Golang - Design Pattern #20: 訪問者模式 (Visitor)

KH Huang

發佈於Golang Lab

更新於 2024/08/29發佈於 2023/08/29閱讀時間約 6 分鐘

👣 操作物件結構 👣

訪問者模式 (Visitor Pattern) 是一種設計模式，它允許開發者將新的操作新增到現有物件結構中，而不必修改該結構的內部代碼。該模式的重點是將操作和物件結構分離，提供一個彈性的方式新增新的行為。

訪問者模式

訪問者模式主要涉及兩種接口：Visitor和Element。Visitor接口定義了訪問不同類型物件的方法，而Element接口定義了accept()方法，該方法接受一個訪問者作為參數。

Go Go Go

🛒 電子商務系統

假設我們的電子商務平台有多種產品，我們想為每種產品計算折扣。而不同的產品，折扣方式可能不同。

// You can edit this code!
// Click here and start typing.
package main

import "fmt"

// Element
type Product interface {
	Accept(visitor Visitor)
}

// Concrete Elements
type Book struct {
	price float64
	title string
}

func (b *Book) Accept(visitor Visitor) {
	visitor.VisitBook(b)
}

type Fruit struct {
	price  float64
	name   string
	weight float64
}

func (f *Fruit) Accept(visitor Visitor) {
	visitor.VisitFruit(f)
}

// Visitor
type Visitor interface {
	VisitBook(book *Book)
	VisitFruit(fruit *Fruit)
}

// Concrete Visitor
type DiscountVisitor struct{}

func (d *DiscountVisitor) VisitBook(book *Book) {
	discountPrice := book.price * 0.9
	fmt.Printf("Discounted price for book '%s': %f\n", book.title, discountPrice)
}

func (d *DiscountVisitor) VisitFruit(fruit *Fruit) {
	discountPrice := fruit.price * 0.85
	fmt.Printf("Discounted price for %s: %f\n", fruit.name, discountPrice)
}

func main() {
	// Client code
	book := &Book{price: 100, title: "Design Patterns"}
	fruit := &Fruit{price: 50, name: "Apple", weight: 0.5}

	discountVisitor := &DiscountVisitor{}

	book.Accept(discountVisitor)  // Discounted price for book 'Design Patterns': 90.000000
	fruit.Accept(discountVisitor) // Discounted price for Apple: 42.500000

}

✉️ 使用者反饋系統

假設我們有一個使用者反饋系統，我們想要對不同類型的反饋執行不同的操作，例如保存到資料庫或發送電子郵件提醒。

package main

// Element
type Feedback interface {
	Accept(visitor FeedbackVisitor)
}

// Concrete Elements
type Comment struct {
	content string
	userID  int
}

func (c *Comment) Accept(visitor FeedbackVisitor) {
	visitor.VisitComment(c)
}

type Report struct {
	description string
	severity    int
}

func (r *Report) Accept(visitor FeedbackVisitor) {
	visitor.VisitReport(r)
}

// Visitor
type FeedbackVisitor interface {
	VisitComment(comment *Comment)
	VisitReport(report *Report)
}

// Concrete Visitor
type DatabaseSaver struct{}

func (d *DatabaseSaver) VisitComment(comment *Comment) {
	// Logic to save comment to database
}

func (d *DatabaseSaver) VisitReport(report *Report) {
	// Logic to save report to database and maybe send an email alert
}

func main() {
	// Client code
	comment := &Comment{content: "Great product!", userID: 101}
	report := &Report{description: "Bug found in checkout process.", severity: 1}

	saver := &DatabaseSaver{}

	comment.Accept(saver)
	report.Accept(saver)
}