<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _snap_cb(self, w):
 """This function is called when the user clicks the 'Snap' button. """

        # Clear the snap image viewer
        self.scrnimage.clear()
        self.scrnimage.redraw_now(whence=0)
        self.fv.update_pending()

        format = self.tosave_type

        if self._screen_size:
            # snap image using actual viewer
            self.fv.error_wrap(self.fitsimage.save_rgb_image_as_file,
                               self.tmpname, format=format)

        else:
            # we will be using shot generator, not actual viewer.
            # check that shot generator size matches UI params
            self.check_and_adjust_dimensions()

            # copy background color of viewer to shot generator
            bg = self.fitsimage.get_bg()
            self.shot_generator.set_bg(*bg)

            # add the main canvas from channel viewer to shot generator
            c1 = self.fitsimage.get_canvas()
            c2 = self.shot_generator.get_canvas()
            c2.delete_all_objects(redraw=False)
            c2.add(c1, redraw=False)
            # hack to fix a few problem graphics
            self.shot_generator._imgobj = self.fitsimage._imgobj

            # scale of the shot generator should be the scale of channel
            # viewer multiplied by the ratio of window sizes
            scale_x, scale_y = self.fitsimage.get_scale_xy()
            c1_wd, c1_ht = self.fitsimage.get_window_size()
            c2_wd, c2_ht = self.shot_generator.get_window_size()

            scale_wd = float(c2_wd) / float(c1_wd)
            scale_ht = float(c2_ht) / float(c1_ht)
            scale = max(scale_wd, scale_ht)
            scale_x *= scale
            scale_y *= scale
            self.shot_generator.scale_to(scale_x, scale_y)

            self.fitsimage.copy_attributes(self.shot_generator,
                                           self.transfer_attrs)

            # snap image
            self.fv.error_wrap(self.shot_generator.save_rgb_image_as_file,
                               self.tmpname, format=format)

            c2.delete_all_objects(redraw=False)
            self.shot_generator._imgobj = None

        self.saved_type = format
        img = RGBImage(logger=self.logger)
        img.load_file(self.tmpname)

        # load the snapped image into the screenshot viewer
        self.scrnimage.set_image(img)