The [observer pattern](en.wikipedia.org/wiki/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)
@return [Object] the added observer
# File lib/concurrent/concern/observable.rb, line 53 def add_observer(*args, &block) observers.add_observer(*args, &block) end
@return [Integer] the observers count
# File lib/concurrent/concern/observable.rb, line 76 def count_observers observers.count_observers end
@return [Object] the deleted observer
# File lib/concurrent/concern/observable.rb, line 65 def delete_observer(*args) observers.delete_observer(*args) end
@return [Observable] self
# File lib/concurrent/concern/observable.rb, line 70 def delete_observers observers.delete_observers self end
as add_observer but it can be used for chaining @return [Observable] self
# File lib/concurrent/concern/observable.rb, line 59 def with_observer(*args, &block) add_observer(*args, &block) self end
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