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Visitor

Visitor in Ruby

Visitor is a behavioral design pattern that allows adding new behaviors to existing class hierarchy without altering any existing code.

Read why Visitors can’t be simply replaced with method overloading in our article Visitor and Double Dispatch.

Learn more about Visitor

Usage of the pattern in Ruby

Complexity:

Popularity:

Usage examples: Visitor isn’t a very common pattern because of its complexity and narrow applicability.

Conceptual Example

This example illustrates the structure of the Visitor design pattern. It focuses on answering these questions:

  • What classes does it consist of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

main.rb: Conceptual Example

# The Component interface declares an `accept` method that should take the base
# visitor interface as an argument.
class Component
  # @abstract
  #
  # @param [Visitor] visitor
  def accept(_visitor)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

# Each Concrete Component must implement the `accept` method in such a way that
# it calls the visitor's method corresponding to the component's class.
class ConcreteComponentA < Component
  # Note that we're calling `visitConcreteComponentA`, which matches the current
  # class name. This way we let the visitor know the class of the component it
  # works with.
  def accept(visitor)
    visitor.visit_concrete_component_a(self)
  end

  # Concrete Components may have special methods that don't exist in their base
  # class or interface. The Visitor is still able to use these methods since
  # it's aware of the component's concrete class.
  def exclusive_method_of_concrete_component_a
    'A'
  end
end

# Same here: visit_concrete_component_b => ConcreteComponentB
class ConcreteComponentB < Component
  # @param [Visitor] visitor
  def accept(visitor)
    visitor.visit_concrete_component_b(self)
  end

  def special_method_of_concrete_component_b
    'B'
  end
end

# The Visitor Interface declares a set of visiting methods that correspond to
# component classes. The signature of a visiting method allows the visitor to
# identify the exact class of the component that it's dealing with.
class Visitor
  # @abstract
  #
  # @param [ConcreteComponentA] element
  def visit_concrete_component_a(_element)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  # @abstract
  #
  # @param [ConcreteComponentB] element
  def visit_concrete_component_b(_element)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

# Concrete Visitors implement several versions of the same algorithm, which can
# work with all concrete component classes.
#
# You can experience the biggest benefit of the Visitor pattern when using it
# with a complex object structure, such as a Composite tree. In this case, it
# might be helpful to store some intermediate state of the algorithm while
# executing visitor's methods over various objects of the structure.
class ConcreteVisitor1 < Visitor
  def visit_concrete_component_a(element)
    puts "#{element.exclusive_method_of_concrete_component_a} + #{self.class}"
  end

  def visit_concrete_component_b(element)
    puts "#{element.special_method_of_concrete_component_b} + #{self.class}"
  end
end

class ConcreteVisitor2 < Visitor
  def visit_concrete_component_a(element)
    puts "#{element.exclusive_method_of_concrete_component_a} + #{self.class}"
  end

  def visit_concrete_component_b(element)
    puts "#{element.special_method_of_concrete_component_b} + #{self.class}"
  end
end

# The client code can run visitor operations over any set of elements without
# figuring out their concrete classes. The accept operation directs a call to
# the appropriate operation in the visitor object.
def client_code(components, visitor)
  # ...
  components.each do |component|
    component.accept(visitor)
  end
  # ...
end

components = [ConcreteComponentA.new, ConcreteComponentB.new]

puts 'The client code works with all visitors via the base Visitor interface:'
visitor1 = ConcreteVisitor1.new
client_code(components, visitor1)

puts 'It allows the same client code to work with different types of visitors:'
visitor2 = ConcreteVisitor2.new
client_code(components, visitor2)

output.txt: Execution result

The client code works with all visitors via the base Visitor interface:
A + ConcreteVisitor1
B + ConcreteVisitor1
It allows the same client code to work with different types of visitors:
A + ConcreteVisitor2
B + ConcreteVisitor2

Visitor in Other Languages

Design Patterns: Visitor in Java Design Patterns: Visitor in C# Design Patterns: Visitor in PHP Design Patterns: Visitor in Python Design Patterns: Visitor in Swift Design Patterns: Visitor in TypeScript