Nibbles
In this part of the Ruby GTK programming tutorial, we will create a Nibbles game clone.Nibbles is an older classic video game. It was first created in late 70s. Later it was brought to PCs. In this game the player controls a snake. The objective is to eat as many apples as possible. Each time the snake eats an apple, its body grows. The snake must avoid the walls and its own body.
Development
The size of each of the joints of a snake is 10px. The snake is controlled with the cursor keys. Initially, the snake has three joints. The game starts immediately. When the game is finished, we display "Game Over" message in the center of the window.board.rb
WIDTH = 300First we will define some globals used in our game.
HEIGHT = 270
DOT_SIZE = 10
ALL_DOTS = WIDTH * HEIGHT / (DOT_SIZE * DOT_SIZE)
RAND_POS = 26
$x = [0] * ALL_DOTS
$y = [0] * ALL_DOTS
class Board < Gtk::DrawingArea
def initialize
super
modify_bg Gtk::STATE_NORMAL, Gdk::Color.new(0, 0, 0)
signal_connect "expose-event" do
on_expose
end
init_game
end
def on_timer
if @inGame
check_apple
check_collision
move
queue_draw
return true
else
return false
end
end
def init_game
@left = false
@right = true
@up = false
@down = false
@inGame = true
@dots = 3
for i in (0..@dots)
$x[i] = 50 - i * 10
$y[i] = 50
end
begin
@dot = Cairo::ImageSurface.from_png "dot.png"
@head = Cairo::ImageSurface.from_png "head.png"
@apple = Cairo::ImageSurface.from_png "apple.png"
rescue Exception => e
puts "cannot load images"
exit
end
locate_apple
GLib::Timeout.add(100) { on_timer }
end
def on_expose
cr = window.create_cairo_context
if @inGame
draw_objects cr
else
game_over cr
end
end
def draw_objects cr
cr.set_source_rgb 0, 0, 0
cr.paint
cr.set_source @apple, @apple_x, @apple_y
cr.paint
for z in (0..@dots)
if z == 0
cr.set_source @head, $x[z], $y[z]
cr.paint
else
cr.set_source @dot, $x[z], $y[z]
cr.paint
end
end
end
def game_over cr
w = allocation.width / 2
h = allocation.height / 2
cr.set_font_size 15
te = cr.text_extents "Game Over"
cr.set_source_rgb 65535, 65535, 65535
cr.move_to w - te.width/2, h
cr.show_text "Game Over"
end
def check_apple
if $x[0] == @apple_x and $y[0] == @apple_y
@dots = @dots + 1
locate_apple
end
end
def move
z = @dots
while z > 0
$x[z] = $x[(z - 1)]
$y[z] = $y[(z - 1)]
z = z - 1
end
if @left
$x[0] -= DOT_SIZE
end
if @right
$x[0] += DOT_SIZE
end
if @up
$y[0] -= DOT_SIZE
end
if @down
$y[0] += DOT_SIZE
end
end
def check_collision
z = @dots
while z > 0
if z > 4 and $x[0] == $x[z] and $y[0] == $y[z]
@inGame = false
end
z = z - 1
end
if $y[0] > HEIGHT - DOT_SIZE
@inGame = false
end
if $y[0] < 0
@inGame = false
end
if $x[0] > WIDTH - DOT_SIZE
@inGame = false
end
if $x[0] < 0
@inGame = false
end
end
def locate_apple
r = rand(RAND_POS)
@apple_x = r * DOT_SIZE
r = rand(RAND_POS)
@apple_y = r * DOT_SIZE
end
def on_key_down event
key = event.keyval
if key == Gdk::Keyval::GDK_Left and not @right
@left = true
@up = false
@down = false
end
if key == Gdk::Keyval::GDK_Right and not @left
@right = true
@up = false
@down = false
end
if key == Gdk::Keyval::GDK_Up and not @down
@up = true
@right = false
@left = false
end
if key == Gdk::Keyval::GDK_Down and not @up
@down = true
@right = false
@left = false
end
end
end
The WIDTH and HEIGHT constants determine the size of the Board. The DOT_SIZE is the size of the apple and the dot of the snake. The ALL_DOTS constant defines the maximum number of possible dots on the Board. The RAND_POS constant is used to calculate a random position of an apple. The DELAY constant determines the speed of the game.
