# observer.rb

Path: | observer.rb
Last Update: | Sat Oct 15 23:44:27 AUS Eastern Standard Time 2005

observer.rb implements the Observer object-oriented design pattern. The following documentation is copied, with modifications, from "Programming Ruby", by Hunt and Thomas; 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

[Validate]