Opal: Ruby in the Browser and the Game of Life
Welcome to the the second part of the article where we look at Opal, a Ruby to Javascript compiler!
In the previous installment, I introduced Opal and showed how to get it set up on your system. We created the first half of Conway’s Game of Life in Opal. In particular, implementing the grid using the HTML 5 canvas element, and adding some interactivity to it.
In this article, we will complete our application by implementing the rest of the logic, and hooking it to the canvas. The completed source can be found at the end of the article.
Let’s dive right in!
Game Rules
As you might recall, Conway’s Game of Life consists of 4 simple rules:
Rule 1
Any live cell with fewer than two live neighbors dies, as if caused by under-population.
Rule 2
Any live cell with two or three live neighbors lives on to the next generation.
Rule 3
Any live cell with more than three live neighbors dies, as if by overcrowding.
Rule 4
Any dead cell with exactly three live neighbors becomes a live cell, as if by reproduction.
Protip: Using pp
as console
.log
Along the way, you might use lots of console.log
statements for debugging. Indeed, that was what I used to do. Then, I learned an awesome trick:
@bentanweihao @opalrb #protip `console.log` is mapped to #pp (https://t.co/5mrxIiOJdV), awesome article btw :)
— EliaSchito(͡° ͜ʖ ͡°) (@elia) April 4, 2014
Implementing the Game Logic
Step 1: Create conway.rb
In the app
directory, go ahead and create conway.rb
:
require 'opal'
require 'opal-jquery'
require 'grid'
class Conway
attr_reader :grid
def initialize(grid)
@grid = grid
end
end
We pass in an instance of Grid
to the Conway
initializer.
Step 2: Discovering Liveness and Population Count
Before the 4 rules can be implemented, we need to find out whether a cell at a particular coordinate is alive or dead:
def is_alive?(x, y)
state[[x, y]] == 1
end
def is_dead?(x, y)
!is_alive?(x, y)
end
Also how many neighbors for a given coordinate:
def population_at(x, y)
[
state[[x-1, y-1]],
state[[x-1, y ]],
state[[x-1, y+1]],
state[[x, y-1]],
state[[x, y+1]],
state[[x+1, y-1]],
state[[x+1, y ]],
state[[x+1, y+1]]
].map(&:to_i).reduce(:+)
end
Notice how we can happily make use of Ruby methods like map
and reduce
, and even Symbol#to_proc
works! It almost makes you forget that you are also working with Javascript.
Step 3: Implementing the Rules
Now that we can check for the liveness and population count at a coordinate, implementing the 4 rules is simple:
# Any live cell with fewer than two live neighbours dies,
# as if caused by under-population.
def is_underpopulated?(state, x, y)
is_alive?(x, y) && population_at(x, y) < 2
end
# Any live cell with two or three live neighbours lives
# on to the next generation.
def is_living_happily?(x, y)
is_alive?(x, y) && ([2, 3].include? population_at(x, y))
end
# Any live cell with more than three live neighbours dies,
# as if by overcrowding.
def is_overpopulated?(x, y)
is_alive?(x, y) && population_at(x, y) > 3
end
# Any dead cell with exactly three live neighbours becomes a live cell,
# as if by reproduction.
def can_reproduce?(x, y)
is_dead?(x, y) && population_at(x, y) == 3
end
Next, check if a cell makes it at the next interval, or tick, in Game of Life parlance:
def get_state_at(x, y)
if is_underpopulated?(x, y)
0
elsif is_living_happily?(x, y)
1
elsif is_overpopulated?(x, y)
0
elsif can_reproduce?(x, y)
1
end
end
Step 4: Accessing the Grid State
Recall how Grid
was implemented:
class Grid
attr_reader :height, :width, :canvas, :context, :max_x, :max_y
attr_accessor :state
CELL_HEIGHT = 15;
CELL_WIDTH = 15;
def initialize
@height = `$(window).height()`
@width = `$(window).width()`
@canvas = `document.getElementById(#{canvas_id})`
@context = `#{canvas}.getContext('2d')`
@max_x = (height / CELL_HEIGHT).floor
@max_y = (width / CELL_WIDTH).floor
@state = blank_state
draw_canvas
end
def blank_state
h = Hash.new
(0..max_x).each do |x|
(0..max_y).each do |y|
h[[x,y]] = 0
end
end
h
end
# ...
end
Note that the state of the grid is stored in the @state
attribute. In order to access the grid state, a Conway
object needs to do something like grid.state
.
