Resizes preview that is currently dragged

def resize_preview(self, dw, dh):
        "Resizes preview that is currently dragged"

        # identify preview
        if self._objects_moving:
            id_ = self._objects_moving[0]
            tags = self.canvas.gettags(id_)
            for tag in tags:
                if tag.startswith('preview_'):
                    _, ident = tag.split('preview_')
                    preview = self.previews[ident]
                    preview.resize_by(dw, dh)
                    self.move_previews()
                    break
        self._update_cregion()