$x = [0] * ALL_DOTSThese two arrays store x, y coordinates of all possible joints of a snake.
$y = [0] * ALL_DOTS
The
init_game method initializes variables, loads images and starts a timeout function. if @inGameInside the
draw_objects cr
else
game_over cr
end
on_expose method, we check the @inGamevariable. If it is true, we draw our objects. The apple and the snake joints. Otherwise we display "Game over" text. def draw_objects crThe
cr.set_source_rgb 0, 0, 0
cr.paint
cr.set_source @apple, @apple_x, @apple_y
cr.paint
for z in (0..@dots)
if z == 0
cr.set_source @head, $x[z], $y[z]
cr.paint
else
cr.set_source @dot, $x[z], $y[z]
cr.paint
end
end
end
draw_objects method draws the apple and the joints of the snake. The first joint of a snake is its head, which is represented by a red circle. def check_appleThe
if $x[0] == @apple_x and $y[0] == @apple_y
@dots = @dots + 1
locate_apple
end
end
check_apple method checks, if the snake has hit the apple object. If so, we add another snake joint and call the locate_apple method, which randomly places a new apple object. In the
move method we have the key algorithm of the game. To understand it, look at how the snake is moving. You control the head of the snake. You can change its direction with the cursor keys. The rest of the joints move one position up the chain. The second joint moves where the first was, the third joint where the second was etc. while z > 0This code moves the joints up the chain.
$x[z] = $x[(z - 1)]
$y[z] = $y[(z - 1)]
z = z - 1
end
if @leftMove the head to the left.
$x[0] -= DOT_SIZE
end
In the
check_collision method, we determine if the snake has hit itself or one of the walls. while z > 0Finish the game, if the snake hits one of its joints with the head.
if z > 4 and $x[0] == $x[z] and $y[0] == $y[z]
@inGame = false
end
z = z - 1
end
if $y[0] > HEIGHT - DOT_SIZEFinish the game, if the snake hits the bottom of the Board.
@inGame = false
end
The
locate_apple method locates an apple randomly on the board. r = rand(RAND_POS)We get a random number from 0 to RAND_POS - 1.
@apple_x = r * DOT_SIZEThese line set the x, y coordinates of the apple object.
...
@apple_y = r * DOT_SIZE
if @inGameEvery 140 ms, the
check_apple
check_collision
move
queue_draw
return true
else
return false
end
on_timer method is called. If we are in the game, we call three methods, that build the logic of the game. Otherwise we return false, which stops the timer event. In the
on_key_down method of the Board class, we determine the keys that were pressed. if key == Gdk::Keyval::GDK_Left and not @rightIf we hit the left cursor key, we set
@left = true
@up = false
@down = false
end
left variable to true. This variable is used in the movemethod to change coordinates of the snake object. Notice also, that when the snake is heading to the right, we cannot turn immediately to the left. nibbles.rb
#!/usr/bin/rubyIn this class, we set up the Nibbles game.
# ZetCode Ruby GTK tutorial
#
# This is a simple nibbles game
# clone
#
# author: jan bodnar
# website: www.zetcode.com
# last modified: June 2009
require 'gtk2'
require 'board'
class RubyApp > Gtk::Window
def initialize
super
set_title "Nibbles"
signal_connect "destroy" do
Gtk.main_quit
end
@board = Board.new
signal_connect "key-press-event" do |w, e|
on_key_down(w, e)
end
add @board
set_default_size 300, 270
set_window_position Gtk::Window::POS_CENTER
show_all
end
def on_key_down widget, event
key = event.keyval
@board.on_key_down event
end
end
Gtk.init
window = RubyApp.new
Gtk.main
def on_key_down widget, eventIn this class, we catch the key press events. And delegate the processing to the
key = event.keyval
@board.on_key_down event
end
on_key_down method of the board class. 
Figure: Nibbles
This was the Nibbles computer game programmed with the GTK library and the Ruby programming language. 
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