def draw_connection_in_progress(touch):
    drawLine([touch.userdata['connection_start'], touch.pos])

class ConnectionButton(MTWidget):
   instances = []

    def __init__(self, **kwargs):
       super().__init__(**kwargs)
       ConnectionButton.instances.append(self)

    def check_connection(self, touch):
        for target in ConnectionButton.instances:
            if target.coolide_point(*touch.pos):
                self.connect(target)

    def on_touch_down(self, touch):
       touch.userdata['connection_start'] = self.to_window(*self.pos)
       touch.push_handler(draw=draw_connection_in_progress)
       touch.push_handler(up=self.check_connection())