CombinedMapView does not maintain keys uniqueness
renatoathaydes opened this issue · comments
The following test fails:
final map = CombinedMapView([{'1': 1, '2': 2, '3': 3}, {'2': 22, '4': 44}]);
expect(map.length, equals(4));
expect(map.keys.toList(), equals(['1', '2', '3', '4']));The first expectation fails because length returns 5.
The second, because keys returns ['1', '2', '3', '2', '4'].
This seems to violate Map's interface, which says:
There is a finite number of keys in the map, and each key has exactly one value associated with it.
This was a problem for me because I needed something that looks just like a normal Map but based on two other Maps, and which lets a map coming first "override" values on the map coming later in the list of maps. I believe that's the main purpose of combining maps (possibly unmodifiable), but due to this issue, it can't be used like that.
My workaround was creating my own type on top of it:
class SnapshotMap<K, V> extends CombinedMapView<K, V> {
Iterable<K> Function() _lazyKeys;
SnapshotMap(Map<K, V> topMap, Map<K, V> bottomMap)
: super([topMap, bottomMap]) {
_lazyKeys = lazy(() => super.keys.toSet());
}
UnmodifiableMapView<K, V> get view => UnmodifiableMapView(this);
@override
Iterable<K> get keys => _lazyKeys();
@override
int get length => keys.length;
}
Ya, I am going to see if I can maintain the lazy behavior of the current implementation though if possible. Otherwise this would likely break some projects.
No the prettiest thing, but my lazy function above:
T Function() lazy<T>(T Function() f) {
T instance;
return () {
if (instance == null) {
instance = f();
if (instance == null) {
throw StateError('lazy function must not return null');
}
}
return instance;
};
}The problem there is the toSet call ends up eagerly iterating all the keys of all the maps as soon as you ask for the first key - I have a simple implementation coming though.
Note that in adding the test for this I also noticed forEach and values were broken, fun!
(these all get fixed by fixing keys though)
That's true, but if you avoid that you'll have to check for already-iterated keys on all maps except the first... which may be more costly?! Unless perhaps if you accumulate keys as you see them into a set?
Yes I am accumulating them as i see them, the impl is actually really easy/small.
@jakemac53 is this going to be released soon?
Yes, should be very soon (waiting for an existing publisher to add me as an uploader or publish it themselves).
This is now published as version 1.14.12