#Visitor

#Overview

Add operations to object structures without changing their classes.

#When to use

• You need to run multiple operations on a stable object structure.
• Adding new operations is more common than adding new node types.

#Java example

interface Visitor {
    void visit(TextNode node);
    void visit(ImageNode node);
}

interface Node {
    void accept(Visitor v);
}

class TextNode implements Node {
    public void accept(Visitor v) { v.visit(this); }
}

#TypeScript example

interface Visitor {
  visitText(node: TextNode): void;
  visitImage(node: ImageNode): void;
}

interface Node {
  accept(v: Visitor): void;
}

class TextNode implements Node {
  accept(v: Visitor): void { v.visitText(this); }
}

#Pros and cons

Pros:

• New operations can be added without modifying nodes.
• Separates algorithms from object structure.

Cons:

• Adding new node types requires updating all visitors.
• Can be verbose.

#Common pitfalls

• Using Visitor for small object graphs.
• Forgetting to update all visitors after new nodes.

#Related patterns

• Iterator
• Composite