How To Get A Button In Kivy To Run A Method Outside App

Pong Game Tutorial¶

Introduction¶

Welcome to the Pong tutorial

This tutorial will teach you how to write pong using Kivy. We'll start with a basic application like the one described in the Create an application and turn it into a playable pong game, describing each step along the way.

Here is a check list before starting this tutorial:

• You have a working Kivy installation. See the Installation section for detailed descriptions
• You know how to run a basic Kivy application. See Create an application if you don't.

If you have read the programming guide, and understand both basic Widget concepts (A Simple Paint App) and basic concepts of the kv language (Kv language), you can probably skip the first 2 steps and go straight to step 3.

Note

You can find the entire source code, and source code files for each step in the Kivy examples directory under tutorials/pong/

Ready? Sweet, let's get started!

Getting Started¶

Getting Started

Let's start by getting a really simple Kivy app up and running. Create a directory for the game and a file named main.py

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from kivy.app import App from kivy.uix.widget import Widget class PongGame ( Widget ): pass class PongApp ( App ): def build ( self ): return PongGame () if __name__ == '__main__' : PongApp () . run ()

Go ahead and run the application. It should just show a black window at this point. What we've done is create a very simple Kivy App , which creates an instance of our PongGame Widget class and returns it as the root element for the applications UI, which you should imagine at this point as a hierarchical tree of Widgets. Kivy places this widget-tree in the default Window. In the next step, we will draw the Pong background and scores by defining how the PongGame widget looks.

Add Simple Graphics¶

Creation of pong.kv

We will use a .kv file to define the look and feel of the PongGame class. Since our App class is called PongApp , we can simply create a file called pong.kv in the same directory that will be automatically loaded when the application is run. So create a new file called ``pong.kv`` and add the following contents.

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#:kivy 1.0.9 <PongGame> : 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"

Note

COMMON ERROR: The name of the kv file, e.g. pong.kv, must match the name of the app, e.g. PongApp (the part before the App ending).

If you run the app now, you should see a vertical bar in the middle, and two zeros where the player scores will be displayed.

Explaining the Kv File Syntax¶

Before going on to the next step, you might want to take a closer look at the contents of the kv file we just created and figure out what is going on. If you understand what's happening, you can probably skip ahead to the next step.

On the very first line we have:

This first line is required in every kv file. It should start with #:kivy followed by a space and the Kivy version it is intended for (so Kivy can make sure you have at least the required version, or handle backwards compatibility later on).

After that, we begin defining rules that are applied to all PongGame instances:

Like Python, kv files use indentation to define nested blocks. A block defined with a class name inside the < and > characters is a Widget rule. It will be applied to any instance of the named class. If you replaced PongGame with Widget in our example, all Widget instances would have the vertical line and the two Label widgets inside them because it would define these rules for all Widget instances.

Inside a rule section, you can add various blocks to define the style and contents of the widgets they will be applied to. You can:

• set property values,
• add child widgets
• define a canvas section in which you can add Graphics instructions that define how the widget is rendered.

The first block inside the <PongGame> rule we have is a canvas block:

                                <PongGame>                :                canvas                :                Rectangle                :                pos                :                self                .                center_x                -                5                ,                0                size                :                10                ,                self                .                height              

So this canvas block says that the PongGame widget should draw some graphics primitives. In this case, we add a rectangle to the canvas. We set the pos of the rectangle to be 5 pixels left of the horizontal center of the widget, and 0 for y. The size of the rectangle is set to 10 pixels in width, and the widget's height in height. The nice thing about defining the graphics like this, is that the rendered rectangle will be automatically updated when the properties of any widgets used in the value expression change.

Note

Try to resize the application window and notice what happens. That's right, the entire UI resizes automatically. The standard behaviour of the Window is to resize an element based on its property size_hint. The default widget size_hint is (1,1), meaning it will be stretched 100% in both x-direction and y-direction and hence fill the available space. Since the pos and size of the rectangle and center_x and top of the score labels were defined within the context of the PongGame class, these properties will automatically update when the corresponding widget properties change. Using the Kv language gives you automatic property binding. :)

The last two sections we add look pretty similar. Each of them adds a Label widget as a child widget to the PongGame widget. For now, the text on both of them is just set to "0". We'll hook that up to the actual score once we have the logic implemented, but the labels already look good since we set a bigger font_size, and positioned them relatively to the root widget. The root keyword can be used inside the child block to refer back to the parent/root widget the rule applies to ( PongGame in this case):

                                <PongGame>                :                # ...                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"              

Add the Ball¶

Add the Ball

Ok, so we have a basic pong arena to play in, but we still need the players and a ball to hit around. Let's start with the ball. We'll add a new PongBall class to create a widget that will be our ball and make it bounce around.

