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#
# 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.
#
# == About
#
# 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
#
# 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.
#
module Observable
#
# Add +observer+ as an observer on this object. +observer+ will now receive
# notifications.
#
def add_observer(observer)
@observer_peers = [] unless defined? @observer_peers
unless observer.respond_to? :update
raise NoMethodError, "observer needs to respond to `update'"
end
@observer_peers.push observer
end
#
# Delete +observer+ as an observer on this object. It will no longer receive
# notifications.
#
def delete_observer(observer)
@observer_peers.delete observer if defined? @observer_peers
end
#
# Delete all observers associated with this object.
#
def delete_observers
@observer_peers.clear if defined? @observer_peers
end
#
# Return the number of observers associated with this object.
#
def count_observers
if defined? @observer_peers
@observer_peers.size
else
0
end
end
#
# Set the changed state of this object. Notifications will be sent only if
# the changed +state+ is +true+.
#
def changed(state=true)
@observer_state = state
end
#
# Query the changed state of this object.
#
def changed?
if defined? @observer_state and @observer_state
true
else
false
end
end
#
# 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
i.update(*arg)
end
end
@observer_state = false
end
end
end
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