Yet Another Android snake with Kivy, Python

  • Tutorial
Hello, there.

A lot of people want to start programming apps for Android, but they prefer not to use Android Studio and/or Java. Why? Because it's an overkill. «I just wanna create Snake and nothing more!»



Let's snake without java! (with a bonus at the end)

Why create yet another snake tutorial?
If you are a python coder and want to learn gamedev for android, you must have already googled «snake on android» and found this (Eng) or its translation (Rus). So did I. Unfortunately, I found the article extremely useless because:

Their code is bad

Minor issues:

  1. Using «tile» and «head» instead of «tiles» or «cells» doesn't make sense to me. The head is not distinct enough from the tile to be different variables.
  2. Clock.schedule for self.update is called from… self.update.
  3. Second-level class Playground is implemented in the beginning, while first-level class SnakeApp is implemented at the end.
  4. Names for directions («up», «down», ...) are used instead of vectors ( (0, 1), (1, 0)… ).

Major issues:

  1. Most dynamic objects (for example, the fruit) are attached to the kv file, so you can't make more than 1 apple because then you should rewrite this part
  2. Weird logic for the snake's motion instead of «cell-after-cell» motion.
  3. The code is very long being more than 350 lines.

The article is not clear for novices

This is my PERSONAL opinion. Moreover, I do not guarantee that my tutorial will be more interesting and clear. But I'll do my best and, for my article, guarantee that:

  1. The code is short
  2. The snake is nice
  3. The instructions will have clear step-by-step implementations and smooth progression from «hello, world» to a ready snake with transitional steps.

The undesirable result


No space between the cells, the triangle is embarrassing, graphics are glitchy.

Get familiarized


First app


Please, confirm that you have already installed Kivy (if not, follow the instructions) and ran
buildozer init in the project directory.

Let's run our first app:

main.py

from kivy.app import App
from kivy.uix.widget import Widget

class WormApp(App):
    def build(self):
        return Widget()

if __name__ == '__main__':
    WormApp().run()



We created a Widget. Analogously, we can create a button or any other UI element:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button

class WormApp(App):
    def build(self):
        self.but = Button()
        self.but.pos = (100, 100)
        self.but.size = (200, 200)
        self.but.text = "Hello, cruel world"

        self.form = Widget()
        self.form.add_widget(self.but)
        return self.form

if __name__ == '__main__':
    WormApp().run()



Wow! Congratulations! You've created a button!

.kv files


However, there's another way to create UI elements. First, we implement our form:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button


class Form(Widget):
    def __init__(self):
        super().__init__()
        self.but1 = Button()
        self.but1.pos = (100, 100)
        self.add_widget(self.but1)


class WormApp(App):
    def build(self):
        self.form = Form()
        return self.form


if __name__ == '__main__':
    WormApp().run()

Then, we create «worm.kv» file.

worm.kv

<Form>:
    but2: but_id

    Button:
        id: but_id
        pos: (200, 200)

What just happened? We created another Button and assign id but_id. Then, but_id was matched to but2 of the form. It means that now we can refer to this button by but2

class Form(Widget):
    def __init__(self):
        super().__init__()
        self.but1 = Button()
        self.but1.pos = (100, 100)
        self.add_widget(self.but1)   #
        self.but2.text = "OH MY"



Graphics


What we do next is creating a graphical element. First, we implement it in worm.kv:

<Form>:

<Cell>:
    canvas:
        Rectangle:
            size: self.size
            pos: self.pos

We linked the rectangle's position to self.pos and its size to self.size. So now those properties are available from Cell, for example, once we create a cell, we can do

class Cell(Widget):
    def __init__(self, x, y, size):
        super().__init__()
        self.size = (size, size)   # As you can see, we can change self.size which is "size" property of a rectangle
        self.pos = (x, y)

class Form(Widget):
    def __init__(self):
        super().__init__()
        self.cell = Cell(100, 100, 30)
        self.add_widget(self.cell)



Ok, we have created a cell.

Necessary methods


Let's try to move it. To do that, we should add Form.update function and schedule it.

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.clock import Clock

class Cell(Widget):
    def __init__(self, x, y, size):
        super().__init__()
        self.size = (size, size)
        self.pos = (x, y)


class Form(Widget):
    def __init__(self):
        super().__init__()
        self.cell = Cell(100, 100, 30)
        self.add_widget(self.cell)

    def start(self):
        Clock.schedule_interval(self.update, 0.01)

    def update(self, _):
        self.cell.pos = (self.cell.pos[0] + 2, self.cell.pos[1] + 3)


class WormApp(App):
    def build(self):
        self.form = Form()
        self.form.start()
        return self.form


if __name__ == '__main__':
    WormApp().run()


The cell will move across the form. As you can see, we can schedule any function with Clock.

