Add attr_evaluated to StaticAssociation objects

[sidane]

attr_evaluated can be used to define an attribute who's return value is evaluated/computed at runtime.

It defines a normal writer method using attr_writer which can take either a static value or a proc/lambda.

If the latter, the reader method will evaluate the proc/lambda when called and return its value. This allows for defining dynamic runtime behaviour for specific record attributes.

A static value can still be assigned like other non-evaluated attributes, and that value will always be returned.

E.g.

class MyClass
  include StaticAssociation

  attr_accessor :toggle
  attr_evaluated :status

  record(id: 1) do
    self.toggle = true
    self.status = -> { toggle ? "on" : "off" }
  end

  record(id: 2) do
    self.toggle = true
    self.status = "static value"
  end
end

record1 = MyClass.find(1)
record1.status # => "on"

record2 = MyClass.find(2)
record2.status # => "static value"

As with attr_accessor you can pass multiple attribute names to a single attr_evaluated call.

attr_evaluated :my_attr_1, :my_attr_2