void invalidate(){
  for (int i=0; i < mCursors.size(); i++) {
    swapCursor(i,null,null);
  }
}

void invalidate ( ) { for ( int i = 0 ; i < mCursors . size ( ) ; i ++ ) { swapCursor ( i , null , null ) ; } }
deactivates all previously bound cursors and unregisters their observers , prepares views for new data bindings
