Slider: check that participants give a response

Hi!!

I have a big problem.. I programmed a slider in Python inline code. I attached my script. But I didn't know how to check whether participants fill in the slider before go on with the experiment.

Thank you so much!!!!

from libopensesame.widgets._widget import Widget
from openexp.canvas_elements import Rect
from libopensesame.widgets import widget_factory
from libopensesame import widgets


class SliderWidget(Widget):
	
	"""A slider widget that integrates into an OpenSesame form."""
	
	def __init__(
		self,
		form,
		orientation=u'vertical',
		initial_fill=0.1,
		fill_color=u'green',
		var=None
	):
		
		Widget.__init__(self, form)
		self.frame = True
		self._orientation = orientation
		self._fill = initial_fill
		self._fill_element = None
		self._fill_color = fill_color
		self.var = var
	
	def coroutine(self):
		
		retval = None
		while True:
			d = yield retval			
			if u'pos' not in d:
				continue
			click_x, click_y = d[u'pos']
			x, y, w, h = self.rect
			if self._orientation == u'vertical':
				self._fill = 1. * (y + h - click_y) / h
			else:
				raise ValueError(u'Invalid orientation: %s' % self._orientation)
			self._set_fill()
			
	def _update_frame(self, rect=None, style=u'empty'):
		
		Widget._update_frame(self, rect=rect, style=style)
			
	def _update(self):
				
		Widget._update(self)
		if self._fill_element is None:
			self._fill_element = Rect(
				*self.rect, color=self._fill_color, fill=True
			).construct(self.form.canvas)
			self.form.canvas.add_element(self._fill_element)
		self._set_fill()
			
	def _set_fill(self):
		
		x, y, w, h = self.rect
		if self._orientation == u'vertical':
			self._fill_element.y = y + h * (1-self._fill)
			self._fill_element.h = h * self._fill
		else:
			raise ValueError(u'Invalid orientation: %s' % self._orientation)
		self.set_var(self._fill)
			
			
			
class Slider(widget_factory.WidgetFactory):
	
	"""A factory for a SliderWidget that provides the clean OpenSesame 3.2
	syntax.
	"""
	
	def construct(self, form):
		
		return SliderWidget(f, *self._args, **self._kwargs)
			
	
# Register the SliderWidget so that it can be used in a form_base
widgets.slider = SliderWidget


and then in a form base I have this script:
set timeout infinite
set spacing 10
set rows "1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1;1"
set only_render no
set margins "25;25;25;25"
set duration 3000
set description "A generic form plug-in"
set cols "1;1;1;1;1;1;1;1"
set _theme gray
widget 0 9.5 2 2 label text="<span style='font-size:28px;'>Quanto è vivida<br>questa immagine?</span>"
widget 3 0 1 1 label text="<span style='font-size:13px;'>Estremamente<br>vivida</span>"
widget 3 19.99 1 1 label text="<span style='font-size:13px;'>Per niente<br>vivida</span>"
widget 3 9.5 1 1 label text="<span style='font-size:13px;'>Mediamente<br>vivida</span>"
widget 4 0 1 20 slider fill_color=white orientation=vertical var=slider_var_int_imag
widget 7 18 8 19 button center=yes text="<span style='font-size:13px;'>Seguente</span>" var=response