# observer.rb
# Define a one-to-many dependency between objects so that when one object
# changes state, all it's dependents are notified and updated automatically.
# 1. observer
class Observer
def initialize(name)
@name = name
end
def update(value)
puts "updated " + @name + ". New value: " + value
end
end
# 2. Observable, serves as a container for observers and takes care of notifying them
module Observable
def initialize
@observers = []
end
def <<(observer)
@observers << observer
end
def >>(observer)
@observers.delete(observer)
end
def self.included(base)
base.extend(ClassMethods)
end
module ClassMethods
def act_as_observable *list
list.each do |observable|
method_name = "#{observable.to_s}="
no_callback_method_name = "no_callback_#{observable.to_s}="
alias_method no_callback_method_name, method_name
define_method method_name do |value|
send no_callback_method_name, value
notify_observers(value)
end
end
end
end
protected
def notify_observers(value)
@observers.clone.each do |observer|
observer.update(value) if observer.kind_of? Observer
observer.call(value) if observer.kind_of? Proc
end
end
end
# 3. test
class Tester
include Observable
attr_accessor :property
act_as_observable :property
end
observer1 = Observer.new("n1")
observer2 = Observer.new("n2")
observer3 = Observer.new("n3")
observer4 = lambda { |value| puts "updated from lambda (update is :#{value})" }
tester = Tester.new
tester << observer1
tester << observer2
tester << observer3
tester << observer4
tester.property = 'red'
puts '----------'
tester >> observer3
tester.property = 'green'
