observer

Behavioral » Observer

Elements

• Observable/Subject - interface that define attach/detach operations
• ConcreteObservable - maintain the state of an object and notifies attached observer
• Observer - interface that defines how to notify
• ConcreteObserver - the object that receive notifications

Description

This pattern uncouple an event from its possible handlers. It is useful to trigger many actions on same events. And it is useful whenever the number of action of this event grows.

Key elements of this patterns are the publisher and the subscriber. The publisher will emit an event. All subscriber that handle that event are called when the particular event is triggered.

Implementation

First of all in this pattern we needs a publisher and a subscriber. The publisher will contain the list of subscribers/observers. The subsciber can be notified (it expose a method Subscriber.Notify(string).

type Subscriber interface {
	Notify(string)
}

type Publisher struct {
	ObserverList []Subscriber
}

As we do not care about the observer behavior now we will take care about some functionalities:

• AddSubscriber
• RemoveObserver
• NotifyObservers

func (p *Publisher) AddSubscriber(o Subscriber) {
	p.ObserverList = append(p.ObserverList, o)
}

func (p *Publisher) RemoveObserver(o Subscriber) {
	var indexToRemove int
	for i, observer := range p.ObserverList {
		if observer == o {
			indexToRemove = i
			break
		}
	}
	p.ObserverList = append(
		p.ObserverList[:indexToRemove],
		p.ObserverList[indexToRemove+1:]...,
	)
}

func (p *Publisher) NotifyObservers(message string) {
	for _, observer := range p.ObserverList {
		observer.Notify(message)
	}
}