Next, let's make a touch event. Rewrite Form:

class Form(Widget):
    def __init__(self):
        super().__init__()
        self.cells = []

    def start(self):
        Clock.schedule_interval(self.update, 0.01)

    def update(self, _):
        for cell in self.cells:
            cell.pos = (cell.pos[0] + 2, cell.pos[1] + 3)

    def on_touch_down(self, touch):
        cell = Cell(touch.x, touch.y, 30)
        self.add_widget(cell)
        self.cells.append(cell)

Each touch_down creates a cell with coordinates = (touch.x, touch.y) and size of 30. Then, we add it as a widget of the form AND to our own array (in order to easily access them).

Now you can tap on your form and generate cells.



Neat settings


Because we want to get a nice snake, we should distinguish the graphical positions and the actual positions of cells.

Why?
A lot of reasons to do so. All logic should be connected with the so-called actual data, while the graphical data is the result of the actual data. For example, if we want to make margins, the actual pos of the cell will be (100, 100) while the graphical pos of the rectangle — (102, 102).

P. S. We wouldn't do it if we dealt with classical on_draw. But here we don't have to program on_draw.

Let's fix the worm.kv file:

<Form>:

<Cell>:
    canvas:
        Rectangle:
            size: self.graphical_size
            pos: self.graphical_pos

and main.py:

...
from kivy.properties import *
...
class Cell(Widget):
    graphical_size = ListProperty([1, 1])
    graphical_pos = ListProperty([1, 1])

    def __init__(self, x, y, size, margin=4):
        super().__init__()
        self.actual_size = (size, size)
        self.graphical_size = (size - margin, size - margin)
        self.margin = margin
        self.actual_pos = (x, y)
        self.graphical_pos_attach()

    def graphical_pos_attach(self):
        self.graphical_pos = (self.actual_pos[0] - self.graphical_size[0] / 2, self.actual_pos[1] - self.graphical_size[1] / 2)
...
class Form(Widget):
    def __init__(self):
        super().__init__()
        self.cell1 = Cell(100, 100, 30)
        self.cell2 = Cell(130, 100, 30)
        self.add_widget(self.cell1)
        self.add_widget(self.cell2)
...



The margin appeared so it looks pretty although we created the second cell with X = 130 instead of 132. Later, we will make smooth motion based on the distance between actual_pos and graphical_pos.

Coding the worm


Implementation


Init config in main.py

class Config:
    DEFAULT_LENGTH = 20
    CELL_SIZE = 25
    APPLE_SIZE = 35
    MARGIN = 4
    INTERVAL = 0.2
    DEAD_CELL = (1, 0, 0, 1)
    APPLE_COLOR = (1, 1, 0, 1)

(Trust me, you'll love it!)

Then, assign config to the app:

class WormApp(App):
    def __init__(self):
        super().__init__()
        self.config = Config()
        self.form = Form(self.config)
    
    def build(self):
        self.form.start()
        return self.form

Rewrite init and start:

class Form(Widget):
    def __init__(self, config):
        super().__init__()
        self.config = config
        self.worm = None

    def start(self):
        self.worm = Worm(self.config)
        self.add_widget(self.worm)
        Clock.schedule_interval(self.update, self.config.INTERVAL)

Then, the Cell:

class Cell(Widget):
    graphical_size = ListProperty([1, 1])
    graphical_pos = ListProperty([1, 1])

    def __init__(self, x, y, size, margin=4):
        super().__init__()
        self.actual_size = (size, size)
        self.graphical_size = (size - margin, size - margin)
        self.margin = margin
        self.actual_pos = (x, y)
        self.graphical_pos_attach()

    def graphical_pos_attach(self):
        self.graphical_pos = (self.actual_pos[0] - self.graphical_size[0] / 2, self.actual_pos[1] - self.graphical_size[1] / 2)

    def move_to(self, x, y):
        self.actual_pos = (x, y)
        self.graphical_pos_attach()

    def move_by(self, x, y, **kwargs):
        self.move_to(self.actual_pos[0] + x, self.actual_pos[1] + y, **kwargs)

    def get_pos(self):
        return self.actual_pos

    def step_by(self, direction, **kwargs):
        self.move_by(self.actual_size[0] * direction[0], self.actual_size[1] * direction[1], **kwargs)

Hopefully, it's more or less clear.

and finally the Worm:

class Worm(Widget):
    def __init__(self, config):
        super().__init__()
        self.cells = []
        self.config = config
        self.cell_size = config.CELL_SIZE
        self.head_init((100, 100))
        for i in range(config.DEFAULT_LENGTH):
            self.lengthen()

    def destroy(self):
        for i in range(len(self.cells)):
            self.remove_widget(self.cells[i])
        self.cells = []

    def lengthen(self, pos=None, direction=(0, 1)):
        # If pos is set, we put the cell in pos, otherwise accordingly to the specified direction
        if pos is None:
            px = self.cells[-1].get_pos()[0] + direction[0] * self.cell_size
            py = self.cells[-1].get_pos()[1] + direction[1] * self.cell_size
            pos = (px, py)
        self.cells.append(Cell(*pos, self.cell_size, margin=self.config.MARGIN))
        self.add_widget(self.cells[-1])

    def head_init(self, pos):
        self.lengthen(pos=pos)

Let's give life to our wormie.



Motion


Now we will make it move.

