Sets the onDone callback if no value has been set yet or calls immediately if the value has already been set or was set during setting the callback.
Crashes with an assert error if attempting to override an existing callback (i.e. calling this function on the same object twice).
Sets the onDone callback if no value has been set yet or calls immediately if the value has already been set or was set during setting the callback. Crashes with an assert error if attempting to override an existing callback (i.e. calling this function on the same object twice).