add preloading to photo list

library/src/com/bumptech/glide/ListPreloader.java

@@ -90,8 +90,10 @@ public abstract class ListPreloader<T> implements AbsListView.OnScrollListener {
 
     private void preload(List<T> items, int position) {
         final T item = items.get(position);
-        int[] dimens = getDimens(item);
-        getRequest(item).into(preloadTargetQueue.next(dimens[0], dimens[1])).with(context);
+        final int[] dimens = getDimens(item);
+        if (dimens != null) {
+            getRequest(item).into(preloadTargetQueue.next(dimens[0], dimens[1])).with(context);
+        }
     }
 
     private void cancelAll() {

samples/flickr/src/com/bumptech/glide/samples/flickr/FlickrPhotoList.java

 package com.bumptech.glide.samples.flickr;
 
+import android.content.Context;
 import android.os.Bundle;
 import android.view.LayoutInflater;
 import android.view.View;
@@ -10,6 +11,7 @@ import android.widget.ListView;
 import android.widget.TextView;
 import com.actionbarsherlock.app.SherlockFragment;
 import com.bumptech.glide.Glide;
+import com.bumptech.glide.ListPreloader;
 import com.bumptech.glide.loader.model.Cache;
 import com.bumptech.glide.samples.flickr.api.Photo;
 
@@ -28,6 +30,7 @@ public class FlickrPhotoList extends SherlockFragment implements PhotoViewer {
     private FlickrPhotoListAdapter adapter;
     private List<Photo> currentPhotos;
     private Cache<URL> urlCache = new Cache<URL>();
+    private FlickrListPreloader preloader;
 
     public static FlickrPhotoList newInstance() {
         return new FlickrPhotoList();
@@ -46,6 +49,8 @@ public class FlickrPhotoList extends SherlockFragment implements PhotoViewer {
         ListView list = (ListView) result.findViewById(R.id.flickr_photo_list);
         adapter = new FlickrPhotoListAdapter();
         list.setAdapter(adapter);
+        preloader = new FlickrListPreloader(getActivity(), 5);
+        list.setOnScrollListener(preloader);
         if (currentPhotos != null)
             adapter.setPhotos(currentPhotos);
         return result;
@@ -61,6 +66,41 @@ public class FlickrPhotoList extends SherlockFragment implements PhotoViewer {
         }
     }
 
+    private class FlickrListPreloader extends ListPreloader<Photo> {
+        private int[] photoDimens = null;
+
+        public FlickrListPreloader(Context context, int maxPreload) {
+            super(context, maxPreload);
+        }
+
+        public boolean isDimensSet() {
+            return photoDimens != null;
+        }
+
+        public void setDimens(int width, int height) {
+            if (photoDimens == null) {
+                photoDimens = new int[] { width, height };
+            }
+        }
+
+        @Override
+        protected int[] getDimens(Photo item) {
+            return photoDimens;
+        }
+
+        @Override
+        protected List<Photo> getItems(int start, int end) {
+            return currentPhotos.subList(start, end);
+        }
+
+        @Override
+        protected Glide.Request<Photo> getRequest(Photo item) {
+            return Glide.using(new FlickrModelLoader(getActivity(), urlCache))
+                    .load(item)
+                    .centerCrop();
+        }
+    }
+
     private class FlickrPhotoListAdapter extends BaseAdapter {
         private final LayoutInflater inflater;
         private List<Photo> photos = new ArrayList<Photo>(0);
@@ -95,14 +135,23 @@ public class FlickrPhotoList extends SherlockFragment implements PhotoViewer {
             final ViewHolder viewHolder;
             if (view == null) {
                 view = inflater.inflate(R.layout.flickr_photo_list_item, container, false);
-                ImageView imageView = (ImageView) view.findViewById(R.id.photo_view);
+                final ImageView imageView = (ImageView) view.findViewById(R.id.photo_view);
                 TextView titleView = (TextView) view.findViewById(R.id.title_view);
                 viewHolder = new ViewHolder(imageView, titleView);
                 view.setTag(viewHolder);
+                if (!preloader.isDimensSet()) {
+                    imageView.post(new Runnable() {
+                        @Override
+                        public void run() {
+                            preloader.setDimens(imageView.getWidth(), imageView.getHeight());
+                        }
+                    });
+                }
             } else {
                 viewHolder = (ViewHolder) view.getTag();
             }
 
+
             Glide.using(new FlickrModelLoader(getActivity(), urlCache))
                     .load(current)
                     .centerCrop()