Kivy: PinchLabel font resize

pinchlabel.py

import kivy
kivy.require('1.9.0')

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.label import Label
from kivy.vector import Vector
from kivy.properties import BoundedNumericProperty

class PinchLabel(Label):
    font_size_min = BoundedNumericProperty(2, min=0)
    font_size_max = BoundedNumericProperty(1e100, min=0)

    def __init__(self, **kwargs):
        super(PinchLabel, self).__init__(**kwargs)
        self._touches = []
        self._last_touch_pos = {}

    def on_touch_down(self, touch):
        if not self.collide_point(*touch.pos):
            return

        if 'multitouch_sim' in touch.profile:
            touch.multitouch_sim = True

        touch.grab(self)
        self._touches.append(touch)
        self._last_touch_pos[touch] = touch.pos
        return True

    def on_touch_move(self, touch):
        if touch in self._touches and touch.grab_current == self:
            self.scale_with_touch(touch)
            self._last_touch_pos[touch] = touch.pos

        if self.collide_point(*touch.pos):
            return True

    def on_touch_up(self, touch):
        if touch in self._touches and touch.grab_state:
            touch.ungrab(self)
            del self._last_touch_pos[touch]
            self._touches.remove(touch)

        if self.collide_point(*touch.pos):
            return True

    def scale_with_touch(self, touch):
        if len(self._touches) == 1:
            return False

        points = [Vector(self._last_touch_pos[t]) for t in self._touches if t is not touch]
        points.append(Vector(touch.pos))

        anchor = max(points[:-1], key=lambda p: p.distance(touch.pos))
        farthest = max(points, key=anchor.distance)
        if farthest is not points[-1]:
            return False

        old_line = Vector(*touch.ppos) - anchor
        new_line = Vector(*touch.pos) - anchor
        if not old_line.length():
            return False

        scale = new_line.length() / old_line.length()
        new_scale = scale * self.font_size
        new_scale = min(self.font_size_max, max(self.font_size_min, new_scale))
        self.font_size = new_scale
        return True

root = Builder.load_string('''
<PinchLabel>:
    size_hint: None, None
    size: self.texture_size

FloatLayout:
    PinchLabel:
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        text: 'hello!'
''')

class TestApp(App):
    def build(self):
        return root

if __name__ == '__main__':
    TestApp().run()