rf-web/vendor/bundle/gems/concurrent-ruby-1.1.5/lib/concurrent/concern/observable.rb
2019-10-21 10:18:17 +02:00

111 lines
3.7 KiB
Ruby

require 'concurrent/collection/copy_on_notify_observer_set'
require 'concurrent/collection/copy_on_write_observer_set'
module Concurrent
module Concern
# The [observer pattern](http://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: {Concurrent::Concern::CopyOnWriteObserverSet}
# and {Concurrent::Concern::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)
module Observable
# @!macro observable_add_observer
#
# Adds an observer to this set. If a block is passed, the observer will be
# created by this method and no other params should be passed.
#
# @param [Object] observer the observer to add
# @param [Symbol] func the function to call on the observer during notification.
# Default is :update
# @return [Object] the added observer
def add_observer(observer = nil, func = :update, &block)
observers.add_observer(observer, func, &block)
end
# As `#add_observer` but can be used for chaining.
#
# @param [Object] observer the observer to add
# @param [Symbol] func the function to call on the observer during notification.
# @return [Observable] self
def with_observer(observer = nil, func = :update, &block)
add_observer(observer, func, &block)
self
end
# @!macro observable_delete_observer
#
# Remove `observer` as an observer on this object so that it will no
# longer receive notifications.
#
# @param [Object] observer the observer to remove
# @return [Object] the deleted observer
def delete_observer(observer)
observers.delete_observer(observer)
end
# @!macro observable_delete_observers
#
# Remove all observers associated with this object.
#
# @return [Observable] self
def delete_observers
observers.delete_observers
self
end
# @!macro observable_count_observers
#
# Return the number of observers associated with this object.
#
# @return [Integer] the observers count
def count_observers
observers.count_observers
end
protected
attr_accessor :observers
end
end
end