Visitor Pattern
Overview
Represent an operation to be performed on the elements of an object structure. Visitor
lets you define a new operation without changing the classes of the elements on which
it operates.
Participants
- declares a Visit operation for each class of ConcreteElement in the object
structure. The operation's name and signature identifies the class that sends the
Visit request to the visitor. That lets the visitor determine the concrete class of
the element being visited. Then the visitor can access the element directly through
its particular interface.
- this participant can not be read-only.
- implements each operation declared by Visitor. Each operation implements a
fragment of the algorithm defined for the corresponding class of object in the structure.
Concrete Visitor provides the context for the algorithm and stores its local state.
This state often accumulates results during the traversal of the structure.
- this participant can not be read-only.
- this participant can not be an interface.
- defines an accept operation that takes a visitor as an argument.
- implements an accept operation that takes a visitor as an argument.
- this participant can not be read-only.
- this participant can not be an interface.
Parameters
-
when selected, will create special JavaDoc links in the code between pattern participants, describing
their relationships.
Applicability
Use the Visitor pattern when
- an object structure contains many classes of objects with differing interfaces,
and you want to perform operations on these objects that depend on their concrete classes.
- many distinct and unrelated operations need to be performed on objects in an object structure, and
you want to avoid "polluting" their classes with these operations. Visitor lets you keep related
operations together by defining them in one class. When the object structure is shared by many
applications, use Visitor to put operations in just those applications that need them.
-
the classes defining the object structure rarely change, but you often want
to define new operations over the structure. Changing the object structure classes requires redefining
the interface to all visitors, which is potentially costly. If the object structure classes change
often, then it's probably better to define the operations in those classes.