visitor pattern

README.md

@@ -42,10 +42,10 @@ A curated collection of idiomatic design & application patterns for Go language.
 | [Memento](/behavioral/memento/main.go) | Generate an opaque token that can be used to go back to a previous state | ✔ |
 | [Observer](/behavioral/observer.md) | Provide a callback for notification of events/changes to data | ✔ |
 | [Registry](/behavioral/registry.md) | Keep track of all subclasses of a given class | ✘ |
-| [State](/behavioral/state.md) | Encapsulates varying behavior for the same object based on its internal state | ✔ |
+| [State](/behavioral/state/main.go) | Encapsulates varying behavior for the same object based on its internal state | ✔ |
 | [Strategy](/behavioral/strategy.md) | Enables an algorithm's behavior to be selected at runtime | ✔ |
-| [Template](/behavioral/template.md) | Defines a skeleton class which defers some methods to subclasses | ✔ |
+| [Template](/behavioral/template/main.go) | Defines a skeleton class which defers some methods to subclasses | ✔ |
-| [Visitor](/behavioral/visitor.md) | Separates an algorithm from an object on which it operates | ✘ |
+| [Visitor](/behavioral/visitor/main.go) | Separates an algorithm from an object on which it operates | ✔ |
 
 ## Synchronization Patterns
 

behavioral/visitor/main.go

@@ -0,0 +1,110 @@
+// 访问者模式	visitor pattern.
+// 该模式用于将数据结构和操作进行分离，同样用于分离操作的还有策略模式(strategy pattern)，但两者存在侧重点的不同.
+// 访问者模式侧重于扩展访问的方法类型，是为了对某一个类及其子类的访问方式进行扩展，允许增加更多不同的访问者，但不宜增加更多的Host.
+// 以下以实体A(仅有玩家、NPC、物体三类)被访问(查看信息、发起挑战等，此处可扩展)的过程
+package main
+
+import "log"
+
+// 如果这里有更多的新类型Host需要扩展，则不宜使用访问者模式
+type Host interface {
+	Accept(Visitor)
+}
+
+type PlayerHost struct {
+	Name string
+	Level int
+}
+
+func (p PlayerHost)Accept(v Visitor){
+	v.VisitPlayer(p)
+}
+
+type NPCHost struct {
+	Name string
+	IsImmortal bool
+}
+
+func (n NPCHost)Accept(v Visitor){
+	v.VisitNPC(n)
+}
+
+type ObjectHost struct {
+	Name string
+	Price int
+}
+
+func (o ObjectHost)Accept(v Visitor){
+	v.VisitObject(o)
+}
+
+type Visitor interface {
+	VisitPlayer(PlayerHost)
+	VisitNPC(NPCHost)
+	VisitObject(ObjectHost)
+}
+
+// 访问者允许有不同类型的访问者
+// 仅查看信息的访问者
+type InfoVisitor struct {}
+
+func (InfoVisitor) VisitPlayer(p PlayerHost) {
+	log.Printf("Player -> Name:%s ,Level:%d\n" ,p.Name ,p.Level)
+}
+
+func (InfoVisitor) VisitNPC(n NPCHost) {
+	log.Printf("NPC -> Name:%s ,Immortal:%v\n" ,n.Name ,n.IsImmortal)
+}
+
+func (InfoVisitor) VisitObject(o ObjectHost) {
+	log.Printf("Object -> Name:%s ,Price:%d\n" ,o.Name ,o.Price)
+}
+
+// 发起攻击的访问者
+type AggressiveVisitor struct {}
+
+func (AggressiveVisitor) VisitPlayer(p PlayerHost) {
+	log.Printf("Attack %s\n" ,p.Name)
+}
+
+func (AggressiveVisitor) VisitNPC(n NPCHost) {
+	log.Printf("Attack NPC %s\n" ,n.Name)
+}
+
+func (AggressiveVisitor) VisitObject(o ObjectHost) {
+	log.Printf("Unsupported target %s\n" ,o.Name)
+}
+
+func main(){
+	infoVst := InfoVisitor{}
+	agrVst := AggressiveVisitor{}
+
+	pA := PlayerHost{"icg" ,1}
+	pB := PlayerHost{"sz" ,2}
+	npc := NPCHost{"nyn" ,true}
+	obj := ObjectHost{"cake" ,19}
+
+	hostList := []Host{pA ,npc ,obj ,pB}
+
+	for _ ,v := range hostList{
+		v.Accept(infoVst)
+	}
+	println()
+	for _ ,v := range hostList{
+		v.Accept(agrVst)
+	}
+	/*
+	output:
+	2019/05/04 10:00:49 Player -> Name:icg ,Level:1
+	2019/05/04 10:00:49 NPC -> Name:nyn ,Immortal:true
+	2019/05/04 10:00:49 Object -> Name:cake ,Price:19
+	2019/05/04 10:00:49 Player -> Name:sz ,Level:2
+
+	2019/05/04 10:00:49 Attack icg
+	2019/05/04 10:00:49 Attack NPC nyn
+	2019/05/04 10:00:49 Unsupported target cake
+	2019/05/04 10:00:49 Attack sz
+	 */
+
+}
+