Bitmapfun Sample: Fix bug where background image loading could hang

Background image loading is paused when the GridView is scrolling,
if the activity is destroyed during a scroll the image loading was
not unpaused causing it to remain running indefinitely. This change
makes sure the background image loading is unpaused.

Change-Id: Iad8093fc2af50fcfc1416c5d9089db0c064711cf

samples/training/bitmapfun/src/com/example/android/bitmapfun/ui/ImageGridFragment.java

@@ -149,6 +149,7 @@ public class ImageGridFragment extends Fragment implements AdapterView.OnItemCli
     @Override
     public void onPause() {
         super.onPause();
+        mImageFetcher.setPauseWork(false);
         mImageFetcher.setExitTasksEarly(true);
         mImageFetcher.flushCache();
     }

samples/training/bitmapfun/src/com/example/android/bitmapfun/util/ImageWorker.java

@@ -150,6 +150,7 @@ public abstract class ImageWorker {
 
     public void setExitTasksEarly(boolean exitTasksEarly) {
         mExitTasksEarly = exitTasksEarly;
+        setPauseWork(false);
     }
 
     /**
@@ -372,6 +373,18 @@ public abstract class ImageWorker {
         }
     }
 
+    /**
+     * Pause any ongoing background work. This can be used as a temporary
+     * measure to improve performance. For example background work could
+     * be paused when a ListView or GridView is being scrolled using a
+     * {@link android.widget.AbsListView.OnScrollListener} to keep
+     * scrolling smooth.
+     * <p>
+     * If work is paused, be sure setPauseWork(false) is called again
+     * before your fragment or activity is destroyed (for example during
+     * {@link android.app.Activity#onPause()}), or there is a risk the
+     * background thread will never finish.
+     */
     public void setPauseWork(boolean pauseWork) {
         synchronized (mPauseWorkLock) {
             mPauseWork = pauseWork;