canvas中的优先级,.after最前，before最底,canvas中间，部件在布局下面

<RelativeWidget>:
    # 画布之后
    canvas.before:
        Color:
            # 白色
            rgba:[1,1,1,1]
        Rectangle:
            pos:self.pos
            # 最大布局
            size:self.size
    canvas:
        Color:
            # 红
            rgba:[.5,.1,.1,1]
        Rectangle:
            pos:self.pos
            size:300,300
    # 之前
    canvas.after:
        Color:
            # 绿
            rgba:[.1,.5,.1,1]
        Rectangle:
            pos:200,200
            size:300,300


    Button:
        text:'Button 0'
        pos:100,100
        size_hint:None,None
        size:300,300
        canvas.after:
            Color:
                # 灰
                rgba:[.1,.1,.5,1]
            Rectangle:
                pos:self.pos
                size:100,100