Android animation not working on ProgressBar

Question

I try to use animations in my Android project, but I try that it does not work. This is the line that I use to animate the progress bar.

ProgressBar progressBar = (ProgressBar)convertView.findViewById(R.id.pb_loading); progressBar.startAnimation(AnimationUtils.loadAnimation(getSherlockActivity().getApplicationContext(),android.R.anim.fade_out)); 

This is my XML.

  <ProgressBar android:id="@+id/pb_loading" android:layout_height="64dp" android:layout_width="64dp" android:indeterminate="true" android:indeterminateDrawable="@drawable/spinner" android:indeterminateDuration="2000" android:indeterminateOnly="true" android:layout_centerInParent="true" />