PongBall Class¶

Here is the Python code for the PongBall class:

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class PongBall ( Widget ): # velocity of the ball on x and y axis velocity_x = NumericProperty ( 0 ) velocity_y = NumericProperty ( 0 ) # referencelist property so we can use ball.velocity as # a shorthand, just like e.g. w.pos for w.x and w.y velocity = ReferenceListProperty ( velocity_x , velocity_y ) # ``move`` function will move the ball one step. This # will be called in equal intervals to animate the ball def move ( self ): self . pos = Vector ( * self . velocity ) + self . pos

And here is the kv rule used to draw the ball as a white circle:

                                <PongBall>                :                size                :                50                ,                50                canvas                :                Ellipse                :                pos                :                self                .                pos                size                :                self                .                size              

To make it all work, you also have to add the imports for the Properties Property classes used and the Vector .

Here is the entire updated python code and kv file for this step:

main.py:

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from kivy.app import App from kivy.uix.widget import Widget from kivy.properties import NumericProperty , ReferenceListProperty from kivy.vector import Vector 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 ): pass class PongApp ( App ): def build ( self ): return PongGame () if __name__ == '__main__' : PongApp () . run ()

pong.kv:

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#:kivy 1.0.9 <PongBall> : size : 50 , 50 canvas : Ellipse : pos : self . pos size : self . size <PongGame> : 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 : center : self . parent . center

Note that not only a <PongBall> widget rule has been added, but also a child widget PongBall in the <PongGame> widget rule.

Adding Ball Animation¶

Making the ball move

Cool, so now we have a ball, and it even has a move function… but it's not moving yet. Let's fix that.

Scheduling Functions on the Clock¶

We need the move method of our ball to be called regularly. Luckily, Kivy makes this pretty easy by letting us schedule any function we want using the Clock and specifying the interval:

                                Clock                .                schedule_interval                (                game                .                update                ,                1.0                /                60.0                )              

This line for example, would cause the update function of the game object to be called once every 60th of a second (60 times per second).

Object Properties/References¶

We have another problem though. We'd like to make sure the PongBall has its move function called regularly, but in our code we don't have any references to the ball object since we just added it via the kv file inside the kv rule for the PongGame class. The only reference to our game is the one we return in the applications build method.

Since we're going to have to do more than just move the ball (e.g. bounce it off the walls and later the players racket), we'll probably need an update method for our PongGame class anyway. Furthermore, given that we have a reference to the game object already, we can easily schedule its new update method when the application gets built:

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class PongGame ( Widget ): def update ( self , dt ): # call ball.move and other stuff pass class PongApp ( App ): def build ( self ): game = PongGame () Clock . schedule_interval ( game . update , 1.0 / 60.0 ) return game

However, that still doesn't change the fact that we don't have a reference to the PongBall child widget created by the kv rule. To fix this, we can add an ObjectProperty to the PongGame class, and hook it up to the widget created in the kv rule. Once that's done, we can easily reference the ball property inside the update method and even make it bounce off the edges:

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class PongGame ( Widget ): ball = ObjectProperty ( None ) 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

Don't forget to hook it up in the kv file, by giving the child widget an id and setting the PongGame's ball ObjectProperty to that id:

                                <PongGame>                :                ball                :                pong_ball                # ... (canvas and Labels)                PongBall                :                id                :                pong_ball                center                :                self                .                parent                .                center              

Note

At this point everything is hooked up for the ball to bounce around. If you're coding along as we go, you might be wondering why the ball isn't moving anywhere. The ball's velocity is set to 0 on both x and y. In the code listing below, a serve_ball method is added to the PongGame class and called in the app's build method. It sets a random x and y velocity for the ball, and also resets the position, so we can use it later to reset the ball when a player has scored a point.

