Kivy ScreenManager 给我一个黑屏

Question

我一直在尝试使用 kivy 创建一个乒乓球游戏。 我按照他们网站上的教程进行操作，效果很好！ 但是我想添加一个开始屏幕。 我在谷歌上查看，发现我应该使用 Kivy 的屏幕管理器。 我把它放在我的代码中，但是当我按下开始按钮时，它什么也不做，给我一个黑屏。 我一直在试图弄清楚，但我不明白有什么问题！ 可能是因为我对 Kivy 没有太多经验。 这是我的代码。
主文件

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import NumericProperty, ReferenceListProperty, ObjectProperty
from kivy.vector import Vector
from kivy.clock import Clock
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen, SlideTransition

class PingPongPaddle(Widget):

    score = NumericProperty(0)

    def bounce_ball(self, ball):
        if self.collide_widget(ball):
            vx, vy = ball.velocity
            offset = (ball.center_y - self.center_y) / (self.height / 2)
            bounced = Vector(-1 * vx, vy)
            vel = bounced * 1.1
            ball.velocity = vel.x, vel.y + offset
class PingPongBall(Widget):
    #Velocity on x and y axis
    velocity_x = NumericProperty(0)
    velocity_y = NumericProperty(0)

    #So ball.velocity can be used
    velocity = ReferenceListProperty(velocity_x, velocity_y)

    #Moves the ball one step
    def move(self):
        self.pos = Vector(*self.velocity) + self.pos

class PingPongGame(Widget):
    ball = ObjectProperty(None)
    player1 = ObjectProperty(None)
    player2 = ObjectProperty(None)


    def serve_ball(self, vel=(8, 0)):
        self.ball.center = self.center
        self.ball.velocity = vel
    def update(self, dt):
        self.ball.move()
        #Bounce off the paddles
        self.player1.bounce_ball(self.ball)
        self.player2.bounce_ball(self.ball)
        #Bounce off top and bottom
        if (self.ball.y < 0) or (self.ball.top > self.height):
            self.ball.velocity_y *= -1

        #Went off the side
        if self.ball.x < self.x:
            self.player2.score += 1
            self.serve_ball(vel=(4, 0))
        if self.ball.x > self.width:
            self.player1.score += 1
            self.serve_ball(vel=(-4, 0))

    def on_touch_move(self, touch):
        #To sense if screen is touched
        if touch.x < self.width / 3:
            self.player1.center_y = touch.y
        if touch.x > self.width - self.width / 3:
            self.player2.center_y = touch.y

class GameScreen(Screen):
    def play(self):
        global game
        game = PingPongGame()
        game.serve_ball()
        #Runs every 60th of a second
        Clock.schedule_interval(game.update, 1.0/60.0)
        return game

class MainScreen(Screen):
    pass



class PingPongApp(App):
    def build(self):
        screen_manager = ScreenManager(transition=SlideTransition())

        screen_manager.add_widget(MainScreen(name="main"))
        screen_manager.add_widget(GameScreen(name="game"))
        return screen_manager




if __name__ == "__main__":
    PingPongApp().run()

乒乓.kv

#:kivy 1.11.1

<MainScreen>:
    BoxLayout:
        Button:
            text: "Start!"
            on_press: root.manager.current = "game"
<GameScreen>:
    on_enter: self.play()

<PingPongBall>:
    size: 75, 75
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size          

<PingPongPaddle>:
    size: 50, 400
    canvas:
        Rectangle:
            pos:self.pos
            size:self.size

<PingPongGame>:
    ball: PingPong_ball
    player1: player_left
    player2: player_right

    canvas:
        Rectangle:
            pos: self.center_x - 5, 0
            size: 20, self.height

    Label:
        font_size: 140  
        center_x: root.width / 4
        top: root.top - 50
        text: str(root.player1.score)

    Label:
        font_size: 140 
        center_x: root.width * 3 / 4
        top: root.top - 50
        text: str(root.player2.score)

    Label:
        font_size: 35
        center_x: root.width * 6/7
        top: root.top - 500
        text: "by Me"

    PingPongBall:
        id: PingPong_ball
        center: self.parent.center

    PingPongPaddle:
        id: player_left
        x: root.x
        center_y: root.center_y

    PingPongPaddle:
        id: player_right
        x: root.width - self.width
        center_y: root.center_y

提前致谢！

Answer 1

我是新来的，但我认为这可能会有所帮助 当我使用 Kivy 时，我也会导入“屏幕”并使用它。 这里是：

        self.removeuser = RemoveUser()
    screen = Screen(name="RemoveUser") # This will be the naem of screen to be called
    screen.add_widget(self.removeuser)
    self.screen_manager.add_widget(screen)

然后应该可以工作，返回 self.screen_manager 然后为您的开始按钮创建一个功能

PingPongApp.screen_manger.current = "<name of start screen>"

Answer 2

我通过在 GameScreen 的播放函数中包含这一行来修复它。

self.add_widget(game)