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- observer.rb
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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 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
Public Instance Methods
Add observer as an observer on this object. so that it will receive notifications.
observer-
the object that will be notified of changes.
func-
Symbol naming the method that will be called when this Observable has changes.
This method must return true for
observer.respond_to?and will receive*argwhen notify_observers is called, where*argis the value passed to notify_observers by this Observable
# File observer.rb, line 126 def add_observer(observer, func=:update) @observer_peers = {} unless defined? @observer_peers unless observer.respond_to? func raise NoMethodError, "observer does not respond to `#{func.to_s}'" end @observer_peers[observer] = func end
Set the changed state of this object. Notifications will be sent only if the changed state is true.
state-
Boolean indicating the changed state of this Observable.
# File observer.rb, line 167 def changed(state=true) @observer_state = state end
Returns true if this object's state has been changed since the last notify_observers call.
# File observer.rb, line 175 def changed? if defined? @observer_state and @observer_state true else false end end
Return the number of observers associated with this object.
# File observer.rb, line 153 def count_observers if defined? @observer_peers @observer_peers.size else 0 end end
Remove observer as an observer on this object so that it will no longer receive notifications.
observer-
An observer of this Observable
# File observer.rb, line 139 def delete_observer(observer) @observer_peers.delete observer if defined? @observer_peers end
Remove all observers associated with this object.
# File observer.rb, line 146 def delete_observers @observer_peers.clear if defined? @observer_peers end
Notify observers of a change in state if this object's changed state is true.
This will invoke the method named in add_observer, passing *arg. The changed state is then set to false.
*arg-
Any arguments to pass to the observers.
# File observer.rb, line 191 def notify_observers(*arg) if defined? @observer_state and @observer_state if defined? @observer_peers @observer_peers.each do |k, v| k.send v, *arg end end @observer_state = false end end