test.py

#!/usr/bin/env python
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import ObjectProperty, NumericProperty
from kivy.lang import Builder

Builder.load_string('''
<ArrowPointer>:
    canvas:
        Color:
            rgb: 0, 0, 255 / 255.
        Line:
            points: self.x - self.offset, self.y - self.offset, self.x + self.offset, self.y + self.offset
''')

class ArrowPointer(Widget):
    arrow_tip = ObjectProperty(None)
    offset = NumericProperty(50)
    
class TestWidget(Widget):
    arrow = ObjectProperty(None)
    def __init__(self, **kwargs):
        super(TestWidget, self).__init__(**kwargs)
        self.arrow = ArrowPointer()
        self.add_widget(self.arrow)

class TestB(App):
    def build(self):
        return TestWidget()

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