It's simple:

class Worm(Widget):
...
    def move(self, direction):
        for i in range(len(self.cells) - 1, 0, -1):
            self.cells[i].move_to(*self.cells[i - 1].get_pos())
        self.cells[0].step_by(direction)

class Form(Widget):
    def __init__(self, config):
        super().__init__()
        self.config = config
        self.worm = None
        self.cur_dir = (0, 0)

    def start(self):
        self.worm = Worm(self.config)
        self.add_widget(self.worm)
        self.cur_dir = (1, 0)
        Clock.schedule_interval(self.update, self.config.INTERVAL)

    def update(self, _):
        self.worm.move(self.cur_dir)



It's alive! It's alive!

Controlling


As you could judge by the preview image, the controls of the snake will be the following:



class Form(Widget):
...
    def on_touch_down(self, touch):
        ws = touch.x / self.size[0]
        hs = touch.y / self.size[1]
        aws = 1 - ws
        if ws > hs and aws > hs:
            cur_dir = (0, -1)         # Down
        elif ws > hs >= aws:
            cur_dir = (1, 0)          # Right
        elif ws <= hs < aws:
            cur_dir = (-1, 0)         # Left
        else:
            cur_dir = (0, 1)           # Up
        self.cur_dir = cur_dir



Cool.

Creating the fruit


First, we initialize it.

class Form(Widget):
...
    def __init__(self, config):
        super().__init__()
        self.config = config
        self.worm = None
        self.cur_dir = (0, 0)
        self.fruit = None
...
    def random_cell_location(self, offset):
        x_row = self.size[0] // self.config.CELL_SIZE
        x_col = self.size[1] // self.config.CELL_SIZE
        return random.randint(offset, x_row - offset), random.randint(offset, x_col - offset)

    def random_location(self, offset):
        x_row, x_col = self.random_cell_location(offset)
        return self.config.CELL_SIZE * x_row, self.config.CELL_SIZE * x_col

    def fruit_dislocate(self):
        x, y = self.random_location(2)
        self.fruit.move_to(x, y)
...
    def start(self):
        self.fruit = Cell(0, 0, self.config.APPLE_SIZE, self.config.MARGIN)
        self.worm = Worm(self.config)
        self.fruit_dislocate()
        self.add_widget(self.worm)
        self.add_widget(self.fruit)
        self.cur_dir = (1, 0)
        Clock.schedule_interval(self.update, self.config.INTERVAL)

The current result:



Now we should implement some Worm methods:

class Worm(Widget):
...
    # Here we get all the positions of our cells
    def gather_positions(self):
        return [cell.get_pos() for cell in self.cells]
    # Just check if our head has the same position as another Cell
    def head_intersect(self, cell):
        return self.cells[0].get_pos() == cell.get_pos()

Other benefits of gather_positions
By the way, after we implemented gather_positions, we can change fruit_dislocate:

class Form(Widget):
    def fruit_dislocate(self):
        x, y = self.random_location(2)
        while (x, y) in self.worm.gather_positions():
            x, y = self.random_location(2)
        self.fruit.move_to(x, y)

At this point, the fruit won't be located in the worm's tile.

...and add this check to update()

class Form(Widget):
...
    def update(self, _):
        self.worm.move(self.cur_dir)
        if self.worm.head_intersect(self.fruit):
            directions = [(0, 1), (0, -1), (1, 0), (-1, 0)]
            self.worm.lengthen(direction=random.choice(directions))
            self.fruit_dislocate()

Detection of self tile hitting


We want to know whether the head has the same position as one of the worm's cells.

class Form(Widget):
...
    def __init__(self, config):
        super().__init__()
        self.config = config
        self.worm = None
        self.cur_dir = (0, 0)
        self.fruit = None
        self.game_on = True

    def update(self, _):
        if not self.game_on:
            return
        self.worm.move(self.cur_dir)
        if self.worm.head_intersect(self.fruit):
            directions = [(0, 1), (0, -1), (1, 0), (-1, 0)]
            self.worm.lengthen(direction=random.choice(directions))
            self.fruit_dislocate()
       if self.worm_bite_self():
            self.game_on = False

    def worm_bite_self(self):
        for cell in self.worm.cells[1:]:
            if self.worm.head_intersect(cell):
                return cell
        return False



Coloring, decorating, and code refactoring


Let's start with code refactoring.

Rewrite and add

class Form(Widget):
...
    def start(self):
        self.worm = Worm(self.config)
        self.add_widget(self.worm)
        if self.fruit is not None:
            self.remove_widget(self.fruit)
        self.fruit = Cell(0, 0, self.config.APPLE_SIZE)
        self.fruit_dislocate()
        self.add_widget(self.fruit)
        Clock.schedule_interval(self.update, self.config.INTERVAL)
        self.game_on = True
        self.cur_dir = (0, -1)

    def stop(self):
        self.game_on = False
        Clock.unschedule(self.update)

    def game_over(self):
        self.stop()
...
    def on_touch_down(self, touch):
        if not self.game_on:
            self.worm.destroy()
            self.start()
            return
        ...

