#
# observer.rb implements the _Observer_ object-oriented design pattern.
#
# == 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
