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Filename : cdesc-Observable.ri
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U:RDoc::NormalModule[iI"Observable:EF@0o:RDoc::Markup::Document:@parts[o;;[o:RDoc::Markup::Paragraph;[I"NThe Observer pattern (also known as publish/subscribe) provides a simple ;TI"Pmechanism for one object to inform a set of interested third-party objects ;TI"when its state changes.;To:RDoc::Markup::BlankLine�S:RDoc::Markup::Heading:
leveli:	textI"Mechanism;T@o;	;[I"3The notifying class mixes in the +Observable+ ;TI"Mmodule, which provides the methods for managing the associated observer ;TI"
objects.;T@o;	;[I" The observable object must:;To:RDoc::Markup::List:
@type:BULLET:@items[o:RDoc::Markup::ListItem:@label0;[o;	;[I""assert that it has +#changed+;To;;0;[o;	;[I"call +#notify_observers+;T@o;	;[I"QAn observer subscribes to updates using Observable#add_observer, which also ;TI"Ospecifies the method called via #notify_observers. The default method for ;TI""#notify_observers is #update.;T@S;;i;
I"Example;T@o;	;[
I"LThe following example demonstrates this nicely.  A +Ticker+, when run, ;TI"Rcontinually receives the stock +Price+ for its <tt>@symbol</tt>.  A +Warner+ ;TI"Mis a general observer of the price, and two warners are demonstrated, a ;TI"P+WarnLow+ and a +WarnHigh+, which print a warning if the price is below or ;TI"*above their set limits, respectively.;T@o;	;[I"NThe +update+ callback allows the warners to run without being explicitly ;TI"Tcalled.  The system is set up with the +Ticker+ and several observers, and the ;TI"Lobservers do their duty without the top-level code having to interfere.;T@o;	;[	I"MNote that the contract between publisher and subscriber (observable and ;TI"Qobserver) is not declared or enforced.  The +Ticker+ publishes a time and a ;TI"Mprice, and the warners receive that.  But if you don't ensure that your ;TI"6contracts are correct, nothing else can warn you.;T@o:RDoc::Markup::Verbatim;[>I"require "observer"
;TI"
;TI"Aclass Ticker          ### Periodically fetch a stock price.
;TI"  include Observable
;TI"
;TI"  def initialize(symbol)
;TI"    @symbol = symbol
;TI"  end
;TI"
;TI"  def run
;TI"    last_price = nil
;TI"    loop do
;TI"(      price = Price.fetch(@symbol)
;TI"-      print "Current price: #{price}\n"
;TI""      if price != last_price
;TI"8        changed                 # notify observers
;TI"         last_price = price
;TI"/        notify_observers(Time.now, price)
;TI"      end
;TI"      sleep 1
;TI"
    end
;TI"  end
;TI"	end
;TI"
;TI"Oclass Price           ### A mock class to fetch a stock price (60 - 140).
;TI"  def self.fetch(symbol)
;TI"    60 + rand(80)
;TI"  end
;TI"	end
;TI"
;TI"Gclass Warner          ### An abstract observer of Ticker objects.
;TI"%  def initialize(ticker, limit)
;TI"    @limit = limit
;TI"#    ticker.add_observer(self)
;TI"  end
;TI"	end
;TI"
;TI"class WarnLow < Warner
;TI"=  def update(time, price)       # callback for observer
;TI"    if price < @limit
;TI"E      print "--- #{time.to_s}: Price below #@limit: #{price}\n"
;TI"
    end
;TI"  end
;TI"	end
;TI"
;TI"class WarnHigh < Warner
;TI"=  def update(time, price)       # callback for observer
;TI"    if price > @limit
;TI"E      print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
;TI"
    end
;TI"  end
;TI"	end
;TI"
;TI"!ticker = Ticker.new("MSFT")
;TI"WarnLow.new(ticker, 80)
;TI"WarnHigh.new(ticker, 120)
;TI"ticker.run
;T:@format0o;	;[I"Produces:;T@o;;[I"Current price: 83
;TI"Current price: 75
;TI":--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75
;TI"Current price: 90
;TI"Current price: 134
;TI"<+++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134
;TI"Current price: 134
;TI"Current price: 112
;TI"Current price: 79
;TI"9--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79;T;0:
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