Retrieve frame data within the current view window.

        This method will adjust the view window if it goes out-of-bounds.

def _fetch_data(self):
        """
        Retrieve frame data within the current view window.

        This method will adjust the view window if it goes out-of-bounds.
        """
        self._view_col0 = clamp(self._view_col0, 0, self._max_col0)
        self._view_row0 = clamp(self._view_row0, 0, self._max_row0)
        self._view_ncols = clamp(self._view_ncols, 0,
                                 self._conn.frame_ncols - self._view_col0)
        self._view_nrows = clamp(self._view_nrows, 0,
                                 self._conn.frame_nrows - self._view_row0)
        self._conn.fetch_data(
            self._view_row0,
            self._view_row0 + self._view_nrows,
            self._view_col0,
            self._view_col0 + self._view_ncols)