add visitor mode

Author: senghoo

23_visitor/README.md

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+# 访问者模式
+
+访问者模式可以给一系列对象透明的添加功能，并且把相关代码封装到一个类中。
+
+对象只要预留访问者接口`Accept`则后期为对象添加功能的时候就不需要改动对象。
+

23_visitor/visitor.go

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+package visitor
+
+import "fmt"
+
+type Customer interface {
+	Accept(Visitor)
+}
+
+type Visitor interface {
+	Visit(Customer)
+}
+
+type EnterpriseCustomer struct {
+	name string
+}
+
+type CustomerCol struct {
+	customers []Customer
+}
+
+func (c *CustomerCol) Add(customer Customer) {
+	c.customers = append(c.customers, customer)
+}
+
+func (c *CustomerCol) Accept(visitor Visitor) {
+	for _, customer := range c.customers {
+		customer.Accept(visitor)
+	}
+}
+
+func NewEnterpriseCustomer(name string) *EnterpriseCustomer {
+	return &EnterpriseCustomer{
+		name: name,
+	}
+}
+
+func (c *EnterpriseCustomer) Accept(visitor Visitor) {
+	visitor.Visit(c)
+}
+
+type IndividualCustomer struct {
+	name string
+}
+
+func NewIndividualCustomer(name string) *IndividualCustomer {
+	return &IndividualCustomer{
+		name: name,
+	}
+}
+
+func (c *IndividualCustomer) Accept(visitor Visitor) {
+	visitor.Visit(c)
+}
+
+type ServiceRequestVisitor struct{}
+
+func (*ServiceRequestVisitor) Visit(customer Customer) {
+	switch c := customer.(type) {
+	case *EnterpriseCustomer:
+		fmt.Printf("serving enterprise customer %s\n", c.name)
+	case *IndividualCustomer:
+		fmt.Printf("serving individual customer %s\n", c.name)
+	}
+}
+
+// only for enterprise
+type AnalysisVisitor struct{}
+
+func (*AnalysisVisitor) Visit(customer Customer) {
+	switch c := customer.(type) {
+	case *EnterpriseCustomer:
+		fmt.Printf("analysis enterprise customer %s\n", c.name)
+	}
+}

23_visitor/visitor_test.go

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+package visitor
+
+func ExampleRequestVisitor() {
+	c := &CustomerCol{}
+	c.Add(NewEnterpriseCustomer("A company"))
+	c.Add(NewEnterpriseCustomer("B company"))
+	c.Add(NewIndividualCustomer("bob"))
+	c.Accept(&ServiceRequestVisitor{})
+	// Output:
+	// serving enterprise customer A company
+	// serving enterprise customer B company
+	// serving individual customer bob
+}
+
+func ExampleAnalysis() {
+	c := &CustomerCol{}
+	c.Add(NewEnterpriseCustomer("A company"))
+	c.Add(NewIndividualCustomer("bob"))
+	c.Add(NewEnterpriseCustomer("B company"))
+	c.Accept(&AnalysisVisitor{})
+	// Output:
+	// analysis enterprise customer A company
+	// analysis enterprise customer B company
+}