Now if the worm is dead (frozen), if you tap again, the game will be reset.

Now let's go to decorating and coloring.

worm.kv

<Form>:
    popup_label: popup_label
    score_label: score_label

    canvas:
        Color:
            rgba: (.5, .5, .5, 1.0)

        Line:
            width: 1.5
            points: (0, 0), self.size

        Line:
            width: 1.5
            points: (self.size[0], 0), (0, self.size[1])


    Label:
        id: score_label
        text: "Score: " + str(self.parent.worm_len)
        width: self.width

    Label:
        id: popup_label
        width: self.width


<Worm>:


<Cell>:
    canvas:
        Color:
            rgba: self.color
        Rectangle:
            size: self.graphical_size
            pos: self.graphical_pos

Rewrite WormApp:

class WormApp(App):
    def build(self):
        self.config = Config()
        self.form = Form(self.config)
        return self.form

    def on_start(self):
        self.form.start()



Let's color it. Rewrite Cell in .kv:

<Cell>:
    canvas:
        Color:
            rgba: self.color

        Rectangle:
            size: self.graphical_size
            pos: self.graphical_pos

Add this to Cell.__init__

self.color = (0.2, 1.0, 0.2, 1.0)    # 

and this to Form.start

self.fruit.color = (1.0, 0.2, 0.2, 1.0)

Great, enjoy your snake



Finally, we will make a «game over» label

class Form(Widget):
...
    def __init__(self, config):
    ...
        self.popup_label.text = ""
...
    def stop(self, text=""):
        self.game_on = False
        self.popup_label.text = text
        Clock.unschedule(self.update)

    def game_over(self):
        self.stop("GAME OVER" + " " * 5 + "\ntap to reset")

and make the hit cell red:

instead of

    def update(self, _):
    ...
        if self.worm_bite_self():
            self.game_over()
    ...

write

    def update(self, _):
        cell = self.worm_bite_self()
        if cell:
            cell.color = (1.0, 0.2, 0.2, 1.0)
            self.game_over()



Are you still paying attention? Coming next is the most interesting part.

Bonus section — smooth motion


Because the worm's step is equal to the cell_size, it's not that smooth. But we want to make it step as frequently as possible, without rewriting the entire logic of the game. So, we need to create a mechanism moving our graphical poses but not our actual poses. So, I wrote a simple file:

smooth.py

from kivy.clock import Clock
import time


class Timing:
    @staticmethod
    def linear(x):
        return x

class Smooth:
    def __init__(self, interval=1.0/60.0):
        self.objs = []
        self.running = False
        self.interval = interval

    def run(self):
        if self.running:
            return
        self.running = True
        Clock.schedule_interval(self.update, self.interval)

    def stop(self):
        if not self.running:
            return
        self.running = False
        Clock.unschedule(self.update)

    def setattr(self, obj, attr, value):
        exec("obj." + attr + " = " + str(value))

    def getattr(self, obj, attr):
        return float(eval("obj." + attr))

    def update(self, _):
        cur_time = time.time()
        for line in self.objs:
            obj, prop_name_x, prop_name_y, from_x, from_y, to_x, to_y, start_time, period, timing = line
            time_gone = cur_time - start_time
            if time_gone >= period:
                self.setattr(obj, prop_name_x, to_x)
                self.setattr(obj, prop_name_y, to_y)
                self.objs.remove(line)
            else:
                share = time_gone / period
                acs = timing(share)
                self.setattr(obj, prop_name_x, from_x * (1 - acs) + to_x * acs)
                self.setattr(obj, prop_name_y, from_y * (1 - acs) + to_y * acs)
        if len(self.objs) == 0:
            self.stop()

    def move_to(self, obj, prop_name_x, prop_name_y, to_x, to_y, t, timing=Timing.linear):
        self.objs.append((obj, prop_name_x, prop_name_y, self.getattr(obj, prop_name_x), self.getattr(obj, prop_name_y), to_x,
                          to_y, time.time(), t, timing))
        self.run()


class XSmooth(Smooth):
    def __init__(self, props, timing=Timing.linear, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.props = props
        self.timing = timing

    def move_to(self, obj, to_x, to_y, t):
        super().move_to(obj, *self.props, to_x, to_y, t, timing=self.timing)


To those who dislike my code
This module is not the most elegant solution ©. It's a bad solution and I acknowledge it. It is an only-hello-world solution.

So you just create smooth.py and copy-paste this code to the file.
Finally, let's make it work:

class Form(Widget):
...
    def __init__(self, config):
    ...
        self.smooth = smooth.XSmooth(["graphical_pos[0]", "graphical_pos[1]"])

Then we replace self.worm.move() with

class Form(Widget):
...
    def update(self, _):
    ...
        self.worm.move(self.cur_dir, smooth_motion=(self.smooth, self.config.INTERVAL))

And this is how methods of Cell should look like

class Cell(Widget):
...
    def graphical_pos_attach(self, smooth_motion=None):
        to_x, to_y = self.actual_pos[0] - self.graphical_size[0] / 2, self.actual_pos[1] - self.graphical_size[1] / 2
        if smooth_motion is None:
            self.graphical_pos = to_x, to_y
        else:
            smoother, t = smooth_motion
            smoother.move_to(self, to_x, to_y, t)

    def move_to(self, x, y, **kwargs):
        self.actual_pos = (x, y)
        self.graphical_pos_attach(**kwargs)

    def move_by(self, x, y, **kwargs):
        self.move_to(self.actual_pos[0] + x, self.actual_pos[1] + y, **kwargs)

That's it, thank you for your attention!

