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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(observer, func=:update) click to toggle source

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 *arg when notify_observers is called, where *arg is 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
            
changed(state=true) click to toggle source

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
            
changed?() click to toggle source

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
            
count_observers() click to toggle source

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
            
delete_observer(observer) click to toggle source

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
            
delete_observers() click to toggle source

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(*arg) click to toggle source

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
            

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