Here is the entire code for this step:

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 ()

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

Connect Input Events¶

Adding Players and reacting to touch input

Sweet, our ball is bouncing around. The only things missing now are the movable player rackets and keeping track of the score. We won't go over all the details of creating the class and kv rules again, since those concepts were already covered in the previous steps. Instead, let's focus on how to move the Player widgets in response to user input. You can get the whole code and kv rules for the PongPaddle class at the end of this section.

In Kivy, a widget can react to input by implementing the on_touch_down , the on_touch_move and the on_touch_up methods. By default, the Widget class implements these methods by just calling the corresponding method on all its child widgets to pass on the event until one of the children returns True .

Pong is pretty simple. The rackets just need to move up and down. In fact it's so simple, we don't even really need to have the player widgets handle the events themselves. We'll just implement the on_touch_move function for the PongGame class and have it set the position of the left or right player based on whether the touch occurred on the left or right side of the screen.

Check the on_touch_move handler:

def on_touch_move ( self , touch ): 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

We'll keep the score for each player in a NumericProperty . The score labels of the PongGame are kept updated by changing the NumericProperty score , which in turn updates the PongGame child labels text property. This binding occurs because Kivy properties automatically bind to any references in their corresponding kv files. When the ball escapes out of the sides, we'll update the score and serve the ball again by changing the update method in the PongGame class. The PongPaddle class also implements a bounce_ball method , so that the ball bounces differently based on where it hits the racket. Here is the code for the PongPaddle class:

class PongPaddle ( Widget ): score = NumericProperty ( 0 ) def bounce_ball ( self , ball ): if self . collide_widget ( ball ): speedup = 1.1 offset = 0.02 * Vector ( 0 , ball . center_y - self . center_y ) ball . velocity = speedup * ( offset - ball . velocity )

Note

This algorithm for ball bouncing is very simple, but will have strange behavior if the ball hits the paddle from the side or bottom…this is something you could try to fix yourself if you like.

And here it is in context. Pretty much done:

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 class PongPaddle ( 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 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 ) player1 = ObjectProperty ( None ) player2 = ObjectProperty ( None ) def serve_ball ( self , vel = ( 4 , 0 )): self . ball . center = self . center self . ball . velocity = vel def update ( self , dt ): self . ball . move () # bounce of paddles self . player1 . bounce_ball ( self . ball ) self . player2 . bounce_ball ( self . ball ) # bounce ball off bottom or top if ( self . ball . y < self . y ) or ( self . ball . top > self . top ): self . ball . velocity_y *= - 1 # went of to a side to score point? 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 ): 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 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 ()

pong.kv:

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#:kivy 1.0.9 <PongBall> : size : 50 , 50 canvas : Ellipse : pos : self . pos size : self . size <PongPaddle> : size : 25 , 200 canvas : Rectangle : pos : self . pos size : self . size <PongGame> : ball : pong_ball player1 : player_left player2 : player_right 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 : str ( root . player1 . score ) Label : font_size : 70 center_x : root . width * 3 / 4 top : root . top - 50 text : str ( root . player2 . score ) PongBall : id : pong_ball center : self . parent . center PongPaddle : id : player_left x : root . x center_y : root . center_y PongPaddle : id : player_right x : root . width - self . width center_y : root . center_y

Where To Go Now?¶

Have some fun

Well, the pong game is pretty much complete. If you understood all of the things that are covered in this tutorial, give yourself a pat on the back and think about how you could improve the game. Here are a few ideas of things you could do:

• Add some nicer graphics / images. (Hint: check out the source property on the graphics instructions like circle or Rectangle , to set an image as the texture.)
• Make the game end after a certain score. Maybe once a player has 10 points, you can display a large "PLAYER 1 WINS" label and/or add a main menu to start, pause and reset the game. (Hint: check out the Button and Label classes, and figure out how to use their add_widget and remove_widget functions to add or remove widgets dynamically.
• Make it a 4 player Pong Game. Most tablets have Multi-Touch support, so wouldn't it be cool to have a player on each side and have four people play at the same time?
• Fix the simplistic collision check so hitting the ball with an end of the paddle results in a more realistic bounce.

Note

You can find the entire source code and source code files for each step in the Kivy examples directory under tutorials/pong/

How To Get A Button In Kivy To Run A Method Outside App

Source: https://kivy.org/doc/stable-1.10.1/tutorials/pong.html

Posted by: mahaffeymersed.blogspot.com

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