Observer Pattern

Observer Design Pattern is a behavioral design pattern. This pattern allows an instance (called subject) to publish events to other multiple instances (called observers). These observers subscribe to the subject and hence get notified by events in case of any change happening in the subject.

Refer to https://swsmile.info/post/design-pattern-observer-pattern/ for Observer Pattern

Demo

subject.go: Subject

package main

type subject interface {
    register(Observer observer)
    deregister(Observer observer)
    notifyAll()
}

item.go: Concrete subject

package main

import "fmt"

type item struct {
    observerList []observer
    name         string
    inStock      bool
}

func newItem(name string) *item {
    return &item{
        name: name,
    }
}
func (i *item) updateAvailability() {
    fmt.Printf("Item %s is now in stock\n", i.name)
    i.inStock = true
    i.notifyAll()
}
func (i *item) register(o observer) {
    i.observerList = append(i.observerList, o)
}

func (i *item) deregister(o observer) {
    i.observerList = removeFromslice(i.observerList, o)
}

func (i *item) notifyAll() {
    for _, observer := range i.observerList {
        observer.update(i.name)
    }
}

func removeFromslice(observerList []observer, observerToRemove observer) []observer {
    observerListLength := len(observerList)
    for i, observer := range observerList {
        if observerToRemove.getID() == observer.getID() {
            observerList[observerListLength-1], observerList[i] = observerList[i], observerList[observerListLength-1]
            return observerList[:observerListLength-1]
        }
    }
    return observerList
}

observer.go: Observer

package main

type observer interface {
    update(string)
    getID() string
}

customer.go: Concrete observer

package main

import "fmt"

type customer struct {
    id string
}

func (c *customer) update(itemName string) {
    fmt.Printf("Sending email to customer %s for item %s\n", c.id, itemName)
}

func (c *customer) getID() string {
    return c.id
}

main.go: Client code

package main

func main() {

    shirtItem := newItem("Nike Shirt")

    observerFirst := &customer{id: "abc@gmail.com"}
    observerSecond := &customer{id: "xyz@gmail.com"}

    shirtItem.register(observerFirst)
    shirtItem.register(observerSecond)

    shirtItem.updateAvailability()
}

output.txt: Execution result

Item Nike Shirt is now in stock
Sending email to customer abc@gmail.com for item Nike Shirt
Sending email to customer xyz@gmail.com for item Nike Shirt

Reference

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【Golang】设计模式 - Structural - Observer