Android ExpandableListView指标/图标始终处于拉伸状态

mListView.setGroupIndicator(null);

   <ImageView
    android:id=\"@+id/help_group_indicator\"
    android:layout_width=\"@dimen/help_group_indicator_dimension\"
    android:layout_height=\"@dimen/help_group_indicator_dimension\"
    android:layout_marginTop=\"@dimen/help_group_indicator_dimension\"
    android:src=\"@drawable/down_icon\" />

mListView.setOnGroupClickListener(new OnGroupClickListener() {
    @Override
    public boolean onGroupClick(ExpandableListView parent, View clickedView, int groupPosition, long rowId) {
        ImageView groupIndicator = (ImageView) clickedView.findViewById(R.id.help_group_indicator);
        if (parent.isGroupExpanded(groupPosition)) {
            parent.collapseGroup(groupPosition);
            groupIndicator.setImageResource(R.drawable.down_icon);
        } else {
            parent.expandGroup(groupPosition);
            groupIndicator.setImageResource(R.drawable.up_icon);
         }
        return true;
    }
});