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Filename : observer.rb
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#
# Implementation of the _Observer_ object-oriented design pattern.  The
# following documentation is copied, with modifications, from "Programming
# Ruby", by Hunt and Thomas; http://www.ruby-doc.org/docs/ProgrammingRuby/html/lib_patterns.html.
#
# See Observable for more info.

# 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
#
# The notifying class mixes in the +Observable+
# module, which provides the methods for managing the associated observer
# objects.
#
# The observers must implement a method called +update+ 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 <tt>@symbol</tt>.  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
module Observable

  #
  # Add +observer+ as an observer on this object. so that it will receive
  # notifications.
  #
  # +observer+:: the object that will be notified of changes.
  # +func+:: Symbol naming the method that will be called when this Observable
  #          has changes.
  #
  #          This method must return true for +observer.respond_to?+ and will
  #          receive <tt>*arg</tt> when #notify_observers is called, where
  #          <tt>*arg</tt> is the value passed to #notify_observers by this
  #          Observable
  def add_observer(observer, func=:update)
    @observer_peers = {} unless defined? @observer_peers
    unless observer.respond_to? func
      raise NoMethodError, "observer does not respond to `#{func.to_s}'"
    end
    @observer_peers[observer] = func
  end

  #
  # Remove +observer+ as an observer on this object so that it will no longer
  # receive notifications.
  #
  # +observer+:: An observer of this Observable
  def delete_observer(observer)
    @observer_peers.delete observer if defined? @observer_peers
  end

  #
  # Remove 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+.
  #
  # +state+:: Boolean indicating the changed state of this Observable.
  #
  def changed(state=true)
    @observer_state = state
  end

  #
  # Returns true if this object's state has been changed since the last
  # #notify_observers call.
  #
  def changed?
    if defined? @observer_state and @observer_state
      true
    else
      false
    end
  end

  #
  # Notify observers of a change in state *if* this object's changed state is
  # +true+.
  #
  # This will invoke the method named in #add_observer, passing <tt>*arg</tt>.
  # The changed state is then set to +false+.
  #
  # <tt>*arg</tt>:: Any arguments to pass to the observers.
  def notify_observers(*arg)
    if defined? @observer_state and @observer_state
      if defined? @observer_peers
        @observer_peers.each do |k, v|
          k.send v, *arg
        end
      end
      @observer_state = false
    end
  end

end