Map as a List helper to make building nested lists in Flutter easier.

flat_map.dart

import 'package:flutter/material.dart';

class FlatMapItem<MapKey, MapValue> {
  MapKey key;
  MapValue value;
  int index;

  FlatMapItem.value(this.value, {@required this.index}) : assert(value != null);
  FlatMapItem.key(this.key, {@required this.index}) : assert(key != null);

  bool get isKey => key != null;
}


class FlatMap<Key, Value> {
  final List<FlatMapItem<Key, Value>> items = [];
  final Map<Key, Iterable<Value>> rawMap;

  FlatMap(this.rawMap) {
    _flattenMapInList(rawMap, items);
  }

  FlatMap.empty() : rawMap = {};
  
  void _flattenMapInList<Key, Value>(
      Map<Key, Iterable<Value>> map, 
      List<FlatMapItem<Key, Value>> outList
  ) {
    int keyIndex = 0;
    int valueIndex = 0;
    for (var entry in map.entries) {
      outList.add(FlatMapItem.key(entry.key, index: keyIndex));
      keyIndex++;
      for (var listItem in entry.value) {
        outList.add(FlatMapItem.value(listItem, index: valueIndex));
        valueIndex++;
      }
    }
  }
}