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Imported Observer from ruby 1.9.2-p290

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commit 3abb42b7beb7f448077bfc8df0b3d2ecbacae145 1 parent 49650f3
@krekoten krekoten authored
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0  lib/observer.rb → lib/18/observer.rb
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193 lib/19/observer.rb
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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. The second optional argument specifies a method to notify
+ # updates, of which default value is +update+.
+ #
+ 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
+
+ #
+ # 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
+ @observer_peers.each { |k, v|
+ k.send v, *arg
+ }
+ end
+ @observer_state = false
+ end
+ end
+
+end
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1  spec/tags/19/ruby/library/observer/count_observers_tags.txt
@@ -1 +0,0 @@
-fails:Observer#count_observers returns the number of unique observers
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