How the final result works:


The final code
main.py
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.clock import Clock
from kivy.properties import *
import random
import smooth


class Cell(Widget):
    graphical_size = ListProperty([1, 1])
    graphical_pos = ListProperty([1, 1])
    color = ListProperty([1, 1, 1, 1])

    def __init__(self, x, y, size, margin=4):
        super().__init__()
        self.actual_size = (size, size)
        self.graphical_size = (size - margin, size - margin)
        self.margin = margin
        self.actual_pos = (x, y)
        self.graphical_pos_attach()
        self.color = (0.2, 1.0, 0.2, 1.0)

    def graphical_pos_attach(self, smooth_motion=None):
        to_x, to_y = self.actual_pos[0] - self.graphical_size[0] / 2, self.actual_pos[1] - self.graphical_size[1] / 2
        if smooth_motion is None:
            self.graphical_pos = to_x, to_y
        else:
            smoother, t = smooth_motion
            smoother.move_to(self, to_x, to_y, t)

    def move_to(self, x, y, **kwargs):
        self.actual_pos = (x, y)
        self.graphical_pos_attach(**kwargs)

    def move_by(self, x, y, **kwargs):
        self.move_to(self.actual_pos[0] + x, self.actual_pos[1] + y, **kwargs)

    def get_pos(self):
        return self.actual_pos

    def step_by(self, direction, **kwargs):
        self.move_by(self.actual_size[0] * direction[0], self.actual_size[1] * direction[1], **kwargs)


class Worm(Widget):
    def __init__(self, config):
        super().__init__()
        self.cells = []
        self.config = config
        self.cell_size = config.CELL_SIZE
        self.head_init((100, 100))
        for i in range(config.DEFAULT_LENGTH):
            self.lengthen()

    def destroy(self):
        for i in range(len(self.cells)):
            self.remove_widget(self.cells[i])
        self.cells = []

    def lengthen(self, pos=None, direction=(0, 1)):
        if pos is None:
            px = self.cells[-1].get_pos()[0] + direction[0] * self.cell_size
            py = self.cells[-1].get_pos()[1] + direction[1] * self.cell_size
            pos = (px, py)
        self.cells.append(Cell(*pos, self.cell_size, margin=self.config.MARGIN))
        self.add_widget(self.cells[-1])

    def head_init(self, pos):
        self.lengthen(pos=pos)

    def move(self, direction, **kwargs):
        for i in range(len(self.cells) - 1, 0, -1):
            self.cells[i].move_to(*self.cells[i - 1].get_pos(), **kwargs)
        self.cells[0].step_by(direction, **kwargs)

    def gather_positions(self):
        return [cell.get_pos() for cell in self.cells]

    def head_intersect(self, cell):
        return self.cells[0].get_pos() == cell.get_pos()


class Form(Widget):
    worm_len = NumericProperty(0)

    def __init__(self, config):
        super().__init__()
        self.config = config
        self.worm = None
        self.cur_dir = (0, 0)
        self.fruit = None
        self.game_on = True
        self.smooth = smooth.XSmooth(["graphical_pos[0]", "graphical_pos[1]"])

    def random_cell_location(self, offset):
        x_row = self.size[0] // self.config.CELL_SIZE
        x_col = self.size[1] // self.config.CELL_SIZE
        return random.randint(offset, x_row - offset), random.randint(offset, x_col - offset)

    def random_location(self, offset):
        x_row, x_col = self.random_cell_location(offset)
        return self.config.CELL_SIZE * x_row, self.config.CELL_SIZE * x_col

    def fruit_dislocate(self):
        x, y = self.random_location(2)
        while (x, y) in self.worm.gather_positions():
            x, y = self.random_location(2)
        self.fruit.move_to(x, y)

    def start(self):
        self.worm = Worm(self.config)
        self.add_widget(self.worm)
        if self.fruit is not None:
            self.remove_widget(self.fruit)
        self.fruit = Cell(0, 0, self.config.APPLE_SIZE)
        self.fruit.color = (1.0, 0.2, 0.2, 1.0)
        self.fruit_dislocate()
        self.add_widget(self.fruit)
        self.game_on = True
        self.cur_dir = (0, -1)
        Clock.schedule_interval(self.update, self.config.INTERVAL)
        self.popup_label.text = ""

    def stop(self, text=""):
        self.game_on = False
        self.popup_label.text = text
        Clock.unschedule(self.update)

    def game_over(self):
        self.stop("GAME OVER" + " " * 5 + "\ntap to reset")

