Kivy异步问题-滚动视图后画布无法更新

Question

I have this piece of code in kivy, after pressing a button, get_liner() function get called. get_liner() then calls get_canvas(). I want the canvas to be updated with a color background after the text in the scrollview gets updated. But seems like there is some asynchronous issue that doesn't let the code run in sequence. But if I press the button the 2nd time, the canvas gets updated after text. How can I make this code work?

myapp.py

def get_liner(self):
    <some code here>
    mythread2 = threading.Thread(target=partial(self.get_canvas),kwargs={'outputtext':outputtext})
    mythread2.start()

def get_canvas(self, outputtext=None):
    self.ids.rst_doc.text = outputtext
    secondhalfview = self.ids.rst_doc
    secondhalfview.canvas.before.clear()
    with secondhalfview.canvas.before:
        Color(1,1,0,1)
        Rectangle(pos=secondhalfview.pos, size=secondhalfview.size)
    self.dismiss_popup()

myapp.kv

Root:
    BoxLayout:
        orientation: 'vertical'
        size: root.size
        pos: root.pos
        BoxLayout:
            id: twohalfviews
            orientation: 'horizontal'          
            size: root.size
            pos: root.pos
            BoxLayout:
                id: secondhalfview
                size_hint_x: 0.7
                ScrollView:
                    id: scrlv
                    size_hint_x: 0.8
                    pos_hint: {'center_x':.5, 'center_y': .5}
                    Label:
                        id: rst_doc
                        text: ' '
                        text_size: self.width,None
                        font_size: '12sp'
                        size_hint_y: None
                        height: self.texture_size[1]
                        color: [1,0,0,1] #red
                        font_name: 'monaco.ttf'

Answer 1

I finally figured out the issue. It's not async issue, but rather a drawing issue. In my old code, the canvas is cleared, then I redraw Rectangle object and Color object which screws up the drawing sequence.

Here is the new code where I don't clear the canvas anymore, and clear the rst document instead then update the Color object alone:

def new_get_canvas(self, outputtext=None):
    self.ids.rst_doc.text = outputtext
    rst_doc = self.ids.rst_doc
    rst_doc.canvas.before.clear()
    self.ids.secondhalfview.canvas.before.children.insert(
        0,Color(211/255.0, 211/255.0, 211/255.0, 0.9))
    self.dismiss_popup()

Debug Image: