FlutterDart Map with Type as key - Stack Overflow

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I have the following map that associates a Type with a list of methods to be executed at some point where the argument is the required type.

final Map<Type, List<Function>> _listeners = {};

void registerListener<T>(void Function(T) listener) {
  final objType = T.runtimeType;
  _listeners[objType] ??= [];
  _listeners[objType]!.add(listener);
}

The problem however is that this does not work... yet the lists are present in the Map though.

registerListener((MyObject x) => {});
final test = _listeners[MyObject];
print(test);
final test2 = _listeners[MyObject().runtimeType];
print(test2);

both logs print null instead of a list of functions. Yet the list is stored with the method present in it as we can see from this debugger statement

I have the following map that associates a Type with a list of methods to be executed at some point where the argument is the required type.

final Map<Type, List<Function>> _listeners = {};

void registerListener<T>(void Function(T) listener) {
  final objType = T.runtimeType;
  _listeners[objType] ??= [];
  _listeners[objType]!.add(listener);
}

The problem however is that this does not work... yet the lists are present in the Map though.

registerListener((MyObject x) => {});
final test = _listeners[MyObject];
print(test);
final test2 = _listeners[MyObject().runtimeType];
print(test2);

both logs print null instead of a list of functions. Yet the list is stored with the method present in it as we can see from this debugger statement

• flutter
• dart
• dictionary

Share Improve this question Follow asked Mar 24 at 16:48 Wouter VandenputteWouter Vandenputte 2,11355 gold badges3232 silver badges5555 bronze badges 4

• most likely you need final objType = T; and not final objType = T.runtimeType; – pskink Commented Mar 24 at 17:08
• That indeed worked! If you add it as an answer I will accept it :D – Wouter Vandenputte Commented Mar 24 at 18:01
• I bet that'll still cause havoc when your code gets symbols mangled in a release version. – Randal Schwartz Commented Mar 24 at 20:50
• It's worth noting that the hashCode of a type isn't guaranteed to be the same value between app runs, app updates, or between instances of the app on different platforms. It should be fine for a map that only lives for the duration of the app, but you shouldn't depend on this for persisted data. – Abion47 Commented Mar 25 at 21:32

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1 Answer 1

Sorted by: Reset to default 0

I believe you may add one more parameters for object initiation, because T it's an abstraction meanwhile T.runtimeType it will display Type

void registerListener<T>(T key, void Function(T data) listener) {
  final objType = key.runtimeType;
  print('RUNTIME: $objType');
  print('RUNTIME: ${T.runtimeType}');
  _listeners[objType] ??= [];
  _listeners[objType]!.add(listener);
}

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