It descripts about Mediator Pattern and give example code in ruby.

1. MEDIATOR DESIGN
PATTERN
Presented by:
Kuyseng Chheoun24-Aug-2014

2. MEDIATOR DESIGN

3. Intent
Define an object that
encapsulates how a set of
objects interact. Mediator
promotes loose coupling by
keeping objects from
referring to each other
explicitly, and it lets you vary
their interaction
independently.
Ref: Head First Design Pattern P.622

4. Applicability
Use the Mediator pattern when:
• a set of objects communicate in well-defined but
complex ways. The resulting interdependencies are
unstructured and difficult to understand.
• reusing an object is difficult because it refers to and
communicates with many other objects.
• a behavior that's distributed between several classes
should be customizable without a lot of subclassing.

5. Structure

6. Consequences
• Comprehension - The mediator encapsulate the logic of
mediation between the colleagues. From this reason it' more
easier to understand this logic since it is kept in only one class.
• Decoupled Colleagues - The colleague classes are totally
decoupled. Adding a new colleague class is very easy due to
this decoupling level.
• Simplified object protocols - The colleague objects need to
communicate only with the mediator objects. Practically the
mediator pattern reduce the required communication
channels(protocols) from many to many to one to many and
many to one.

7. • Limits Subclassing - Because the entire
communication logic is encapsulated by the
mediator class, when this logic need to be extended
only the mediator class need to be extended.
• Complexity - in practice the mediators tends to
become more complex and complex. A good
practice is to take care to make the mediator classes
responsible only for the communication part. For
example when implementing different screens the
the screen class should not contain code which is
not a part of the screen operations. It should be put
in some other classes.

8. Examples
Example 1 - Air traffic controller.
Air traffic controller (ATC) is a
mediator between flights. It helps in
communication between flights and
coordinates/controls landing, take-
off. Two flights need not interact
directly and there is no dependency
between them. This dependency is
solved by the mediator ATC. If
ATC is not there all the flights have
to interact with one another and
managing the show will be very
difficult and things may go wrong.

9. Example 2 - Chat application
In a chat application we can have several participants.
It's not a good idea to connect each participant to all
the others because the number of connections would
be really high, there would be technical problems due
to proxies and firewalls, etc... . The most appropriate
solution is to have a hub where all participants will
connect; this hub is just the mediator class.

10. Sample Codes (other example)
class Buyer!
attr_accessor :budget!
!
def initialize budget: 0!
@budget = budget!
end!
!
def follow_agency agency!
@agency = agency!
end!
!
def buy house!
@agency.mediate_purchase house, self!
end !
end!
class Seller!
attr_accessor :balance!
!
def initialize balance: 0!
@balance = balance!
end!
!
def publish agency, house!
agency.register house!
end!
!
end!

11. class Agency!
def initialize!
@houses = []!
end!
!
def register house!
@houses << house!
end!
!
def mediate_purchase house, buyer!
if conditions_are_met? house, buyer!
handle_money house, buyer!
house.owner = buyer!
end!
end!
!
private!
!
def conditions_are_met? house, buyer!
@houses.include?(house) && buyer.budget >=
house.price!
end!
!
def pay seller, amount!
seller.balance += amount!
end!
!
def charge buyer, amount!
buyer.budget -= amount!
end!
!
def handle_money house, buyer!
pay house.owner, house.price!
charge buyer, house.price!
end!
end!
class House!
attr_accessor :owner!
attr_reader :price!
!
def initialize rooms: 0, price: 0, owner: nil!
@rooms = rooms!
@price = price!
@owner = owner!
end!
end!

12. References
• http://www.oodesign.com/mediator-pattern.html
• http://code.tutsplus.com/courses/gang-of-four-
design-patterns-in-ruby
• http://www.amazon.com/Design-Patterns-
Elements-Reusable-Object-Oriented/dp/
0201633612

13. ありがとう