Visitor: separate elements from operations on them

3a:I[286652,["$20","$21","$22","$23","$24","$2e"],"default"] 3b:T3722,{"type":"doc","content":[{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Overview"}]},{"type":"paragraph","content":[{"type":"text","text":"A document of text and image nodes gains rendering, counting and validation operations. Instead of adding another operation to every node class, collect each operation in its own object."}]},{"type":"paragraph","content":[{"type":"text","text":"A Visitor provides a method for each element kind. An element accepts the visitor and calls the method for its own type, allowing text and images to receive different treatment."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"UML"}]},{"type":"animatedDiagramBlock","attrs":{"title":"Visitor: separate elements from operations on them","scene":{"svg":"","css":"","javascript":""},"width":0,"height":590,"autoplay":false,"showControls":false,"fitMode":"contain","frameMode":"content"}},{"type":"paragraph","content":[{"type":"text","text":"TextNode and ImageNode implement IDocNode and accept a visitor. TextVisitor and CountVisitor implement IVisitor for rendering and counting respectively."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"History"}]},{"type":"paragraph","content":[{"type":"text","text":"GoF cataloged Visitor as a behavioral pattern in Design Patterns in 1994. It separates operations from the classes that form an object structure."},{"type":"footnote","attrs":{"id":"visitor-gof","body":"Gamma, Helm, Johnson, Vlissides. *Design Patterns* (1994). [Publisher record](https://www.informit.com/store/design-patterns-elements-of-reusable-object-oriented-software-9780201633610). Behavioral patterns, Visitor."}}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Why use it?"}]},{"type":"paragraph","content":[{"type":"text","text":"A new rendering or counting operation can be another visitor without changing existing element classes. One visitor collects the element-specific parts of that operation."}]},{"type":"paragraph","content":[{"type":"text","text":"Adding a new element kind instead changes IVisitor and existing visitors. This favors stable element kinds with frequently added operations."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Example"}]},{"type":"paragraph","content":[{"type":"text","text":"Render each document element with TextVisitor."}]},{"type":"codeBlock","attrs":{"language":"csharp"},"content":[{"type":"text","text":"using System;\nusing System.Collections.Generic;\nusing System.Linq;\n\nIDocNode[] nodes = { new TextNode(\"hello\"), new ImageNode(\"cover\") };\nvar visitor = new TextVisitor();\nforeach (IDocNode node in nodes) node.Accept(visitor);\nforeach (string line in visitor.Lines) Console.WriteLine(line);\n\ninterface IDocNode { void Accept(IVisitor visitor); }\n// Collect the element-specific parts of one operation in its visitor.\ninterface IVisitor\n{\n void Visit(TextNode node);\n void Visit(ImageNode node);\n}\n// The node selects its matching visitor method without implementing the operation itself.\nsealed record TextNode(string Value) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed record ImageNode(string Name) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed class TextVisitor : IVisitor\n{\n public List Lines { get; } = new();\n public void Visit(TextNode node) => Lines.Add(node.Value);\n public void Visit(ImageNode node) => Lines.Add(\"[image: \" + node.Name + \"]\");\n}\nsealed class CountVisitor : IVisitor\n{\n public int Texts { get; private set; }\n public int Images { get; private set; }\n public void Visit(TextNode node) => Texts++;\n public void Visit(ImageNode node) => Images++;\n}\n"}]},{"type":"codeBlock","attrs":{"language":"text"},"content":[{"type":"text","text":"hello\n[image: cover]"}]},{"type":"paragraph","content":[{"type":"text","text":"The caller holds node as IDocNode, so Visit(node) cannot select either the TextNode or ImageNode overload. Each concrete Accept() passes its own typed this to Visit(), selecting the matching operation without type checks in the caller."