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Observer

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 observable object must:

  • assert that it has +#changed+
  • call +#notify_observers+

An observer subscribes to updates using Observable#add_observer, which also specifies the method called via #notify_observers. The default method for notify_observers is #update.

Installation

Add this line to your application's Gemfile:

gem 'observer'

And then execute:

$ bundle

Or install it yourself as:

$ gem install observer

Usage

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
    last_price = nil
    loop do
      price = Price.fetch(@symbol)
      print "Current price: #{price}\n"
      if price != last_price
        changed                 # notify observers
        last_price = 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 self.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

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/observer.

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The Observer pattern provides a simple mechanism for one object to inform a set of interested third-party objects when its state changes.

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