module Garcon::Observable
The [observer pattern] is one of the most useful design patterns.
The workflow is very simple:
-
an ‘observer` can register itself to a `subject` via a callback
-
many ‘observers` can be registered to the same `subject`
-
the ‘subject` notifies all registered observers when its status changes
-
an ‘observer` can deregister itself when is no more interested to receive event notifications
In a single threaded environment the whole pattern is very easy: the ‘subject` can use a simple data structure to manage all its subscribed `observer`s and every `observer` can react directly to every event without caring about synchronization.
In a multi threaded environment things are more complex. The ‘subject` must synchronize the access to its data structure and to do so currently we’re using two specialized ObserverSet: CopyOnWriteObserverSet
and CopyOnNotifyObserverSet
.
When implementing and ‘observer` there’s a very important rule to remember: **there are no guarantees about the thread that will execute the callback**
Let’s take this example “‘ class Observer
def initialize @count = 0 end def update @count += 1 end
end
obs = Observer.new [obj1, obj2, obj3, obj4].each { |o| o.add_observer(obs) } # execute [obj1, obj2, obj3, obj4] “‘
‘obs` is wrong because the variable `@count` can be accessed by different threads at the same time, so it should be synchronized (using either a Mutex or an AtomicFixum)
Public Instance Methods
@return [Object] the added observer
# File lib/garcon/task/observable.rb, line 70 def add_observer(*args, &block) observers.add_observer(*args, &block) end
@return [Integer] the observers count
# File lib/garcon/task/observable.rb, line 93 def count_observers observers.count_observers end
@return [Object] the deleted observer
# File lib/garcon/task/observable.rb, line 82 def delete_observer(*args) observers.delete_observer(*args) end
@return [Observable] self
# File lib/garcon/task/observable.rb, line 87 def delete_observers observers.delete_observers self end
as add_observer
but it can be used for chaining @return [Observable] self
# File lib/garcon/task/observable.rb, line 76 def with_observer(*args, &block) add_observer(*args, &block) self end