Using Observer in Ruby

By: Hunt and Thomas  

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.


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


The following example demonstrates this nicely. A Ticker, when run, continually receives the stock Price for its [email protected]+. 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

    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(, price)
        sleep 1

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

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

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

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

  ticker ="MSFT"), 80), 120)


  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

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