Observable

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 #add_observer, which also specifies the method called via notify_observers. The default method for notify_observers is update.

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 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