Question

I recently started learning Scala and started a little project to create a simple roguelike game. However, I’m stuck at trying to implement the observer pattern.
This answer touches the subject, but I can’t figure out how to make it work. Below is the code from the answer linked above. I’m mostly puzzled by the “this: S =>” part of the code, I suppose I should have some kind of a function there, but I’m not sure. I would like to have it return a tuple from the class that will extend the Subject trait.

trait Observer[S] {
 def receiveUpdate(subject: S);
}

trait Subject[S] {
 this: S =>
 private var observers: List[Observer[S]] = Nil
 def addObserver(observer: Observer[S]) = observers = observer :: observers

 def notifyObservers() = observers.foreach(_.receiveUpdate(this))
}

Answer 1

See Steve’s answer about the self type and for another code example.

Here is some sample code using an observer. The ObservedAccount is the Subject that is observed by an AccountReporter observer.

trait Observer[S] {
    def receiveUpdate(subject: S);
}

trait Subject[S] { 
    this: S =>
    private var observers: List[Observer[S]] = Nil
    def addObserver(observer: Observer[S]) = observers = observer :: observers

    def notifyObservers() = observers.foreach(_.receiveUpdate(this))
}

class Account(initialBalance: Double) {
    private var currentBalance = initialBalance
    def balance = currentBalance
    def deposit(amount: Double)  = currentBalance += amount
    def withdraw(amount: Double) = currentBalance -= amount
}

class ObservedAccount(initialBalance: Double) extends Account(initialBalance) with Subject[Account] {
    override def deposit(amount: Double) = {
        super.deposit(amount)
        notifyObservers()
    }
    override def withdraw(amount: Double) = {
        super.withdraw(amount)
        notifyObservers()
    }
}


class AccountReporter extends Observer[Account] {
    def receiveUpdate(account: Account) =
        println("Observed balance change: "+account.balance)
}

Let’s see it in action:

scala> val oa = new ObservedAccount(100.0)
oa: ObservedAccount = ObservedAccount@3f947e20

scala> val ar = new AccountReporter
ar: AccountReporter = AccountReporter@6ea70a98

scala> oa.addObserver(ar)

scala> oa.deposit(40.0)
Observed balance change: 140.0

scala> oa.withdraw(40.0)
Observed balance change: 100.0