Turns out, there’s a more idiomatic way to do this in Ruby. Enter the Forwardable
module. What does this do exactly?
The Forwardable module provides delegation of specified methods to a designated object, using the methods
def_delegator
anddef_delegators
.
In the code:
require 'opal'
require 'opal-jquery'
require 'grid'
require 'forwardable'
class Conway
attr_reader :grid
extend Forwardable
def_delegators :@grid, :state, :state=
def initialize(grid)
@grid = grid
end
# ...
end
First, require
and extend
Forwardable. Then, declare which object (@grid
) and which methods we want delegated. This means that both of these are now the same thing:
Conway.new(Grid.new).grid.state
Conway.new(Grid.new).state
Step 5: Computing the Next State
At each tick
, the next state for each cell needs to be computed. In other words, find out whether that cell is alive or dead. The computed new state is then saved into grid
. The entire canvas is redrawn with these new coordinates.
def tick
# def_delegators at work again!
# This call is delegate to grid.state=
self.state = new_state
redraw_canvas
end
def new_state
new_state = Hash.new
state.each do |cell, _|
new_state[cell] = get_state_at(cell[0], cell[1])
end
new_state
end
Redrawing the Canvas
We have yet to implement redraw_canvas
. Let’s rectify that. Open up grid.rb
and add this:
class Grid
# ...
def redraw_canvas
draw_canvas
state.each do |cell, liveness|
fill_cell(cell[0], cell[1]) if liveness == 1
end
end
# ...
end
redraw_canvas
redraws a blank canvas and iterates through state
. If the coordinate value is 1 (alive), it fills it, otherwise it is left unfilled.
Head back to conway.rb
and make an addition to the def_delegators
:
def_delegators :@grid, :state, :state=, :redraw_canvas
Step 6. Looping
There is no way to transition into the next tick.
In Opal, you cannot write an infinite loop in the traditional Ruby way. So, doing something like this does not work:
loop do
tick
end
Neither will this:
while true do
tick
end
Let’s think for a bit. What we really want is to run tick
at certain, fixed intervals. Javascript has setInterval
! How can we use this in our code?
Create yet another file called interval.rb
in app
.
class Interval
# Note that time is in ms.
def initialize(time=0, &block)
@interval = `setInterval(function(){#{block.call}}, time)`
end
def stop
`clearInterval(#@interval)`
end
end
I hope your mind is blown after you realize what is going on here.
The Interval
initializer takes in a time
(no surprise), and a block! Then it call
s the block in the Javascript setInterval
function!
This means that we can now implement our infinite loop. Remember to require interval
and then pass in a block containing the tick
method like so:
require 'interval'
#...
class Conway
# ...
def run
Interval.new do
tick
end
end
# ...
end
run
kickstarts the loop which drives the entire animation. But before we get to that, we have a bit more work left to do.
Step 7: Seeding
We have yet to tackle the issue of the initial state of the board, also known as the seed
. seed
should contain the coordinates of all the clicked positions of the board.
When the grid is first loaded, it is blank. The user gets to click the cells to fill them up. This makes up the seed
. The seed
coordinates are recorded. When the user is done, he/she presses the Enter key, triggering Conway#run
. The user now gets to enjoy a lovely animation.
