Last Modified
2014-03-27 09:01:43 -0700


observer.rb implements the Observer object-oriented design pattern. The following documentation is copied, with modifications, from “Programming 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:


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

  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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