Example
Proxy object is often used to ensure guarded access to another object, which internal business logic we don't want to pollute with safety requirements.
Suppose we'd like to guarantee that only user of specific permissions can access resource.
Proxy definition: (it ensure that only users which actually can see reservations will be able to consumer reservation_service)
class Proxy
def initialize(current_user, reservation_service)
@current_user = current_user
@reservation_service = reservation_service
end
def highest_total_price_reservations(date_from, date_to, reservations_count)
if @current_user.can_see_reservations?
@reservation_service.highest_total_price_reservations(
date_from,
date_to,
reservations_count
)
else
[]
end
end
end
Models and ReservationService:
class Reservation
attr_reader :total_price, :date
def initialize(date, total_price)
@date = date
@total_price = total_price
end
end
class ReservationService
def highest_total_price_reservations(date_from, date_to, reservations_count)
# normally it would be read from database/external service
reservations = [
Reservation.new(Date.new(2014, 5, 15), 100),
Reservation.new(Date.new(2017, 5, 15), 10),
Reservation.new(Date.new(2017, 1, 15), 50)
]
filtered_reservations = reservations.select do |reservation|
reservation.date.between?(date_from, date_to)
end
filtered_reservations.take(reservations_count)
end
end
class User
attr_reader :name
def initialize(can_see_reservations, name)
@can_see_reservations = can_see_reservations
@name = name
end
def can_see_reservations?
@can_see_reservations
end
end
Consumer service:
class StatsService
def initialize(reservation_service)
@reservation_service = reservation_service
end
def year_top_100_reservations_average_total_price(year)
reservations = @reservation_service.highest_total_price_reservations(
Date.new(year, 1, 1),
Date.new(year, 12, 31),
100
)
if reservations.length > 0
sum = reservations.reduce(0) do |memo, reservation|
memo + reservation.total_price
end
sum / reservations.length
else
0
end
end
end
Test:
def test(user, year)
reservations_service = Proxy.new(user, ReservationService.new)
stats_service = StatsService.new(reservations_service)
average_price = stats_service.year_top_100_reservations_average_total_price(year)
puts "#{user.name} will see: #{average_price}"
end
test(User.new(true, "John the Admin"), 2017)
test(User.new(false, "Guest"), 2017)
BENEFITS
- we're avoiding any changes in
ReservationServicewhen access restrictions are changed. - we're not mixing business related data (
date_from,date_to,reservations_count) with domain unrelated concepts (user permissions) in service. - Consumer (
StatsService) is free from permissions related logic as well
CAVEATS
- Proxy interface is always exactly the same as the object it hides, so that user that consumes service wrapped by proxy wasn't even aware of proxy presence.
