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# frozen_string_literal: true
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#
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# Implementation of the _Observer_ object-oriented design pattern. The
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# following documentation is copied, with modifications, from "Programming
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# Ruby", by Hunt and Thomas; http://www.ruby-doc.org/docs/ProgrammingRuby/html/lib_patterns.html.
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#
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# See Observable for more info.
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# The Observer pattern (also known as publish/subscribe) provides a simple
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# mechanism for one object to inform a set of interested third-party objects
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# when its state changes.
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#
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# == Mechanism
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#
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# The notifying class mixes in the +Observable+
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# module, which provides the methods for managing the associated observer
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# objects.
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#
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# The observable object must:
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# * assert that it has +#changed+
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# * call +#notify_observers+
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#
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# An observer subscribes to updates using Observable#add_observer, which also
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# specifies the method called via #notify_observers. The default method for
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# #notify_observers is #update.
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#
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# === Example
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#
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# The following example demonstrates this nicely. A +Ticker+, when run,
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# continually receives the stock +Price+ for its <tt>@symbol</tt>. A +Warner+
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# is a general observer of the price, and two warners are demonstrated, a
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# +WarnLow+ and a +WarnHigh+, which print a warning if the price is below or
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# above their set limits, respectively.
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#
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# The +update+ callback allows the warners to run without being explicitly
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# called. The system is set up with the +Ticker+ and several observers, and the
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# observers do their duty without the top-level code having to interfere.
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#
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# Note that the contract between publisher and subscriber (observable and
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# observer) is not declared or enforced. The +Ticker+ publishes a time and a
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# price, and the warners receive that. But if you don't ensure that your
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# contracts are correct, nothing else can warn you.
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#
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# require "observer"
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#
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# class Ticker ### Periodically fetch a stock price.
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# include Observable
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#
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# def initialize(symbol)
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# @symbol = symbol
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# end
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#
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# def run
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# last_price = nil
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# loop do
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# price = Price.fetch(@symbol)
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# print "Current price: #{price}\n"
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# if price != last_price
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# changed # notify observers
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# last_price = price
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# notify_observers(Time.now, price)
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# end
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# sleep 1
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# end
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# end
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# end
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#
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# class Price ### A mock class to fetch a stock price (60 - 140).
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# def self.fetch(symbol)
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# 60 + rand(80)
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# end
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# end
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#
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# class Warner ### An abstract observer of Ticker objects.
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# def initialize(ticker, limit)
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# @limit = limit
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# ticker.add_observer(self)
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# end
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# end
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#
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# class WarnLow < Warner
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# def update(time, price) # callback for observer
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# if price < @limit
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# print "--- #{time.to_s}: Price below #@limit: #{price}\n"
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# end
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# end
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# end
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#
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# class WarnHigh < Warner
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# def update(time, price) # callback for observer
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# if price > @limit
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# print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
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# end
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# end
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# end
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#
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# ticker = Ticker.new("MSFT")
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# WarnLow.new(ticker, 80)
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# WarnHigh.new(ticker, 120)
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# ticker.run
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#
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# Produces:
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#
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# Current price: 83
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# Current price: 75
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# --- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75
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# Current price: 90
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# Current price: 134
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# +++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134
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# Current price: 134
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# Current price: 112
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# Current price: 79
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# --- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79
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#
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# === Usage with procs
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#
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# The +#notify_observers+ method can also be used with +proc+s by using
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# the +:call+ as +func+ parameter.
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#
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# The following example illustrates the use of a lambda:
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#
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# require 'observer'
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#
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# class Ticker
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# include Observable
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#
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# def run
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# # logic to retrieve the price (here 77.0)
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# changed
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# notify_observers(77.0)
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# end
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# end
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#
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# ticker = Ticker.new
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# warner = ->(price) { puts "New price received: #{price}" }
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# ticker.add_observer(warner, :call)
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# ticker.run
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module Observable
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VERSION = "0.1.0"
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#
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# Add +observer+ as an observer on this object. So that it will receive
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# notifications.
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#
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# +observer+:: the object that will be notified of changes.
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# +func+:: Symbol naming the method that will be called when this Observable
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# has changes.
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#
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# This method must return true for +observer.respond_to?+ and will
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# receive <tt>*arg</tt> when #notify_observers is called, where
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# <tt>*arg</tt> is the value passed to #notify_observers by this
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# Observable
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def add_observer(observer, func=:update)
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@observer_peers = {} unless defined? @observer_peers
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unless observer.respond_to? func
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raise NoMethodError, "observer does not respond to `#{func}'"
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end
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@observer_peers[observer] = func
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end
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#
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# Remove +observer+ as an observer on this object so that it will no longer
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# receive notifications.
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#
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# +observer+:: An observer of this Observable
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def delete_observer(observer)
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@observer_peers.delete observer if defined? @observer_peers
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end
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#
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# Remove all observers associated with this object.
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#
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def delete_observers
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@observer_peers.clear if defined? @observer_peers
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end
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#
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# Return the number of observers associated with this object.
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#
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def count_observers
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if defined? @observer_peers
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@observer_peers.size
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else
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0
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end
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end
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#
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# Set the changed state of this object. Notifications will be sent only if
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# the changed +state+ is +true+.
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#
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# +state+:: Boolean indicating the changed state of this Observable.
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#
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def changed(state=true)
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@observer_state = state
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end
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#
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# Returns true if this object's state has been changed since the last
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# #notify_observers call.
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#
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def changed?
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if defined? @observer_state and @observer_state
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true
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else
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false
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end
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end
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#
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# Notify observers of a change in state *if* this object's changed state is
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# +true+.
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#
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# This will invoke the method named in #add_observer, passing <tt>*arg</tt>.
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# The changed state is then set to +false+.
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#
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# <tt>*arg</tt>:: Any arguments to pass to the observers.
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def notify_observers(*arg)
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if defined? @observer_state and @observer_state
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if defined? @observer_peers
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@observer_peers.each do |k, v|
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k.__send__(v, *arg)
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end
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end
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@observer_state = false
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end
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end
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end
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