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"An extended example"}]},{"type":"paragraph","content":[{"type":"text","text":"Keep the element classes unchanged and count text and image nodes with CountVisitor."}]},{"type":"codeBlock","attrs":{"language":"csharp"},"content":[{"type":"text","text":"using System;\nusing System.Collections.Generic;\nusing System.Linq;\n\nIDocNode[] nodes = { new TextNode(\"hello\"), new ImageNode(\"cover\"), new TextNode(\"world\") };\nvar visitor = new CountVisitor();\nforeach (IDocNode node in nodes) node.Accept(visitor);\nConsole.WriteLine(\"text nodes: \" + visitor.Texts);\nConsole.WriteLine(\"image nodes: \" + visitor.Images);\n\ninterface IDocNode { void Accept(IVisitor visitor); }\n// Collect the element-specific parts of one operation in its visitor.\ninterface IVisitor\n{\n void Visit(TextNode node);\n void Visit(ImageNode node);\n}\n// The node selects its matching visitor method without implementing the operation itself.\nsealed record TextNode(string Value) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed record ImageNode(string Name) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed class TextVisitor : IVisitor\n{\n public List Lines { get; } = new();\n public void Visit(TextNode node) => Lines.Add(node.Value);\n public void Visit(ImageNode node) => Lines.Add(\"[image: \" + node.Name + \"]\");\n}\nsealed class CountVisitor : IVisitor\n{\n public int Texts { get; private set; }\n public int Images { get; private set; }\n public void Visit(TextNode node) => Texts++;\n public void Visit(ImageNode node) => Images++;\n}\n"}]},{"type":"codeBlock","attrs":{"language":"text"},"content":[{"type":"text","text":"text nodes: 2\nimage nodes: 1"}]},{"type":"paragraph","content":[{"type":"text","text":"The caller foreach owns traversal, while the visitor owns the operation on each element. On a Composite tree, decide who traverses children to avoid visiting them twice."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"TypeScript example"}]},{"type":"paragraph","content":[{"type":"text","text":"Nodes connect their concrete kind to the visitor; visitors own operations such as rendering and counting. A new operation can then be added without another method in every node class."}]},{"type":"codeBlock","attrs":{"language":"typescript"},"content":[{"type":"text","text":"interface IDocNode { accept(visitor: IVisitor): void; }\n// Collect the element-specific parts of one operation in its visitor.\ninterface IVisitor {\n visitText(node: TextNode): void;\n visitImage(node: ImageNode): void;\n}\n// The node selects its matching visitor method without implementing the operation itself.\nclass TextNode implements IDocNode {\n constructor(readonly text: string) {}\n accept(visitor: IVisitor): void { visitor.visitText(this); }\n}\nclass ImageNode implements IDocNode {\n constructor(readonly caption: string) {}\n accept(visitor: IVisitor): void { visitor.visitImage(this); }\n}\nclass TextVisitor implements IVisitor {\n readonly lines: string[] = [];\n visitText(node: TextNode): void { this.lines.push(node.text); }\n visitImage(node: ImageNode): void { this.lines.push(\"[image: \" + node.caption + \"]\"); }\n}\nclass CountVisitor implements IVisitor {\n texts = 0;\n images = 0;\n visitText(node: TextNode): void { this.texts++; }\n visitImage(node: ImageNode): void { this.images++; }\n}\nconst nodes: IDocNode[] = [new TextNode(\"hello\"), new ImageNode(\"cover\"), new TextNode(\"world\")];\nconst render = new TextVisitor();\nconst count = new CountVisitor();\nfor (const node of nodes) { node.accept(render); node.accept(count); }\nfor (const line of render.lines) console.log(line);\nconsole.log(\"text nodes: \" + count.texts);\nconsole.log(\"image nodes: \" + count.images);"}]},{"type":"codeBlock","attrs":{"language":"text"},"content":[{"type":"text","text":"hello\n[image: cover]\nworld\ntext nodes: 2\nimage nodes: 1"}]},{"type":"paragraph","content":[{"type":"text","text":"TypeScript overload signatures share a single implementation. This version gives the C# Visit(TextNode) and Visit(ImageNode) operations distinct names, visitText and visitImage, called by the matching accept()."