Observer / Tk Demo

observe.rb

require 'observer'

class TestObservable
  include Observable

  def initialize(name)
    @name = name
  end

  def change_with(value)
    changed
    notify_observers @name, value
  end
end

class TestObserver
  def initialize(name)
    @name = name
  end

  def update(observable, value)
    puts "Observer #{@name} observered #{observable} change with #{value}"
  end
end

# Make observables
bill = TestObservable.new('Bill')
fred = TestObservable.new('Fred')

# Make observers
john = TestObserver.new('John')
bill.add_observer(john)
fred.add_observer(john)

will = TestObserver.new('Will')
bill.add_observer(will)
fred.add_observer(will)


# Change the observables
bill.change_with(12)
fred.change_with(9001)

stock_watcher.rb

require 'tk'
require 'observer'

class StockPrice
  def self.fetch(symbol)
    60 + rand(80)
  end
end

class Ticker
  include Observable

  @mutex = Mutex.new
  @tickers = {}

  class << self
    def ticker(symbol)
      @mutex.synchronize do
        return @tickers[symbol] if @tickers[symbol]
        @tickers[symbol] = Ticker.new(symbol)
      end
    end

    def run
      Thread.new do
        loop do
          fetch_stocks
          sleep 2
        end
      end
    end

    def fetch_stocks
      @mutex.synchronize do
        @tickers.each do |symbol, ticker|
          ticker.price = StockPrice.fetch(symbol)
        end
      end
    end
  end

  attr_reader :symbol, :price
  def initialize(symbol)
    @symbol = symbol
  end

  def price=(price)
    unless @price == price
      changed # Must call changed in order for observers to actually be notified
      @price = price
      notify_observers(@price)
    end
  end
end

class StockViewer
  def initialize(root, row, ticker)
    @symbol_label = TkLabel.new(root) do
      text ticker.symbol
      grid({
        :row    => row,
        :column => 1,
        :stick  => 'nsew'
      })
    end
    @price_label = TkLabel.new(root) do
      text 'waiting...'
      grid({
        :row    => row,
        :column => 2,
        :stick  => 'nsew'
      })
    end
    ticker.add_observer(self)
  end

  def update(price)
    @price_label.text price
  end
end

class Application
  def add_stock(entry)
    ticker = Ticker.ticker(entry.value)
    StockViewer.new(@root, @row, ticker)
    @row += 1
    entry.value = ""
  end

  def initialize
    @row = 2

    @root = TkRoot.new do
      title "Stock Watcher"
    end
    TkGrid.columnconfigure(@root, 1, :weight => 1)
    TkGrid.columnconfigure(@root, 2, :weight => 1)

    entry = TkEntry.new(@root) do
      width 10
      grid({
        :row    => 1,
        :column => 1,
        :sticky => 'nsew'
      })
    end

    handler = proc { add_stock(entry) }

    TkButton.new(@root) do
      text "Add Stock"
      grid({
        :row    => 1,
        :column => 2,
        :sticky => 'nsew'
      })

      command handler
    end

    Ticker.run
    Tk.mainloop
  end
end

Application.new