Add Observer Pattern 🎉 (#20)

observer.go

@@ -0,0 +1,79 @@
+package patterns
+
+import "fmt"
+
+// Subject interface
+type Subject interface {
+	Register(Subscriber)
+	Deregister(Subscriber)
+	NotifyAll()
+}
+
+// Product struct
+type Product struct {
+	SubscriberList []Subscriber
+	name           string
+	inStock        bool
+}
+
+// NewProduct function create a brand-new Product
+func NewProduct(name string) *Product {
+	return &Product{
+		name: name,
+	}
+}
+
+// UpdateAvailability method on Product struct
+func (i *Product) UpdateAvailability() {
+	i.inStock = true
+	i.NotifyAll()
+}
+
+// Register method on Product struct
+func (i *Product) Register(o Subscriber) {
+	i.SubscriberList = append(i.SubscriberList, o)
+}
+
+// Deregister method on Product struct
+func (i *Product) Deregister(o Subscriber) {
+	i.SubscriberList = removeFromslice(i.SubscriberList, o)
+}
+
+// NotifyAll method on Product struct
+func (i *Product) NotifyAll() {
+	for _, Subscriber := range i.SubscriberList {
+		Subscriber.Update(i.name)
+	}
+}
+
+func removeFromslice(SubscriberList []Subscriber, observerToRemove Subscriber) []Subscriber {
+	observerListLength := len(SubscriberList)
+	for i, Subscriber := range SubscriberList {
+		if observerToRemove.GetID() == Subscriber.GetID() {
+			SubscriberList[observerListLength-1], SubscriberList[i] = SubscriberList[i], SubscriberList[observerListLength-1]
+			return SubscriberList[:observerListLength-1]
+		}
+	}
+	return SubscriberList
+}
+
+// Subscriber interface
+type Subscriber interface {
+	Update(string)
+	GetID() string
+}
+
+// Customer struct
+type Customer struct {
+	id string
+}
+
+// Update method on Custome struct
+func (c *Customer) Update(itemName string) {
+	fmt.Printf("Tell customer %s for new product %s\n", c.id, itemName)
+}
+
+// GetID method on Customer struct
+func (c *Customer) GetID() string {
+	return c.id
+}

observer_test.go

@@ -0,0 +1,13 @@
+package patterns
+
+func ExampleSubscriber() {
+	shirtItem := NewProduct("Go React")
+	firstCustomer := &Customer{id: "vue@yahoo.com"}
+	secondCustomer := &Customer{id: "angular@yahoo.com"}
+	shirtItem.Register(firstCustomer)
+	shirtItem.Register(secondCustomer)
+	shirtItem.UpdateAvailability()
+	// Output:
+	// Tell customer vue@yahoo.com for new product Go React
+	// Tell customer angular@yahoo.com for new product Go React
+}