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 observers must implement a method called update 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