Observer pattern

The subscribers listen to publishers. Once there is a change, publishers will notify subscribers.

trait Subscriber {
  def handle(publisher: Publisher): Unit
}

trait Publisher {
  private subscribers: Set[Subscriber] = Set()

  def subscribe(subscriber: Subscriber): Unit = {
    subscribers += subscriber
  }

  def unsubscribe(subscriber: Subscriber): Unit = {
    subscribers -= subscriber
  }

  def publish(): Unit = {
    subscribers.foreach(_.handle(this))
  }
}

Observer pattern provides a eazy way to decouple the view from state. However, it has some downsides.

• Forces imperative style. A lot of functions return Unit type.
• Concurrency makes things more complicated.

These downsides cause bugs.