在kivy中旋转触摸事件上的对象

我正在制作一个像大表盘一样旋转的圆圈.目前,我在顶部有一个箭头,显示表盘朝向哪个方向.我希望它的行为有点像一个老式的旋转手机,这样当你的手指/光标向下时你可以旋转它,但它会(放慢)在你松开后慢慢地回到顶部.

这是我的对象的样子:

这是我的代码:

#!/usr/bin/kivy
import kivy
kivy.require('1.7.2')
import math

from random import random
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.gridlayout import GridLayout
from kivy.uix.anchorlayout import AnchorLayout
from kivy.uix.relativelayout import RelativeLayout
from kivy.graphics import Color, Ellipse, Rectangle

class MinimalApp(App):
    title = 'My App'
    def build(self):
        root = RootLayout()
        return(root)

class RootLayout(AnchorLayout):
    pass

class Circley(RelativeLayout):
    angle = 0
    def on_touch_down(self, touch):
        ud = touch.ud
        ud['group'] = g = str(touch.uid)
        return True
    def on_touch_move(self, touch):
        ud = touch.ud
#        print(touch.x, 0)
#        print(self.center)
#        print(0, touch.y)
#        print(touch.x - self.center[0], touch.y - self.center[1])
        y = (touch.y - self.center[1])
        x = (touch.x - self.center[0])
        calc = math.degrees(math.atan2(y,x))
        angle = calc if calc > 0 else (360 + calc)
        print(angle)
    def on_touch_up(self, touch):
        touch.ungrab(self)
        ud = touch.ud
        return True

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

和kv:

#:kivy 1.7.2
#:import kivy kivy

:
    anchor_x: 'center'                              # I think this /is/ centered
    anchor_y: 'center' 
    canvas.before:
        Color:
            rgba: 0.4, 0.4, 0.4, 1
        Rectangle:
            pos: self.pos
            size: self.size
    Circley:
        anchor_x: 'center'                          # this is /not/ centered.
        anchor_y: 'center' 
        canvas.before:
            PushMatrix
            Color:
                rgba: 0.94, 0.94, 0.94, 1
            Rotate:
                angle: self.angle
                axis: 0, 0, 1
                origin: self.center
            Ellipse:
                source: 'arrow.png'
                size: min(self.size), min(self.size)
                pos: 0.5*self.size[0] - 0.5*min(self.size), 0.5*self.size[1] - 0.5*min(self.size)
                Label:
                    text: unicode(self.size)    # this is /not/ appearing
                    color: 1,0,0,1
        canvas.after:
            PopMatrix

部分内容来自kivy touchtracer演示,以及SO问题.

你可以看到我的计算正确地打印了圆的原点和触摸事件之间的角度(不知道这对多个手指有什么反应,没想到那么远),但不知道如何整合这成为界面中的"旋转"反馈事件.