A function that determines a value for an async compute.
function(lastSetValue, setVal)
The function callback to can.compute.async that determines
the value of the compute.
Parameters
lastSetValue
{*}Optional
The last set value of the compute. This should be returned
if you are doing an in-place compute.
setVal
{function(newVal)}Optional
Called to update the value
of the compute at a later time.
Returns
{*}
If a setVal argument is not provided, the return value
is set as the current value of the compute. If setVal is provided and
undefined is returned, the current value remains until setVal is called.