# observer.rb implements the _Observer_ object-oriented design pattern. The
# following documentation is copied, with modifications, from "Programming
# Ruby", by Hunt and Thomas; http://www.rubycentral.com/book/lib_patterns.html.
# 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
# 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 +@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.
# class Ticker ### Periodically fetch a stock price.
# price = Price.fetch(@symbol)
# print "Current price: #{price}\n"
# changed # notify observers
# notify_observers(Time.now, price)
# class Price ### A mock class to fetch a stock price (60 - 140).
# def Price.fetch(symbol)
# class Warner ### An abstract observer of Ticker objects.
# def initialize(ticker, limit)
# ticker.add_observer(self)
# def update(time, price) # callback for observer
# print "--- #{time.to_s}: Price below #@limit: #{price}\n"
# class WarnHigh < Warner
# def update(time, price) # callback for observer
# print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
# ticker = Ticker.new("MSFT")
# WarnLow.new(ticker, 80)
# WarnHigh.new(ticker, 120)
# --- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75
# +++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134
# --- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79
# Implements the Observable design pattern as a mixin so that other objects can
# be notified of changes in state. See observer.rb for details and an example.
# Add +observer+ as an observer on this object. +observer+ will now receive
def add_observer(observer)
@observer_peers = [] unless defined? @observer_peers
unless observer.respond_to? :update
raise NoMethodError, "observer needs to respond to `update'"
@observer_peers.push observer
# Delete +observer+ as an observer on this object. It will no longer receive
def delete_observer(observer)
@observer_peers.delete observer if defined? @observer_peers
# Delete all observers associated with this object.
@observer_peers.clear if defined? @observer_peers
# Return the number of observers associated with this object.
if defined? @observer_peers
# Set the changed state of this object. Notifications will be sent only if
# the changed +state+ is +true+.
# Query the changed state of this object.
if defined? @observer_state and @observer_state
# If this object's changed state is +true+, invoke the update method in each
# currently associated observer in turn, passing it the given arguments. The
# changed state is then set to +false+.
def notify_observers(*arg)
if defined? @observer_state and @observer_state
if defined? @observer_peers
for i in @observer_peers.dup
It is recommended that you Edit text format, this type of Fix handles quite a lot in one request
