Overriding setter on var

Question

A tiny question with hopefully a tiny answer:

I have a var in my class that needs to trigger some kind of update whenever it is set. I know that a var implicitly gets two methods with it, a getter and a setter. Is it possible to somehow override the setter method to make sure the update is triggered, without getting recursive? I mean

def a_=(x: Any) = { a = x; update }

Will probably be an infinite recursion, right?

The var is only set outside the class and read only inside the class, maybe that helps.

Thanks for listening.

Answer 1

Your code will never be an infinite recursion because it won't compile. Due to implicit creation of a Getter and a Setter by the compiler you can't create such methods twice. I don't know if there is a reason why the compiler does not check if a Getter or a Setter exists and only if there are no such methods it create ones.

You can avoid this problem by renaming the private variable:

class X(private var _i: Int) {
  def i = _i
  def i_=(i: Int) {
    println(i)
    _i = i
  }
}

These methods have the same signature as the methods generated by the compiler.

If the update method has only be called once you can do this inside the companion object:

object X {
  def apply(i: Int) = {
    update
    new X(i)
  }
}
class X(i: Int)

Is there a reason why you don't prefer an immutable object? If no, you can copy the old one and set a new value at the same time:

case class X(i: Int, j: Int)
val x1 = X(3, 6)
val x2 = x1.copy(i = 1) // x2 = X(1,6)