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Related reading"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","content":[{"type":"text","text":"Programming patterns","marks":[{"type":"link","attrs":{"href":"https://www.makonea.com/en-US/wiki/programming-patterns","target":null,"rel":null}}]}]}]},{"type":"listItem","content":[{"type":"paragraph","content":[{"type":"text","text":"Composite Pattern","marks":[{"type":"link","attrs":{"href":"https://www.makonea.com/en-US/wiki/composite-pattern","target":null,"rel":null}}]}]}]},{"type":"listItem","content":[{"type":"paragraph","content":[{"type":"text","text":"Iterator Pattern","marks":[{"type":"link","attrs":{"href":"https://www.makonea.com/en-US/wiki/iterator-pattern","target":null,"rel":null}}]}]}]}]},{"type":"paragraph","content":[{"type":"text","text":"Each C# example is a separate .NET 10 console program."}]},{"type":"paragraph","content":[{"type":"text","text":"Compile the TypeScript examples for ES2022 or later and run the emitted JavaScript with Node.js."}]}]}3c:T370a,{"type":"doc","content":[{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Overview"}]},{"type":"paragraph","content":[{"type":"text","text":"A document of text and image nodes gains rendering, counting and validation operations. Instead of adding another operation to every node class, collect each operation in its own object."}]},{"type":"paragraph","content":[{"type":"text","text":"A Visitor provides a method for each element kind. An element accepts the visitor and calls the method for its own type, allowing text and images to receive different treatment."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"UML"}]},{"type":"animatedDiagramBlock","attrs":{"title":"Visitor: separate elements from operations on them","scene":{"svg":"","css":"","javascript":""},"width":0,"height":590,"autoplay":false,"showControls":false,"fitMode":"contain","frameMode":"content"}},{"type":"paragraph","content":[{"type":"text","text":"TextNode and ImageNode implement IDocNode and accept a visitor. TextVisitor and CountVisitor implement IVisitor for rendering and counting respectively."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"History"}]},{"type":"paragraph","content":[{"type":"text","text":"GoF cataloged Visitor as a behavioral pattern in Design Patterns in 1994. It separates operations from the classes that form an object structure."},{"type":"footnote","attrs":{"id":"visitor-gof","body":"Gamma, Helm, Johnson, Vlissides. *Design Patterns* (1994). [Publisher record](https://www.informit.com/store/design-patterns-elements-of-reusable-object-oriented-software-9780201633610). Behavioral patterns, Visitor."}}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Why use it?"}]},{"type":"paragraph","content":[{"type":"text","text":"A new rendering or counting operation can be another visitor without changing existing element classes. One visitor collects the element-specific parts of that operation."}]},{"type":"paragraph","content":[{"type":"text","text":"Adding a new element kind instead changes IVisitor and existing visitors. This favors stable element kinds with frequently added operations."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Example"}]},{"type":"paragraph","content":[{"type":"text","text":"Render each document element with TextVisitor."}]},{"type":"codeBlock","attrs":{"language":"csharp"},"content":[{"type":"text","text":"using System;\nusing System.Collections.Generic;\nusing System.Linq;\n\nIDocNode[] nodes = { new TextNode(\"hello\"), new ImageNode(\"cover\") };\nvar visitor = new TextVisitor();\nforeach (IDocNode node in nodes) node.Accept(visitor);\nforeach (string line in visitor.Lines) Console.WriteLine(line);\n\ninterface IDocNode { void Accept(IVisitor visitor); }\n// Collect the element-specific parts of one operation in its visitor.