How to develop a simple Python Kivy bounce ball application

Here in this article we will try to build a very basic Kivy bounce ball application.

Table of Contents

Test Environment

Fedora 32
Kivy 1.11.1

Python with kivy installed (This example is tested with python 3 and kivy 1.11 which are the latest available as of this writing)

What is Kivy

Kivy is a python framework for GUI based Application development. It is used to develop multitouch application interfaces which are compatible on various devices. Kivy uses a special language called .kv which is used to define the presentation logic and to separate it from the Python which can used for writing business logic and dynamic content generation for interfaces.

Here is the complete source code for this simple game available on github.

Procedure –

Step1: main.py which extends the App base class and other Widget class to define the business logic

[admin@feddesk brick]$ cat main.py

#!/usr/bin/env python
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 PongPaddle(Widget):
    score = NumericProperty(0)
    pass

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()
    player = ObjectProperty()
    #score = NumericProperty()

    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()

        if (self.ball.y < self.player.top) or (self.ball.top > self.height):
            self.ball.velocity_y *= -1

        if (self.ball.x < 0) or (self.ball.right > self.width):
            self.ball.velocity_x *= -1

        if self.ball.y < self.player.top:
            if self.player.x < self.ball.x < self.player.right:
                #print(self.player.score)
                self.player.score += 1
                #self.serve_ball()

    def on_touch_move(self, touch):
        if touch.x < 50:
            self.player.x = touch.x
        if touch.x > 50:
            self.player.right = touch.x

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()

Step2: kivy file pong.kv which defines the presentation logic for the Widgets

Main Widget - PongGame
Ball Widget - PongBall
Player Widget - PongPaddle

pong.kv

[admin@feddesk brick]$ cat pong.kv

# File name: pong.kv
:
  size: 20, 20
  canvas:
    Ellipse:
      pos: self.pos
      size: self.size

:
  size: 100, 10
  pos: self.center_x - 50, 0
  canvas:
    Rectangle
      pos: self.pos
      size: self.size

:
  ball: pong_ball
  player: player1

  PongPaddle:
    id: player1
    y: 20
    center_x: root.center_x

  Label:
    font_size: 20
    center_x: root.width - 30
    top: root.top - 50
    text: "Score"

  Label:
    font_size: 20
    center_x: root.width - 30
    top: root.top - 70
    text: str(root.player.score)

  PongBall:
    id: pong_ball
    pos: self.parent.center_x - 10, 10

Step3: Screenshot of the Game demo

For more detailed explanation of kivy concepts please go through the Kivy documentation for Pong Game Application as a starting point – https://kivy.org/doc/stable/tutorials/pong.html

Hope you enjoyed reading this article. Thank you..

How to develop a simple Python Kivy bounce ball application