Union/Intersection for Dart Lists - dart

Question

In a Flutter app for iOS/Android that I am writing one of the tasks I need the app to handle is that of subscribing/unsubscribing the app to/from multiple Push notification topics as the user's behavior/location or other conditions change.
In itself this is not such a big issue. I am using the excellent Firebase Cloud Messaging plugin for this purpose and its subscribeToTopic + unSubscribeFromTopic methods do just what I need. However, there do not appear to be plural versions of these methods - I mean subscribeToTopic*s* and unSubscribeFromTopic*s*.
Doing a wholesale unsubscribeFrom for each currently subscribed topic followed by a subscribeTo... when ony a few of the topics have changed strikes me as being inefficient. So here is what I do instead
void performUnSubscribe(shared,e){if (!shared.contains(e)) print('Unsubscribe from $e');}
void performSubscribe(shared,e){if (!shared.contains(e)) print('Subscribe to $e');}
void main()
{
List<List<String>> topics = [
['X311','Y739','C0','V1','x43','y5','D3179'],
//currently subscribed list of topics
['X319','Y732','C0','V1','x43','y5','D3183']
//new list of topics to be subscribed to
];
List<String> shared = List.from(topics.fold<Set>(topics.first.toSet(),(a, b) =>
a.intersection(b.toSet())));
print(shared);
print(topics);
topics[0].forEach((e)=> performUnSubscribe(shared,e));
topics[1].forEach((e)=> performSubscribe(shared,e));
}
which works and yields the output
Unsubscribe from X311
Unsubscribe from Y739
Unsubscribe from D3179
Subscribe to X319
Subscribe to Y732
Subscribe to D3183
While this works I find the code rather clumsy. Is there a better way of accomplishing the same result?

Answer 1

I am properly misunderstanding your problem but it seems you can achieve the same output by doing this:
void performUnSubscribe(String e) => print('Unsubscribe from $e');
void performSubscribe(String e) => print('Subscribe to $e');
void main() {
final topics = [
['X311', 'Y739', 'C0', 'V1', 'x43', 'y5', 'D3179'],
//currently subscribed list of topics
['X319', 'Y732', 'C0', 'V1', 'x43', 'y5', 'D3183']
//new list of topics to be subscribed to
];
topics[0]
.where((element) => !topics[1].contains(element))
.forEach(performUnSubscribe);
topics[1]
.where((element) => !topics[0].contains(element))
.forEach(performSubscribe);
}
If your lists are containing a lot of elements, it can be more efficient to use a Set like you are already doing. So it would look like this:
void performUnSubscribe(String e) => print('Unsubscribe from $e');
void performSubscribe(String e) => print('Subscribe to $e');
void main() {
final topics = [
['X311', 'Y739', 'C0', 'V1', 'x43', 'y5', 'D3179'],
//currently subscribed list of topics
['X319', 'Y732', 'C0', 'V1', 'x43', 'y5', 'D3183']
//new list of topics to be subscribed to
];
var set = topics[1].toSet();
topics[0]
.where((element) => !set.contains(element))
.forEach(performUnSubscribe);
set = topics[0].toSet();
topics[1]
.where((element) => !topics[0].contains(element))
.forEach(performSubscribe);
}
Or put the logic inside its own method like this:
void performUnSubscribe(String e) => print('Unsubscribe from $e');
void performSubscribe(String e) => print('Subscribe to $e');
void main() {
final topics = [
['X311', 'Y739', 'C0', 'V1', 'x43', 'y5', 'D3179'],
//currently subscribed list of topics
['X319', 'Y732', 'C0', 'V1', 'x43', 'y5', 'D3183']
//new list of topics to be subscribed to
];
runFunctionOnDifference(topics[0], topics[1], performUnSubscribe);
runFunctionOnDifference(topics[1], topics[0], performSubscribe);
}
/// Execute [function] on elements in [list1] which are not on [list2].
void runFunctionOnDifference<T>(
List<T> list1, List<T> list2, void Function(T) function) {
final set = list2.toSet();
list1.where((element) => !set.contains(element)).forEach(function);
}
All three solutions gives:
Unsubscribe from X311
Unsubscribe from Y739
Unsubscribe from D3179
Subscribe to X319
Subscribe to Y732
Subscribe to D3183