Open up grid.rb
, and add the seed
attribute. For convenience, we call add_mouse_event_listener
when Grid
is initialized.
class Grid
attr_accessor :state, :seed
def initialize
# ...
@seed = []
# ...
add_mouse_event_listener
end
# ...
end
When a click occurs, add the coordinates into seed
. Similarly, for double-click, remove the coordinates from seed
.
def add_mouse_event_listener
Element.find("##{canvas_id}").on :click do |event|
# ...
seed << [x, y]
end
Element.find("##{canvas_id}").on :dblclick do |event|
# ...
seed.delete([x, y])
end
end
Turn your attention now to conway.rb
. First, add seed
to def_delegators
:
def_delegators :@grid, :state, :state=, :redraw_canvas, :seed
We need to detect when the ‘Enter’ key is pressed.
def add_enter_event_listener
Document.on :keypress do |event|
if enter_pressed?(event)
seed.each do |x, y|
state[[x, y]] = 1
end
run
end
end
end
When ‘Enter’ is detected, this method iterates through the coordinates of seed
and populate the grid
‘s initial state
. After that, the game is run.
How do we detect if an ‘Enter’ is pressed? Just like in JavaScript/jQuery, check event.which
for the ‘Enter’ key code:
def enter_pressed?(event)
event.which == 13
end
Again for convenience, we run add_enter_event_listener
when Conway
is initialized:
class Conway
# ...
def initialize(grid)
@grid = grid
add_enter_event_listener
end
# ...
end
Step 8: Bonus! The Glider Gun Pattern
Drop this code into grid.rb
:
class Grid
def initialize
# ...
add_demo_event_listener
end
# Ctrl+D displays a demo of the Glider Gun:
# https://en.wikipedia.org/wiki/File:Game_of_life_glider_gun.svg
def add_demo_event_listener
Document.on :keypress do |event|
if ctrl_d_pressed?(event)
[
[25, 1],
[23, 2], [25, 2],
[13, 3], [14, 3], [21, 3], [22, 3],
[12, 4], [16, 4], [21, 4], [22, 4], [35, 4], [36, 4],
[1, 5], [2, 5], [11, 5], [17, 5], [21, 5], [22, 5], [35, 5], [36, 5],
[1, 6], [2, 6], [11, 6], [15, 6], [17, 6], [18, 6], [23, 6], [25, 6],
[11, 7], [17, 7], [25, 7],
[12, 8], [16, 8],
[13, 9], [14, 9]
].each do |x, y|
fill_cell(x, y)
seed << [x, y]
end
end
end
end
def ctrl_d_pressed?(event)
event.ctrl_key == true && event.which == 4
end
# ...
end
Here, when you type Ctrl
+ D
together, the Glider Gun pattern would be drawn. Press ‘Enter’ to start the animation.
Step 9: All Done!
Give yourself a big pat on the back. We have reached the end. Go ahead and try out a couple of patterns. If you need any inspiration for starting patterns, a quick search will yield many interesting results.
Concluding Remarks
I hope you had a bit of fun experimenting with Opal and, along the way, maybe even found ways to integrate Opal into your existing project. Rails developers will be pleased to know that Opal plays nice with the asset pipeline with the opal-rails
gem.
In my short experience playing with Opal, I didn’t find debugging that big of an issue. Usually, inspecting the error log in the browser was sufficient in figuring out what went wrong. This is especially true since during compilation, the Javascript function has the same name as the Ruby method. Most of the times the error messages were descriptive enough.
You will notice that this implementation is not exactly performant, and depends on the size of the canvas. I chose to go for the most straightforward implementation, therefore, some performance had to be sacrificed. Nonetheless, I hope you had lot of fun building this application!
Another thing to note is that there are a few libraries already written for Opal, such as opal-browser, opal-rails and opal-rspec. You can see even more on the Opal github page. We could make our application shorter with opal-browser, for example, but I wanted to demonstrate what could be achieved with the bare minimum.
Opal is very promising, and I hope it gains more widespread adoption. It will be very interesting to see what other developers do with this.
Thanks for reading!
Resources
- Opal, A new hope (for Ruby programmers) (Ruby Conf 2013) – video
- Opal mailing list is pretty active, and a good place to ask questions if you get stuck.
The Complete Source
For your convenience, the complete source is on Github