In object-oriented systems, we often need to perform different operations on complex data structures. If the operation logic is directly embedded in the data structure, it will not only increase the complexity of the class, but also make the code difficult to maintain and expand.Visitor PatternBy separating operations from data structures, it provides an elegant way to define new operations for these structures. This article will introduce in detail the concept of the visitor pattern, the difference from other patterns, the problems it solves, implementation examples in Golang, as well as its application and precautions in actual development.

What is Visitor Pattern?

Visitor Patternis aBehavioral design patterns, which allows usSeparate operations from object structureIn the visitor pattern,VisitorThe operations to be performed on the object are defined, and the object structure itself only needs to be accessed by the visitor. In this way, new operations can be added to it without modifying the original object structure.

Components of the Visitor Pattern

1. Visitor interface: Defines the method of accessing elements. There are different access methods for different element types.
2. Concrete Visitor: Implement the visitor interface and define specific operation logic.
3. Element interface: Defines methods for accepting visitors for implementation by specific elements.
4. Concrete Element: Implement the element interface and call the visitor's methods in it.

Differences between the Visitor pattern and other patterns

1. Strategy Pattern

• Target: The strategy pattern replaces the behavior of the same type of operation through different algorithms.
• the difference: The strategy pattern is mainly used to replace algorithms, while the visitor pattern is used to add new operations without modifying the class.
  For more information about the strategy mode, please refer to the article:In-depth analysis of the implementation and application of Go design pattern strategy pattern in Golang

2. Command Pattern

• Target: The command pattern encapsulates requests as objects and supports functions such as cancellation and queuing.
• the difference: The command pattern encapsulates specific requests, while the visitor pattern separates operations from data structures and supports different types of objects.
  For more information about command mode, please refer to the article:Select In-depth analysis of Go design pattern Command Pattern (Command Pattern) implementation and application in Golang In-depth analysis of Go design pattern Command Pattern (Command Pattern) implementation and application in Golang

3. Iterator Pattern

• Target: The iterator pattern is used to sequentially access the elements in a collection without exposing its internal structure.

• the difference: The iterator pattern is used to traverse the collection, while the visitor pattern is used to define new operation logic.
  For details about the iterator mode, please refer to the article:In-depth analysis of the implementation and application of the Iterator Pattern in Golang

What problem does the Visitor pattern solve?

1. Separating operations from data structures: The visitor pattern separates operations from data structures, avoiding modifying the code of the original class.
2. Increase flexibility for new operations: New operations can be implemented by adding new visitor classes without modifying the existing data structure.
3. Simplify class structure: Move the operation logic out of the class itself to make the data structure class more concise.

Application scenarios of visitor pattern

1. Compiler syntax tree traversal: Perform different operations on different nodes in the syntax tree.
2. File system scan: Perform different operations, such as compression or encryption, based on the file type.
3. Report generation system: Perform different statistical and calculation operations on different types of financial data.
4. Bulk operations on object structures: Such as performing different maintenance operations on a group of different types of equipment.

Visitor pattern implementation in Golang

Next, we will go through a specificGraphics drawing systemThis example shows how to implement the visitor pattern in Golang. There are different types of graphic objects in the system (such as circles and rectangles), and the visitor pattern allows us to add new operations to them (such as drawing or calculating the area) without modifying these graphic classes.

Example: Graphics drawing system

1. Define the visitor interface

package main import "fmt" // Visitor interface: defines operations for visiting different graphics type Visitor interface { VisitCircle(c *Circle) VisitRectangle(r *Rectangle) }

2. Define element interface

// Shape interface: defines the method for accepting visitors type Shape interface { Accept(v Visitor) }

3. Implement specific elements (circle and rectangle)

// Circle structure: specific element, representing a circle type Circle struct { Radius float64 } // Accept method: accept visitors func (c *Circle) Accept(v Visitor) { v.VisitCircle(c) } // Rectangle structure: specific element, representing a rectangle type Rectangle struct { Width float64 Height float64 } // Accept method: accept visitors func (r *Rectangle) Accept(v Visitor) { v.VisitRectangle(r) }

4. Implementing specific visitors

// DrawVisitor structure: specific visitor, implements drawing operations type DrawVisitor struct{} func (d *DrawVisitor) VisitCircle(c *Circle) { fmt.Printf("Drawing a circle with radius %.2f\n", c.Radius) } func (d *DrawVisitor) VisitRectangle(r *Rectangle) { fmt.Printf("Drawing a rectangle with width %.2f and height %.2f\n", r.Width, r.Height) } // AreaVisitor structure: specific visitor, implements area calculation operations type AreaVisitor struct{} func (a *AreaVisitor) VisitCircle(c *Circle) { area := 3.14 * c.Radius * c.Radius fmt.Printf("Area of the circle: %.2f\n", area) } func (a *AreaVisitor) VisitRectangle(r *Rectangle) { area := r.Width * r.Height fmt.Printf("Area of the rectangle: %.2f\n", area) }

5. Sample code using the visitor pattern

func main() { // Create specific elements circle := &Circle{Radius: 5} rectangle := &Rectangle{Width: 4, Height: 3} // Create visitors drawVisitor := &DrawVisitor{} areaVisitor := &AreaVisitor{} // Use drawing visitors fmt.Println("Drawing shapes:") circle.Accept(drawVisitor) rectangle.Accept(drawVisitor) // Use area calculation visitors fmt.Println("\nCalculating areas:") circle.Accept(areaVisitor) rectangle.Accept(areaVisitor) }

Output

Drawing shapes: Drawing a circle with radius 5.00 Drawing a rectangle with width 4.00 and height 3.00 Calculating areas: Area of the circle: 78.50 Area of the rectangle: 12.00

Code Analysis

1. Visitor Interface: Defines methods for accessing circles and rectangles.
2. Shape Interface: DefinedAcceptMethods for specific elements to implement.
3. Circle and Rectangle structures: Specific elements that implementAcceptmethod.
4. DrawVisitor and AreaVisitor structures: Specific visitors implement the operations of drawing and calculating area respectively.
5. main function: Shows how to use the visitor pattern to draw and calculate the area of a graph.

Application in actual development

1. Compiler: The visitor pattern is used to traverse the syntax tree and generate target code.
2. File System: Perform different operations, such as compression or encryption, based on the file type.
3. Business reporting system: Generate different reports based on different types of data.
4. Equipment maintenance system: Perform different maintenance operations on different types of equipment.

Notes on using the visitor pattern

1. Increased number of element classes: Each time a new visitor is added, the element class needs to be modifiedAcceptmethod, which may increase the complexity of the code.
2. Coupling of visitors to elements：Visitors need to understand the internal structure of the element, which may lead to a certain degree of coupling.
3. Not suitable for frequently changing classes: If the element class structure changes frequently, using the visitor pattern will increase maintenance costs.

Visitor pattern vs. strategy pattern

Summarize

Visitor PatternIt is a powerful design pattern that is suitable for scenarios where different operations need to be performed on object structures. Through the visitor pattern, we can add new operation logic to it without modifying the data structure. In Golang, the visitor pattern can be easily implemented through the combination of interfaces and structures, and applied to scenarios such as compilers, file systems, and business reports. In actual development, the reasonable use of the visitor pattern can improve the scalability and maintainability of the code, but it is necessary to pay attention to its applicable scenarios and the complexity that may be brought. If you need to perform different operations on the object structure in your project, the visitor pattern will be a very suitable choice.

Reference Links

• Design Patterns - Visitor
• Go Design Patterns