Nibbles
In this part of the Ruby Qt 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.rbWIDTH = 300 HEIGHT = 300 DOT_SIZE = 10 ALL_DOTS = WIDTH * HEIGHT / (DOT_SIZE * DOT_SIZE) RAND_POS = 29 DELAY = 140 $x = [0] * ALL_DOTS $y = [0] * ALL_DOTS class Board < Qt::Widget def initialize(parent) super(parent) setFocusPolicy Qt::StrongFocus initGame end def initGame @left = false @right = true @up = false @down = false @inGame = true @dots = 3 begin @ball = Qt::Image.new "dot.png" @apple = Qt::Image.new "apple.png" @head = Qt::Image.new "head.png" rescue puts "cannot load images" end for i in (0..@dots) $x[i] = 50 - i * 10 $y[i] = 50 end locateApple setStyleSheet "QWidget { background-color: #000000 }" @timer = Qt::BasicTimer.new @timer.start(140, self) end def paintEvent event painter = Qt::Painter.new painter.begin self if @inGame drawObjects painter else gameOver painter end painter.end end def drawObjects painter painter.drawImage @apple_x, @apple_y, @apple for z in (0..@dots) if z == 0 painter.drawImage $x[z], $y[z], @head else painter.drawImage $x[z], $y[z], @ball end end end def gameOver painter msg = "Game Over" small = Qt::Font.new "Helvetica", 12, Qt::Font::Bold.value metr = Qt::FontMetrics.new small textWidth = metr.width msg h = height w = width painter.setPen Qt::Color.new Qt::white painter.setFont small painter.translate Qt::Point.new w/2, h/2 painter.drawText -textWidth/2, 0, msg end def checkApple if $x[0] == @apple_x and $y[0] == @apple_y @dots = @dots + 1 locateApple 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 checkCollision 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 @inGame = false end if $y[0] < 0 @inGame = false end if $x[0] > WIDTH @inGame = false end if $x[0] < 0 @inGame = false end end def locateApple r = rand RAND_POS @apple_x = r * DOT_SIZE r = rand RAND_POS @apple_y = r * DOT_SIZE end def timerEvent event if @inGame checkApple checkCollision move else @timer.stop end repaint end def keyPressEvent event key = event.key if key == Qt::Key_Left.value and not @right @left = true @up = false @down = false end if key == Qt::Key_Right.value and not @left @right = true @up = false @down = false end if key == Qt::Key_Up.value and not @down @up = true @right = false @left = false end if key == Qt::Key_Down.value and not @up @down = true @right = false @left = false end end end
First we will define some constants used in our game.
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_DOTS $y = [0] * ALL_DOTS
These two arrays store x, y coordinates of all possible joints of a snake.
The initGame method initializes variables, loads images and starts a timeout function.
if @inGame
drawObjects painter
else
gameOver painter
end
Inside the paintEvent method, we check the @inGame variable. If it is true, we draw our objects. The apple and the snake joints. Otherwise we display "Game over" text.
def drawObjects painter
painter.drawImage @apple_x, @apple_y, @apple
for z in (0..@dots)
if z == 0
painter.drawImage $x[z], $y[z], @head
else
painter.drawImage $x[z], $y[z], @ball
end
end
end
The drawObjects 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 checkApple
if $x[0] == @apple_x and $y[0] == @apple_y
@dots = @dots + 1
locateApple
end
end
The checkApple method checks, if the snake has hit the apple object. If so, we add another snake joint and call the locateApple 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 > 0
$x[z] = $x[(z - 1)]
$y[z] = $y[(z - 1)]
z = z - 1
end
This code moves the joints up the chain.
if @left
$x[0] -= DOT_SIZE
end
Move the head to the left.
In the checkCollision method, we determine if the snake has hit itself or one of the walls.
while z > 0
if z > 4 and $x[0] == $x[z] and $y[0] == $y[z]
@inGame = false
end
z = z - 1
end
Finish the game, if the snake hits one of its joints with the head.
if $y[0] > HEIGHT
@inGame = false
end
Finish the game, if the snake hits the bottom of the Board.
The locateApple 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_SIZE ... @apple_y = r * DOT_SIZE
These line set the x, y coordinates of the apple object.
if @inGame
checkApple
checkCollision
move
else
@timer.stop
end
Every 140 ms, the timerEvent method is called. If we are in the game, we call three methods, that build the logic of the game. Otherwise we stop the timer.
In the keyPressEvent method of the Board class, we determine the keys that were pressed.
if key == Qt::Key_Left.value and not @right
@left = true
@up = false
@down = false
end
If we hit the left cursor key, we set @left variable to true. This variable is used in the move method 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/ruby # ZetCode Ruby Qt tutorial # # In this program, we create # a Nibbles game clone. # # author: jan bodnar # website: www.zetcode.com # last modified: July 2009 require 'Qt' require 'Board' class QtApp < Qt::MainWindow def initialize super setWindowTitle "Nibbles" setCentralWidget Board.new(self) resize 310, 310 move 300, 300 show end end app = Qt::Application.new ARGV QtApp.new app.exec
In this class, we set up the Nibbles game.
This was the Nibbles computer game programmed with the Qt library and the Ruby programming language.