Let's say I have a class called Person and person has an attribute called partner. When I call partner= on one of the Person objects, I want to set the @partner instance variable of both objects. Here's an example with invalid syntax:
class Person
attr_reader :partner
def partner=(person)
# reset the old partner instance variable if it exists
partner.@partner = nil if partner
# set the partner attributes
@partner = person
person.@partner = self
end
end
Change the attr_reader to an attr_accessor and add a helper method:
class Person
attr_accessor :partner
def link_partners(person)
@partner = person
person.partner = self
end
end
Update for visibility. Based on suggestion from Frederick below. This is a little more verbose, but will prevent partner from being set directly:
class Person
protected
attr_writer :partner
public
attr_reader :partner
def link_partners(person)
@partner = person
person.partner = self
end
end
Both implementations works like this:
p1, p2 = Person.new, Person.new
p1.link_partners(p2)
# p2.link_partners(p1)
You could provide a protected helper method which gets called by your partner= method to do the actual work. Since it can't be called by "outsiders", all of your check and balances can be maintained in your implementation of partner=:
class Person
attr_reader :partner
def partner=(person)
@partner.set_partner(nil) if @partner
set_partner(person)
person.set_partner(self) if person
end
def set_partner(person)
@partner = person
end
protected :set_partner
end
Never mind, I just discovered instance_variable_set.
class Person
attr_reader :partner
def partner=(person)
# reset the old partner instance variable if it exists
partner.instance_variable_set(:@partner, nil) if partner
# set the partner attributes
@partner = person
person.instance_variable_set(:@partner, self)
end
end
In theory, you could do this as follows:
def partner=(person)
@partner = person
person.instance_variable_set(:@partner, self)
end
However, I would consider this to be magic. (That's not a good thing.) Instead, make the attr_reader into an attr_accessor and write a different method to set two persons' partners to each other.
This will solve your issue with recursive set and looks better than your solution:
class Partner
attr_reader :partner
def set_partner(person, recursive = true)
# reset previous partner
@partner.set_partner(nil, false) if recursive && @partner
# set new partner
@partner = person
@partner.set_partner(self, false) if recursive
end
alias_method :partner=, :set_partner
end
