Different Vars require different mechanisms for synchronization.
A non-side-effecting test if the value was initialized
Set the session variable
-- the value to set the session variable to
Perform an atomic update of this Settable.
Change the value of the Var for the lifespan of the function
Shadow of the 'is' method
The current value of the variable
On any change to this Var, invoke the function.
the function to execute on change
Perform an atomic operation on the Settable.
Shadow of the apply method
Set the Var if it has not been calculated
Has this Var been set or accessed and had its default value calculated
Applies the given function to the contents of this variable and sets the variable to the resulting value.
-- the function to apply and set the result from.