Capture default value from somewhere dynamic (e.g. callable)

[ntoll]

On start-up it should be possible for values in the datastore to be dynamically populated via a function. Furthermore, such functions should also be able to be called in some scheduled way so, for example, a GPS reading to be stored in the datastore is created by calling a function that uses the browser's GPS reading API, after some period of time (say 5 seconds).