Dart / Flutter如何在地图中初始化列表？

Question

如何在地图中初始化列表？

Map<String,Map<int,List<String>>> myMapList = Map();

我得到一个错误：

The method '[]' was called on null.
I/flutter (16433): Receiver: null
I/flutter (16433): Tried calling: [](9)

Answer 1

只需在创建地图时添加元素，或稍后add将其添加。例如：

var myMapList = <String, Map<int, List<String>>>{
  'someKey': <int, List<String>>{
    0: <String>['foo', 'bar'],
  },
};

或

var m2 = <String, Map<int, List<String>>>{}; // empty map
m2['otherKey'] = <int, List<String>>{}; // add an empty map at the top level
m2['otherKey'][2] = <String>[]; // and an empty list to that map
m2['otherKey'][2].add('baz'); // and a value to that list
print(m2);

打印{otherKey: {2: [baz]}}

Answer 2

尝试使用{}

进行初始化

Map<String,Map<int,List<String>>> myMapList = {}; // just add {}

它将创建一个空地图，并且可以正常工作。