DART：合并两个具有相同键的地图

Question

假设我有这些地图：

Map<int,List<String>>  firstMap   = {1:["a", "b"]};
Map<int,List<String>>  secondMap  = {2:["c"]};
Map<int,List<String>>  thirdMap   = {1:["d"]};

我想合并它们而不用相同的键覆盖值以获得此输出：

{1: [a, b, d], 2: [c]

我同时使用了 spread 运算符和 adAll 方法，并且都将键 1 的值覆盖为 {1: [d], 2: [c]} 而不是 {1: [a, b, d], 2: [c]。

Answer 1

void main() {
  Map<int, List<String>> firstMap = {1: ["a", "b"]};
  Map<int, List<String>> secondMap = {2: ["c"]};
  Map<int, List<String>> thirdMap = {1: ["d"]};

  var mergedMap = <int, List<String>>{};
  for (var map in [firstMap, secondMap, thirdMap]) {
    for (var entry in map.entries) {
      // Add an empty `List` to `mergedMap` if the key doesn't already exist
      // and then merge the `List`s.
      (mergedMap[entry.key] ??= []).addAll(entry.value);
    }
  }
  print(mergedMap); // Prints: {1: [a, b, d], 2: [c]}
}