\ninterface IVisitor\n{\n void Visit(TextNode node);\n void Visit(ImageNode node);\n}\n// The node selects its matching visitor method without implementing the operation itself.\nsealed record TextNode(string Value) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed record ImageNode(string Name) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed class TextVisitor : IVisitor\n{\n public List Lines { get; } = new();\n public void Visit(TextNode node) => Lines.Add(node.Value);\n public void Visit(ImageNode node) => Lines.Add(\"[image: \" + node.Name + \"]\");\n}\nsealed class CountVisitor : IVisitor\n{\n public int Texts { get; private set; }\n public int Images { get; private set; }\n public void Visit(TextNode node) => Texts++;\n public void Visit(ImageNode node) => Images++;\n}\n"}]},{"type":"codeBlock","attrs":{"language":"text"},"content":[{"type":"text","text":"hello\n[image: cover]"}]},{"type":"paragraph","content":[{"type":"text","text":"The caller holds node as IDocNode, so Visit(node) cannot select either the TextNode or ImageNode overload. Each concrete Accept() passes its own typed this to Visit(), selecting the matching operation without type checks in the caller."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"An extended example"}]},{"type":"paragraph","content":[{"type":"text","text":"Keep the element classes unchanged and count text and image nodes with CountVisitor."}]},{"type":"codeBlock","attrs":{"language":"csharp"},"content":[{"type":"text","text":"using System;\nusing System.Collections.Generic;\nusing System.Linq;\n\nIDocNode[] nodes = { new TextNode(\"hello\"), new ImageNode(\"cover\"), new TextNode(\"world\") };\nvar visitor = new CountVisitor();\nforeach (IDocNode node in nodes) node.Accept(visitor);\nConsole.WriteLine(\"text nodes: \" + visitor.Texts);\nConsole.WriteLine(\"image nodes: \" + visitor.Images);\n\ninterface IDocNode { void Accept(IVisitor visitor); }\n// Collect the element-specific parts of one operation in its visitor.\ninterface IVisitor\n{\n void Visit(TextNode node);\n void Visit(ImageNode node);\n}\n// The node selects its matching visitor method without implementing the operation itself.\nsealed record TextNode(string Value) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed record ImageNode(string Name) : IDocNode\n{\n public void Accept(IVisitor visitor) => visitor.Visit(this);\n}\nsealed class TextVisitor : IVisitor\n{\n public List Lines { get; } = new();\n public void Visit(TextNode node) => Lines.Add(node.Value);\n public void Visit(ImageNode node) => Lines.Add(\"[image: \" + node.Name + \"]\");\n}\nsealed class CountVisitor : IVisitor\n{\n public int Texts { get; private set; }\n public int Images { get; private set; }\n public void Visit(TextNode node) => Texts++;\n public void Visit(ImageNode node) => Images++;\n}\n"}]},{"type":"codeBlock","attrs":{"language":"text"},"content":[{"type":"text","text":"text nodes: 2\nimage nodes: 1"}]},{"type":"paragraph","content":[{"type":"text","text":"The caller foreach owns traversal, while the visitor owns the operation on each element. On a Composite tree, decide who traverses children to avoid visiting them twice."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"TypeScript example"}]},{"type":"paragraph","content":[{"type":"text","text":"Nodes connect their concrete kind to the visitor; visitors own operations such as rendering and counting. A new operation can then be added without another method in every node class."}]},{"type":"codeBlock","attrs":{"language":"typescript"},"content":[{"type":"text","text":"interface IDocNode { accept(visitor: IVisitor): void; }\n// Collect the element-specific parts of one operation in its visitor.