    def align_labels(self):
        try:
            self.popup_label.pos = ((self.size[0] - self.popup_label.width) / 2, self.size[1] / 2)
            self.score_label.pos = ((self.size[0] - self.score_label.width) / 2, self.size[1] - 80)
        except:
            print(self.__dict__)
            assert False

    def update(self, _):
        if not self.game_on:
            return
        self.worm.move(self.cur_dir, smooth_motion=(self.smooth, self.config.INTERVAL))
        if self.worm.head_intersect(self.fruit):
            directions = [(0, 1), (0, -1), (1, 0), (-1, 0)]
            self.worm.lengthen(direction=random.choice(directions))
            self.fruit_dislocate()
        cell = self.worm_bite_self()
        if cell:
            cell.color = (1.0, 0.2, 0.2, 1.0)
            self.game_over()
        self.worm_len = len(self.worm.cells)
        self.align_labels()

    def on_touch_down(self, touch):
        if not self.game_on:
            self.worm.destroy()
            self.start()
            return
        ws = touch.x / self.size[0]
        hs = touch.y / self.size[1]
        aws = 1 - ws
        if ws > hs and aws > hs:
            cur_dir = (0, -1)
        elif ws > hs >= aws:
            cur_dir = (1, 0)
        elif ws <= hs < aws:
            cur_dir = (-1, 0)
        else:
            cur_dir = (0, 1)
        self.cur_dir = cur_dir

    def worm_bite_self(self):
        for cell in self.worm.cells[1:]:
            if self.worm.head_intersect(cell):
                return cell
        return False


class Config:
    DEFAULT_LENGTH = 20
    CELL_SIZE = 25
    APPLE_SIZE = 35
    MARGIN = 4
    INTERVAL = 0.3
    DEAD_CELL = (1, 0, 0, 1)
    APPLE_COLOR = (1, 1, 0, 1)


class WormApp(App):
    def build(self):
        self.config = Config()
        self.form = Form(self.config)
        return self.form

    def on_start(self):
        self.form.start()


if __name__ == '__main__':
    WormApp().run()




smooth.py
from kivy.clock import Clock
import time


class Timing:
    @staticmethod
    def linear(x):
        return x


class Smooth:
    def __init__(self, interval=1.0/60.0):
        self.objs = []
        self.running = False
        self.interval = interval

    def run(self):
        if self.running:
            return
        self.running = True
        Clock.schedule_interval(self.update, self.interval)

    def stop(self):
        if not self.running:
            return
        self.running = False
        Clock.unschedule(self.update)

    def setattr(self, obj, attr, value):
        exec("obj." + attr + " = " + str(value))

    def getattr(self, obj, attr):
        return float(eval("obj." + attr))

    def update(self, _):
        cur_time = time.time()
        for line in self.objs:
            obj, prop_name_x, prop_name_y, from_x, from_y, to_x, to_y, start_time, period, timing = line
            time_gone = cur_time - start_time
            if time_gone >= period:
                self.setattr(obj, prop_name_x, to_x)
                self.setattr(obj, prop_name_y, to_y)
                self.objs.remove(line)
            else:
                share = time_gone / period
                acs = timing(share)
                self.setattr(obj, prop_name_x, from_x * (1 - acs) + to_x * acs)
                self.setattr(obj, prop_name_y, from_y * (1 - acs) + to_y * acs)
        if len(self.objs) == 0:
            self.stop()

    def move_to(self, obj, prop_name_x, prop_name_y, to_x, to_y, t, timing=Timing.linear):
        self.objs.append((obj, prop_name_x, prop_name_y, self.getattr(obj, prop_name_x), self.getattr(obj, prop_name_y), to_x,
                          to_y, time.time(), t, timing))
        self.run()


class XSmooth(Smooth):
    def __init__(self, props, timing=Timing.linear, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.props = props
        self.timing = timing

    def move_to(self, obj, to_x, to_y, t):
        super().move_to(obj, *self.props, to_x, to_y, t, timing=self.timing)




worm.kv
<Form>:
    popup_label: popup_label
    score_label: score_label

    canvas:
        Color:
            rgba: (.5, .5, .5, 1.0)

        Line:
            width: 1.5
            points: (0, 0), self.size

        Line:
            width: 1.5
            points: (self.size[0], 0), (0, self.size[1])
    Label:
        id: score_label
        text: "Score: " + str(self.parent.worm_len)
        width: self.width

    Label:
        id: popup_label
        width: self.width

<Worm>:


<Cell>:
    canvas:
        Color:
            rgba: self.color

        Rectangle:
            size: self.graphical_size
            pos: self.graphical_pos




