Ruby Proc: Invoking method from Class A from within Class B, and using Class B's 'method'

Question

I am not sure whether this is actually possible, but I wasn't able to find a clear answer anywhere. Also I find it hard to define my question in mere 'search terms'. So I am sorry if this has already been answered somewhere else, I could not find it.

What I would like to know is if it is possible to create a Proc that holds a method that isn't defined in the location where the Proc is being defined. Then I would like to put that instance inside another class that does have the method, and run THAT one with the provided arguments.

Here is a sample of what I want to accomplish, but don't know how.

class MyClassA

  # This class does not have the #run method
  # but I want Class B to run the #run method that
  # I invoke from within the Proc within this initializer
  def initialize
    Proc.new { run 'something great' }
  end

end

class MyClassB

  def initialize(my_class_a_object)
    my_class_a_object.call
  end

  # This is the #run method I want to invoke
  def run(message)
    puts message
  end

end

# This is what I execute
my_class_a_object = MyClassA.new
MyClassB.new(my_class_a_object)

The following error is produced

NoMethodError: undefined method  for #<MyClassA:0x10017d878>

And I think I understand why, it is because it is trying to invoke the run method on the MyClassA instance rather than the one in MyClassB. However, is there a way I could make the run command invoke MyClassB's run instance method?

Answer 1

There are two problems with your code:

1. MyClassA.new does not return the value of initialize it ALWAYS returns an instance of MyClassA.

2. You cannot just call the proc, you have to use the instance_eval method to run it in the context of MyClassB

Here is your code corrected to work as you want:

class MyClassA    
  def self.get_proc
    Proc.new { run 'something great' }
  end
end

class MyClassB

  def initialize(my_class_a_object)
   instance_eval(&my_class_a_object)
  end

  # This is the #run method I want to invoke
  def run(message)
    puts message
  end

end

# This is what I execute
my_class_a_object = MyClassA.get_proc
MyClassB.new(my_class_a_object) #=> "something great"