Module: Observable
Relationships & Source Files | |
Defined in: | lib/observer.rb |
Overview
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
Instance Attribute Summary
-
#changed? ⇒ Boolean
readonly
Returns true if this object's state has been changed since the last #notify_observers call.
Instance Method Summary
-
#add_observer(observer, func = :update)
Add
observer
as an observer on this object. -
#changed(state = true)
readonly
Set the changed state of this object.
-
#count_observers
Return the number of observers associated with this object.
-
#delete_observer(observer)
Remove
observer
as an observer on this object so that it will no longer receive notifications. -
#delete_observers
Remove all observers associated with this object.
-
#notify_observers(*arg)
Notify observers of a change in state if this object's changed state is
true
.
Instance Attribute Details
#changed? ⇒ Boolean
(readonly)
Returns true if this object's state has been changed since the last #notify_observers call.
# File 'lib/observer.rb', line 177
def changed? if defined? @observer_state and @observer_state true else false end end
Instance Method Details
#add_observer(observer, func = :update)
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 'lib/observer.rb', line 128
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}'" end @observer_peers[observer] = func end
#changed(state = true) (readonly)
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 'lib/observer.rb', line 169
def changed(state=true) @observer_state = state end
#count_observers
Return the number of observers associated with this object.
# File 'lib/observer.rb', line 155
def count_observers if defined? @observer_peers @observer_peers.size else 0 end end
#delete_observer(observer)
Remove observer
as an observer on this object so that it will no longer receive notifications.
observer
-
An observer of this
Observable
# File 'lib/observer.rb', line 141
def delete_observer(observer) @observer_peers.delete observer if defined? @observer_peers end
#delete_observers
Remove all observers associated with this object.
# File 'lib/observer.rb', line 148
def delete_observers @observer_peers.clear if defined? @observer_peers end
#notify_observers(*arg)
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 'lib/observer.rb', line 193
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