Passing reference to Reference Class object between two other Reference Class objects (soccer example)

Question

I am trying to "pass" a reference to an Reference Class object (say, a ball) between two other Reference Class objects (say, two soccer [football] players) with the following example:

# create Reference classes
b <- setRefClass('Ball', fields = list(size = 'numeric'))

    p <- setRefClass('Player', fields = list(name = 'character', possession = 'Ball'), 
methods = list(pass = function(){
        tmp <- possession$copy()
        possession <<- NULL
        return(tmp)
    }, receive = function(newBall){
        possession <<- newBall
} 
))


# initialize pretend all-star team
# p1 gets initial possession of a new ball
p1 <- p$new(name = 'Ronaldinho', possession = b$new(size=5) )

p2 <- p$new(name = 'Beckham')

# now pass the ball from p1 to p2
p2$receive(p1$pass())

However I get the following error:

Error in function (value)  : 
  invalid replacement for field ‘possession’, should be from class “Ball” or a subclass (was class “NULL”)

Theoretically I am trying to retutn a reference to the ball object, and then add that reference to the other player, but obviously that is not working. I know it is possible to achieve the same results by accessing the fields directly, but I would like to find a way to accomplish this "pass" using internal methods of the class only. Is this possible? Why am I getting this error?

Answer 1

You can get an error because when you define your Player class, you set the possession field to have type Ball. But in your pass function, you set possession to be NULL:

possession <<- NULL

If you change the initialisation to:

p = setRefClass('Player', fields = list(name = 'character', possession = 'ANY')

then everything works as expect:

R> p2$receive(p1$pass())
R> p1
Reference class object of class "Player"
Field "name":
[1] "Ronaldinho"
Field "possession":
NULL
R> p2
Reference class object of class "Player"
Field "name":
[1] "Beckham"
Field "possession":
Reference class object of class "Ball"
Field "size":
[1] 5