Code, adjusted by @tshirtman
I received some issues with my code, for example, tshirtman, one of the Kivy project contributers, suggested me not to make Cells as Widgets but instead make a Point instruction. However, I don't find this code easier to understand than mine, even though it is definetely nicer in terms of UI and game development. Anyway, the code:
main.py
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.clock import Clock
from kivy.properties import *
import random
import smooth


class Cell:
    def __init__(self, x, y):
        self.actual_pos = (x, y)

    def move_to(self, x, y):
        self.actual_pos = (x, y)

    def move_by(self, x, y):
        self.move_to(self.actual_pos[0] + x, self.actual_pos[1] + y)

    def get_pos(self):
        return self.actual_pos


class Fruit(Cell):
    def __init__(self, x, y):
        super().__init__(x, y)


class Worm(Widget):
    margin = NumericProperty(4)
    graphical_poses = ListProperty()
    inj_pos = ListProperty([-1000, -1000])
    graphical_size = NumericProperty(0)

    def __init__(self, config, **kwargs):
        super().__init__(**kwargs)
        self.cells = []
        self.config = config
        self.cell_size = config.CELL_SIZE
        self.head_init((self.config.CELL_SIZE * random.randint(3, 5), self.config.CELL_SIZE * random.randint(3, 5)))
        self.margin = config.MARGIN
        self.graphical_size = self.cell_size - self.margin
        for i in range(config.DEFAULT_LENGTH):
            self.lengthen()

    def destroy(self):
        self.cells = []
        self.graphical_poses = []
        self.inj_pos = [-1000, -1000]

    def cell_append(self, pos):
        self.cells.append(Cell(*pos))
        self.graphical_poses.extend([0, 0])
        self.cell_move_to(len(self.cells) - 1, pos)

    def lengthen(self, pos=None, direction=(0, 1)):
        if pos is None:
            px = self.cells[-1].get_pos()[0] + direction[0] * self.cell_size
            py = self.cells[-1].get_pos()[1] + direction[1] * self.cell_size
            pos = (px, py)
        self.cell_append(pos)

    def head_init(self, pos):
        self.lengthen(pos=pos)

    def cell_move_to(self, i, pos, smooth_motion=None):
        self.cells[i].move_to(*pos)
        to_x, to_y = pos[0], pos[1]
        if smooth_motion is None:
            self.graphical_poses[i * 2], self.graphical_poses[i * 2 + 1] = to_x, to_y
        else:
            smoother, t = smooth_motion
            smoother.move_to(self, "graphical_poses[" + str(i * 2) + "]", "graphical_poses[" + str(i * 2 + 1) + "]",
                             to_x, to_y, t)

    def move(self, direction, **kwargs):
        for i in range(len(self.cells) - 1, 0, -1):
            self.cell_move_to(i, self.cells[i - 1].get_pos(), **kwargs)
        self.cell_move_to(0, (self.cells[0].get_pos()[0] + self.cell_size * direction[0], self.cells[0].get_pos()[1] +
                              self.cell_size * direction[1]), **kwargs)

    def gather_positions(self):
        return [cell.get_pos() for cell in self.cells]

    def head_intersect(self, cell):
        return self.cells[0].get_pos() == cell.get_pos()


class Form(Widget):
    worm_len = NumericProperty(0)
    fruit_pos = ListProperty([0, 0])
    fruit_size = NumericProperty(0)

    def __init__(self, config, **kwargs):
        super().__init__(**kwargs)
        self.config = config
        self.worm = None
        self.cur_dir = (0, 0)
        self.fruit = None
        self.game_on = True
        self.smooth = smooth.Smooth()

    def random_cell_location(self, offset):
        x_row = self.size[0] // self.config.CELL_SIZE
        x_col = self.size[1] // self.config.CELL_SIZE
        return random.randint(offset, x_row - offset), random.randint(offset, x_col - offset)

    def random_location(self, offset):
        x_row, x_col = self.random_cell_location(offset)
        return self.config.CELL_SIZE * x_row, self.config.CELL_SIZE * x_col

    def fruit_dislocate(self, xy=None):
        if xy is not None:
            x, y = xy
        else:
            x, y = self.random_location(2)
            while (x, y) in self.worm.gather_positions():
                x, y = self.random_location(2)
        self.fruit.move_to(x, y)
        self.fruit_pos = (x, y)

    def start(self):
        self.worm = Worm(self.config)
        self.add_widget(self.worm)
        self.fruit = Fruit(0, 0)
        self.fruit_size = self.config.APPLE_SIZE
        self.fruit_dislocate()
        self.game_on = True
        self.cur_dir = (0, -1)
        Clock.schedule_interval(self.update, self.config.INTERVAL)
        self.popup_label.text = ""

    def stop(self, text=""):
        self.game_on = False
        self.popup_label.text = text
        Clock.unschedule(self.update)

    def game_over(self):
        self.stop("GAME OVER" + " " * 5 + "\ntap to reset")

    def align_labels(self):
        self.popup_label.pos = ((self.size[0] - self.popup_label.width) / 2, self.size[1] / 2)
        self.score_label.pos = ((self.size[0] - self.score_label.width) / 2, self.size[1] - 80)

    def update(self, _):
        if not self.game_on:
            return
        self.worm.move(self.cur_dir, smooth_motion=(self.smooth, self.config.INTERVAL))
        if self.worm.head_intersect(self.fruit):
            directions = [(0, 1), (0, -1), (1, 0), (-1, 0)]
            self.worm.lengthen(direction=random.choice(directions))
            self.fruit_dislocate()
        cell = self.worm_bite_self()
        if cell is not None:
            self.worm.inj_pos = cell.get_pos()
            self.game_over()
        self.worm_len = len(self.worm.cells)
        self.align_labels()

