Scala Design Patterns - Second Edition

Code example

Now, let's go through all the code that represents the preceding diagram. First, let's see the Observer interface. We have decided to have it as a trait that can be mixed in any class:

trait Observer[T] {  def handleUpdate(subject: T)}

This is extremely simple. Next, we will take a look at the Observable class. It is a trait that can also be mixed in and can make classes observable:

trait Observable[T] {  this: T =>  private val observers = ListBuffer[Observer[T]]()  def addObserver(observer: Observer[T]): Unit = {    observers.+=:(observer)  }  def notifyObservers(): Unit = {    observers.foreach(_.handleUpdate(this))  }}

In the preceding code, we have used a self-type in order to make sure that we limit how the Observable trait is mixed ...