Visitor Design Pattern
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The Visitor design motif
The informal description: Excerpts from [Gamma et al. 1995] (adapted for
this purpose):
Intent: 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.
Structure: Original OMT diagram converted to UML (Why
and How?):
Participants:
- Visitor: 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.
- ConcreteVisitor: implements each operation declared by Visitor. Each operation implements a fragment of the algorithm defined for the corresponding class of object in the structure. ConcreteVisitor 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 that takes a visitor as an argument.
- ObjectStructure:
- Can enumerate its elements.
- May provide a high-level interface to allow the visitor to visit its elements.
- May either be a composite or a collection such as a list or a set.
Collaborations:
- A client that uses the Visitor pattern must create a ConcreteVisitor object and then traverse the object structure, visiting each element with the visitor.
- When an element is visited, it calls the Visitor operation that corresponds to its class. The element supplies itself as an argument to this operation to let the visitor access its state, if necessary.
Formal specification
See also:
