first commit

kivy_venv/share/kivy-examples/tutorials/pong/steps/step4/main.py

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+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 random import randint
+
+
+class PongBall(Widget):
+    velocity_x = NumericProperty(0)
+    velocity_y = NumericProperty(0)
+    velocity = ReferenceListProperty(velocity_x, velocity_y)
+
+    def move(self):
+        self.pos = Vector(*self.velocity) + self.pos
+
+
+class PongGame(Widget):
+    ball = ObjectProperty(None)
+
+    def serve_ball(self):
+        self.ball.center = self.center
+        self.ball.velocity = Vector(4, 0).rotate(randint(0, 360))
+
+    def update(self, dt):
+        self.ball.move()
+
+        # bounce off top and bottom
+        if (self.ball.y < 0) or (self.ball.top > self.height):
+            self.ball.velocity_y *= -1
+
+        # bounce off left and right
+        if (self.ball.x < 0) or (self.ball.right > self.width):
+            self.ball.velocity_x *= -1
+
+
+class PongApp(App):
+    def build(self):
+        game = PongGame()
+        game.serve_ball()
+        Clock.schedule_interval(game.update, 1.0 / 60.0)
+        return game
+
+
+if __name__ == '__main__':
+    PongApp().run()

kivy_venv/share/kivy-examples/tutorials/pong/steps/step4/pong.kv

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+#:kivy 1.0.9
+
+<PongBall>:
+    size: 50, 50 
+    canvas:
+        Ellipse:
+            pos: self.pos
+            size: self.size          
+
+<PongGame>:
+    ball: pong_ball
+    
+    canvas:
+        Rectangle:
+            pos: self.center_x - 5, 0
+            size: 10, self.height
+    
+    Label:
+        font_size: 70  
+        center_x: root.width / 4
+        top: root.top - 50
+        text: "0"
+        
+    Label:
+        font_size: 70  
+        center_x: root.width * 3 / 4
+        top: root.top - 50
+        text: "0"
+    
+    PongBall:
+        id: pong_ball
+        center: self.parent.center
+        
+