main.dart

typedef GenMethod<T> = String Function(T value);

class CustomValue {
  final String data1;
  final String data2;
  
  CustomValue(this.data1, this.data2);
}

GenMethod<int> getNumToString() {
  return (value) => value.toString();
}

GenMethod<CustomValue> getCustomValueToString() {
  return (value) => '${value.data1} ${value.data2}';
}

class MyData<T> {
  final GenMethod<T> method;
  
  MyData(this.method);
}

String dataToString<T>(MyData<T> anyData, T value) {
  return anyData.method(value);
}

void main() {
  final datas = [
    MyData<int>(getNumToString()),
    MyData<CustomValue>(getCustomValueToString()),
  ];
  print(datas.runtimeType);
  
  final data = datas[0];
  print(data.runtimeType);
  
  final value = dataToString(data, 2);
  print(value);
}