Programming Tutorials

Using Observer in Ruby

By: Hunt and Thomas in Ruby Tutorials on 2009-03-03  

The Observer pattern, also known as Publish/Subscribe, provides a simple mechanism for one object to inform a set of interested third-party objects when its state changes.

Mechanism

In the Ruby implementation, the notifying class mixes in the Observable module, which provides the methods for managing the associated observer objects.

The observers must implement the update method to receive notifications.

The observable object must:

• assert that it has changed
• call notify_observers

Example

The following example demonstrates this nicely. A Ticker, when run, continually receives the stock Price for its +@symbol+. A Warner is a general observer of the price, and two warners are demonstrated, a WarnLow and a WarnHigh, which print a warning if the price is below or above their set limits, respectively.

The update callback allows the warners to run without being explicitly called. The system is set up with the Ticker and several observers, and the observers do their duty without the top-level code having to interfere.

Note that the contract between publisher and subscriber (observable and observer) is not declared or enforced. The Ticker publishes a time and a price, and the warners receive that. But if you don't ensure that your contracts are correct, nothing else can warn you.

  require "observer"

  class Ticker          ### Periodically fetch a stock price.
    include Observable

    def initialize(symbol)
      @symbol = symbol
    end

    def run
      lastPrice = nil
      loop do
        price = Price.fetch(@symbol)
        print "Current price: #{price}\n"
        if price != lastPrice
          changed                 # notify observers
          lastPrice = price
          notify_observers(Time.now, price)
        end
        sleep 1
      end
    end
  end

  class Price           ### A mock class to fetch a stock price (60 - 140).
    def Price.fetch(symbol)
      60 + rand(80)
    end
  end

  class Warner          ### An abstract observer of Ticker objects.
    def initialize(ticker, limit)
      @limit = limit
      ticker.add_observer(self)
    end
  end

  class WarnLow < Warner
    def update(time, price)       # callback for observer
      if price < @limit
        print "--- #{time.to_s}: Price below #@limit: #{price}\n"
      end
    end
  end

  class WarnHigh < Warner
    def update(time, price)       # callback for observer
      if price > @limit
        print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
      end
    end
  end

  ticker = Ticker.new("MSFT")
  WarnLow.new(ticker, 80)
  WarnHigh.new(ticker, 120)
  ticker.run

Produces:

  Current price: 83
  Current price: 75
  --- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75
  Current price: 90
  Current price: 134
  +++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134
  Current price: 134
  Current price: 112
  Current price: 79
  --- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79