dart - Sort a list of futures - sorting

Question

So I have a set of options that each contain an int value representing their ordinal.
These options are stored in a remote database with each option being a record.
As such when I fetch them from the db I end up with a list of future:
e.g. List<Future<Option>>
I need to be able to sort these Options.
The following dart pad shows a simplified view of what I'm trying to achieve:
https://dartpad.dartlang.org/a5175401516dbb9242a0edec4c89fef6
The Options MUST be futures.
My original solution was to copy the Options into a list, 'complete' them, then sort the list.
This however caused other problems and as such I need to do an 'insitu' sort on the original list.

Answer 1

You cannot sort the futures before they have completed, and even then, you need to extract the values first.
If you need to have a list of futures afterwards, this is what I would do:
List<Future<T>> sortFutures<T>(List<Future<T>> input, [int compare(T a, T b)]) {
var completers = [for (var i = 0; i < input.length; i++) Completer<T>()];
Future.wait(input).then((values) {
values.sort(compare);
for (int i = 0; i < values.length; i++) {
completers[i].complete(values[i]);
}
});
return [for (var c in completers) c.future];
}
This does not return the original futures because you don't know the ordering at the time you have to return them. It does return futures which complete with the same value.
If any of the futures completes with an error, then this blows up. You'll need more error handling if that is possible.

Answer 2

Gentlfolk,
thanks for the help.
julemand101 suggestion of using Future.wait() lead me to the answer.
It also helped me better understand the problem.
I've done a new gist that more accurately shows what I was attempting to do.
Essentilly when we do a db request over the network we get an entity back.
The problem is that the entity will often have references to other entities.
This can end in a whole tree of entities needing to be returned.
Often you don't need any of these entities.
So the solution we went for is to only return the database 'id' of each child entity (only the immediate children).
We then store those id's in a class RefId (see below).
The RefId is essentially a future that has the entities id and knows how to fetch the entity from the db.
When we actually need to access a child entity we force the RefId to complete (i.e. retrieve the entity across the network boundary).
We have a whole caching scheme to keep this performant as well as the ability to force the fetching of child elements, as part of the parent request, where we know up front they will be needed.
The options in my example are essentially menu items that need to be sorted.
But of course I can't sort them until they have been retrieved.
So a re-written example and answer:
https://dartpad.dartlang.org/369e71bb173ba3c19d28f6d6fec2072a
Here is the actual IdRef class we use:
https://dartpad.dartlang.org/ba892873a94d9f6f3924436e9fcd1b42
It now has a static resolveList method to help with this type of problem.
Thanks for your assistance.