README.md

Visitor Pattern

Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Problem

• An object structure contains many unrelated classes with different interfaces. You want to perform operations on those objects that depend on their concrete classes.
• You need to add new opeations frequently and it is inflexible to add new subclasses each time a new operation is required.
• Distributing all these operations across the various node classes leads to a system that's hard to understand, maintain and change.

Solution

• Define a separate Visitor object that implements an operation to be performed on elements of an object structure.
• Clients traverse the object structure and calls the visitor to perform the operation on the element.

Common Structure

• Visitor
  • declares a visit operation for each class of ConcreteElement, e.g. VisitConcreteElementA().
• ConcreteVisitor
  • implements each operation declared by Visitor.
  • provides the context for the algorithm and stores its local state. This state often accumulates results during the traversal of the structure.
• Element
  • defines an Accept() operation that takes a visitor as an argument.
• ConcreteElement
  • implements an Accept operation
• ObjectStructure
  • can enumerate its elements
  • may provide a high-level interface to allow the Client to visit its elements.

Collaboration

• A client must create a ConcreteVisitor object and then traverse the object structure and visits each element with that concrete visitor.
• When an element is visited, it calls the Visitor operation and supplies itself as an argument to let the visitor access its state, if necessary.

Benefits

• New operations can be added easily.
• Moves related behaviors into a visitor.
  • That simplifies both the elements and the algorithms defined in the visitors.
• Accumulating state
  • Without a visitor, the state should be passed as extra argument or stored as global variables.

Drawbacks

• Adding new ConcreteElement classes is hard.
  • For each new ConcreteElement, new operations VisitConcreteElementX() should be implemented in ConcreteVisitor.
• Breaking encapsulation
  • Element should provide public operations to access its internal state, so that the Visitors can do their job.

Example

Definition

Usage

Comparison with other patterns

• Composite - Visitors can be used to apply an operation over entire Composite tree.
• Interpreter - Visitor may be applied to do the interpretation.