    def on_touch_down(self, touch):
        if not self.game_on:
            self.worm.destroy()
            self.start()
            return
        ws = touch.x / self.size[0]
        hs = touch.y / self.size[1]
        aws = 1 - ws
        if ws > hs and aws > hs:
            cur_dir = (0, -1)
        elif ws > hs >= aws:
            cur_dir = (1, 0)
        elif ws <= hs < aws:
            cur_dir = (-1, 0)
        else:
            cur_dir = (0, 1)
        self.cur_dir = cur_dir

    def worm_bite_self(self):
        for cell in self.worm.cells[1:]:
            if self.worm.head_intersect(cell):
                return cell
        return None


class Config:
    DEFAULT_LENGTH = 20
    CELL_SIZE = 26  # DO NOT FORGET THAT CELL_SIZE - MARGIN WILL BE DIVIDED BY 4
    APPLE_SIZE = 36
    MARGIN = 2
    INTERVAL = 0.3
    DEAD_CELL = (1, 0, 0, 1)
    APPLE_COLOR = (1, 1, 0, 1)


class WormApp(App):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.form = None

    def build(self, **kwargs):
        self.config = Config()
        self.form = Form(self.config, **kwargs)
        return self.form

    def on_start(self):
        self.form.start()


if __name__ == '__main__':
    WormApp().run()


smooth.py
from kivy.clock import Clock
import time


class Timing:
    @staticmethod
    def linear(x):
        return x


class Smooth:
    def __init__(self, interval=1.0/60.0):
        self.objs = []
        self.running = False
        self.interval = interval

    def run(self):
        if self.running:
            return
        self.running = True
        Clock.schedule_interval(self.update, self.interval)

    def stop(self):
        if not self.running:
            return
        self.running = False
        Clock.unschedule(self.update)

    def set_attr(self, obj, attr, value):
        exec("obj." + attr + " = " + str(value))

    def get_attr(self, obj, attr):
        return float(eval("obj." + attr))

    def update(self, _):
        cur_time = time.time()
        for line in self.objs:
            obj, prop_name_x, prop_name_y, from_x, from_y, to_x, to_y, start_time, period, timing = line
            time_gone = cur_time - start_time
            if time_gone >= period:
                self.set_attr(obj, prop_name_x, to_x)
                self.set_attr(obj, prop_name_y, to_y)
                self.objs.remove(line)
            else:
                share = time_gone / period
                acs = timing(share)
                self.set_attr(obj, prop_name_x, from_x * (1 - acs) + to_x * acs)
                self.set_attr(obj, prop_name_y, from_y * (1 - acs) + to_y * acs)
        if len(self.objs) == 0:
            self.stop()

    def move_to(self, obj, prop_name_x, prop_name_y, to_x, to_y, t, timing=Timing.linear):
        self.objs.append((obj, prop_name_x, prop_name_y, self.get_attr(obj, prop_name_x), self.get_attr(obj, prop_name_y), to_x,
                          to_y, time.time(), t, timing))
        self.run()


class XSmooth(Smooth):
    def __init__(self, props, timing=Timing.linear, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.props = props
        self.timing = timing

    def move_to(self, obj, to_x, to_y, t):
        super().move_to(obj, *self.props, to_x, to_y, t, timing=self.timing)


worm.kv
<Form>:
    popup_label: popup_label
    score_label: score_label

    canvas:
        Color:
            rgba: (.5, .5, .5, 1.0)

        Line:
            width: 1.5
            points: (0, 0), self.size

        Line:
            width: 1.5
            points: (self.size[0], 0), (0, self.size[1])

        Color:
            rgba: (1.0, 0.2, 0.2, 1.0)

        Point:
            points: self.fruit_pos
            pointsize: self.fruit_size / 2

    Label:
        id: score_label
        text: "Score: " + str(self.parent.worm_len)
        width: self.width

    Label:
        id: popup_label
        width: self.width

<Worm>:
    canvas:
        Color:
            rgba: (0.2, 1.0, 0.2, 1.0)
        Point:
            points: self.graphical_poses
            pointsize: self.graphical_size / 2
        Color:
            rgba: (1.0, 0.2, 0.2, 1.0)
        Point:
            points: self.inj_pos
            pointsize: self.graphical_size / 2


Feel free to ask any questions.

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Comments 2

    +1
    you'll be able to build Android Apps victimization Python. Kivy would be a decent choice if you wish to create straight-forward games. there's an obstacle too, you will not be ready to leverage excellent commonplace and different open supply Android libraries with Kivy. they're obtainable via grade build(in Android Studio) or as jars.

      0

      Yeah. I'd never create complicated 3d-games, browsers, video editors and so on, with Kivy. The lack of supported libraries is not the only reason. When an update for android is released, new features are presented. But, of course, if I develop an app with such far-from-Java frameworks, I'll never get up-to-date support of new features.

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