\ninterface IVisitor {\n visitText(node: TextNode): void;\n visitImage(node: ImageNode): void;\n}\n// The node selects its matching visitor method without implementing the operation itself.\nclass TextNode implements IDocNode {\n constructor(readonly text: string) {}\n accept(visitor: IVisitor): void { visitor.visitText(this); }\n}\nclass ImageNode implements IDocNode {\n constructor(readonly caption: string) {}\n accept(visitor: IVisitor): void { visitor.visitImage(this); }\n}\nclass TextVisitor implements IVisitor {\n readonly lines: string[] = [];\n visitText(node: TextNode): void { this.lines.push(node.text); }\n visitImage(node: ImageNode): void { this.lines.push(\"[image: \" + node.caption + \"]\"); }\n}\nclass CountVisitor implements IVisitor {\n texts = 0;\n images = 0;\n visitText(node: TextNode): void { this.texts++; }\n visitImage(node: ImageNode): void { this.images++; }\n}\nconst nodes: IDocNode[] = [new TextNode(\"hello\"), new ImageNode(\"cover\"), new TextNode(\"world\")];\nconst render = new TextVisitor();\nconst count = new CountVisitor();\nfor (const node of nodes) { node.accept(render); node.accept(count); }\nfor (const line of render.lines) console.log(line);\nconsole.log(\"text nodes: \" + count.texts);\nconsole.log(\"image nodes: \" + count.images);"}]},{"type":"codeBlock","attrs":{"language":"text"},"content":[{"type":"text","text":"hello\n[image: cover]\nworld\ntext nodes: 2\nimage nodes: 1"}]},{"type":"paragraph","content":[{"type":"text","text":"TypeScript overload signatures share a single implementation. This version gives the C# Visit(TextNode) and Visit(ImageNode) operations distinct names, visitText and visitImage, called by the matching accept()."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Related reading"}]},{"type":"bulletList","content":[{"type":"listItem","content":[{"type":"paragraph","content":[{"type":"text","text":"Programming patterns","marks":[{"type":"link","attrs":{"href":"https://www.makonea.com/en-US/wiki/programming-patterns","target":null,"rel":null}}]}]}]},{"type":"listItem","content":[{"type":"paragraph","content":[{"type":"text","text":"Composite Pattern","marks":[{"type":"link","attrs":{"href":"https://www.makonea.com/en-US/wiki/composite-pattern","target":null,"rel":null}}]}]}]},{"type":"listItem","content":[{"type":"paragraph","content":[{"type":"text","text":"Iterator Pattern","marks":[{"type":"link","attrs":{"href":"https://www.makonea.com/en-US/wiki/iterator-pattern","target":null,"rel":null}}]}]}]}]},{"type":"paragraph","content":[{"type":"text","text":"Each C# example is a separate .NET 10 console program."}]},{"type":"paragraph","content":[{"type":"text","text":"Compile the TypeScript examples for ES2022 or later and run the emitted JavaScript with Node.js."}]}]}1c:["$","div",null,{"className":"mx-auto max-w-6xl space-y-5 px-4 py-8 sm:py-12","children":[["$","$L2f",null,{"href":"/en-US/wiki/visitor-pattern/history","className":"underline","children":["← ","Revision history"]}],["$","h1",null,{"className":"text-2xl font-bold","children":"Compare revisions"}],[["$","h2",null,{"className":"break-words text-xl font-bold","children":"Visitor Pattern"}],["$","p",null,{"className":"text-sm","children":"Body changes between the two selected stored revisions. Code is not executed."}],["$","div",null,{"className":"flex flex-wrap gap-4 text-sm","children":[["$","$L2f","Before: full revision",{"prefetch":false,"href":"/en-US/wiki/visitor-pattern/history/rev_38aecf51a5d447c5af259f3ef1ab00de","className":"underline","children":["Before: full revision",": ","2026-10-25 02:10:19 UTC"]}],["$","$L2f","After: full revision",{"prefetch":false,"href":"/en-US/wiki/visitor-pattern/history/rev_22010a0f0de74593957862c9c731d5d6","className":"underline","children":["After: full revision",": ","2026-10-25 03:11:05 UTC"]}]]}],["$","$L3a","rev_38aecf51a5d447c5af259f3ef1ab00de:rev_22010a0f0de74593957862c9c731d5d6",{"before":"$3b","after":"$3c","locale":"en-US"}]]]}]

Compare revisions

Visitor Pattern

1 changed sections

Content blocks 5 → 5