view
    imageView.setImageResource(mThumbIds[position]);
    return imageView;
}

Answer 2

The answer is view recycling.

In general your getView should always be something of the form:

public class ImageAdapter extends BaseAdapter {

    private List<String> mUrls; // put your urls here
    private Map<String, Drawable> mImages; // cache your images here

    public ImageAdapter() {
        ...
        mUrls = new ArrayList<String>();
        mImages = new HashMap<String, Drawable>();
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder; // Use the ViewHolder pattern for efficiency

        if (convertView == null) {
            // first time this view has been created so inflate our layout
            convertView = View.inflate(this, R.layout.my_grid_item, null);
            holder = new ViewHolder();
            holder.image = convertView.findViewById(R.id.image);
            holder.text = convertView.findViewById(R.id.text);
            convertView.setTag(holder); // set the View holder
        } else {
           holder = (ViewHolder) convertView.getTag();
        }

        // update the current view - this must be done EVERY
        // time getView is called due to view recycling
        holder.text.setText(Integer.toString(position));

        // check our cache for the downloaded image
        final String url = mUrls.get(position);
        if (mImages.get(url) != null)
            holder.image.setImageDrawable(mImages.get(url));
        else
            loadImage(url, holder.image);

        // return our view
        return convertView;
    }

    public loadImage(final String url, final ImageView image) {
        // load an image (maybe do this using an AsyncTask
        // if you're loading from network
    }

    ...
}

Where your ViewHolder class would look something like

public class ViewHolder {
    ImageView thumbImage;
    TextView text;
}

Then you shouldn't run into any problems. Also I'm not sure why you needed to sleep in your getView? That will slow down scrolling of your GridView.

Answer 3

Use the standard Lazy Loader to get the images but never update onpostexecute, which causes the images to update too fast and get the randomness you don't desire.

I even used the code:

if (result != null && !mFlinging && myPosition < 12)
{   
   imageView.setImageBitmap(result);
}

int the onpostexecute which would then update the images correctly for the first screen, but if you fling the images screen then come back too quickly to the first screen get the randomness again so I now use the :

mPhotoView.setOnScrollListener(new OnScrollListener() { 



            public void onScroll(AbsListView view, int firstVisibleItem,
                    int visibleItemCount, int totalItemCount) {
                // TODO Auto-generated method stub

            }

            public void onScrollStateChanged(AbsListView view, int scrollState) {
                // TODO Auto-generated method stub
                if(scrollState != SCROLL_STATE_IDLE) {
                  mAdapter.setFlinging(true);
                } else {
                  mAdapter.setFlinging(false);   
                }

                int first = view.getFirstVisiblePosition(); 
                int count = view.getChildCount(); 

                if (scrollState == SCROLL_STATE_IDLE || (first + count > mAdapter.getCount()) ) { 
                    mPhotoView.invalidateViews(); 
                }

            } 
        }); 

to update the view.