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Using Observer in Ruby

By: Hunt and Thomas in Ruby Tutorials on 2009年03月03日 [フレーム]

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

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