Kivy:实时更新小部件的颜色

我发现了一个类似的问题:

How do I change the color of my widget in Kivy at run time?

我正在使用类似的方法来尝试在拖动和移动小部件时更改它们的颜色。

class GraphNode(Widget):
    r = NumericProperty(1.0)

def __init__(self, **kwargs):
    self.size= [50,50]
    self.pos = [175,125]
    super(GraphNode, self).__init__(**kwargs)

def on_touch_down(self, touch):
    if self.collide_point(*touch.pos):
        if touch.grab_current == None:
            self.r = 0.6
            touch.grab(self)             
            return True                
    return super(GraphNode, self).on_touch_down(touch)



def on_touch_move(self, touch):
    if touch.grab_current is self:
        self.pos=[touch.x-25,touch.y-25]
    # now we only handle moves which we have grabbed


def on_touch_up(self, touch):
    if touch.grab_current is self:
        touch.ungrab(self)
        self.r = 1.0
        # and finish up here


def onTouchMove(self, touch):
    if self.collide_point(touch.x, touch.y):
        self.pos=[touch.x-25, touch.y-25]
pass

我正在尝试使用以下(kv)文件更新数字属性以更改颜色:

#:kivy 1.0.9

<GraphInterface>:
    node: graph_node

    GraphNode:
        id: graph_node
        center: self.parent.center

<GraphApp>:
    canvas:

<GraphNode>:
    size: 50, 50
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size
        Color:
            rgba: (root.r,1,1,1)

<GraphEdge>:
    size: 10, 10
    canvas:

然而,当我抓住它们时,颜色不会改变。如果我不在on_ch_drop()方法中更改颜色,那么我可以用新颜色生成节点。知道如何解决这个问题吗?